Checkpoint AV1 inter encoding and shared search operators

Preserve the current encoder, container sequence, and decoder integration work. Record focused Release verification and the remaining interpolation/conformance work in the implementation plan. Non-regular interpolation runtime verification remains open.

src/ImageSharp/Formats/Heif/Av1/Av1BitStreamWriter.cs

@@ -173,6 +173,84 @@ internal ref struct Av1BitStreamWriter
         }
     }
 
+    /// <summary>
+    /// Writes a finite subexponential value recentered around a signed reference value.
+    /// </summary>
+    /// <param name="value">The signed value to write.</param>
+    /// <param name="valueMagnitude">One greater than the maximum absolute value in the signed domain.</param>
+    /// <param name="groupBitCount">The bit width of the first subexponential group.</param>
+    /// <param name="reference">The signed reference value around which smaller codewords are concentrated.</param>
+    public void WriteSignedReferenceSubexponential(int value, int valueMagnitude, int groupBitCount, int reference)
+    {
+        int shiftedReference = reference + valueMagnitude - 1;
+        int shiftedValue = value + valueMagnitude - 1;
+        int scaledValueCount = (valueMagnitude << 1) - 1;
+        int recenteredValue = RecenterFiniteNonNegative(scaledValueCount, shiftedReference, shiftedValue);
+
+        this.WriteSubexponential(recenteredValue, scaledValueCount, groupBitCount);
+    }
+
+    /// <summary>
+    /// Writes one value using a finite sequence of exponentially growing code groups.
+    /// </summary>
+    private void WriteSubexponential(int value, int valueCount, int groupBitCount)
+    {
+        int groupIndex = 0;
+        int groupStart = 0;
+        while (true)
+        {
+            // The first two groups retain the initial width. Later groups grow one bit at a time until the
+            // finite tail is small enough for the exact non-symmetric alphabet.
+            int bitCount = groupIndex == 0 ? groupBitCount : groupBitCount + groupIndex - 1;
+            int groupSize = 1 << bitCount;
+            if (valueCount <= groupStart + (3 * groupSize))
+            {
+                this.WriteNonSymmetric((uint)(value - groupStart), (uint)(valueCount - groupStart));
+
+                return;
+            }
+
+            bool useLaterGroup = value >= groupStart + groupSize;
+            this.WriteBoolean(useLaterGroup);
+            if (!useLaterGroup)
+            {
+                this.WriteLiteral((uint)(value - groupStart), bitCount);
+                return;
+            }
+
+            groupIndex++;
+            groupStart += groupSize;
+        }
+    }
+
+    /// <summary>
+    /// Maps an unsigned value to increasing distance from a reference inside a finite domain.
+    /// </summary>
+    private static int RecenterFiniteNonNegative(int valueCount, int reference, int value)
+    {
+        if ((reference << 1) <= valueCount)
+        {
+            return RecenterNonNegative(reference, value);
+        }
+
+        return RecenterNonNegative(valueCount - 1 - reference, valueCount - 1 - value);
+    }
+
+    /// <summary>
+    /// Maps an unsigned value to alternating positions around a nonnegative reference.
+    /// </summary>
+    private static int RecenterNonNegative(int reference, int value)
+    {
+        if (value > (reference << 1))
+        {
+            return value;
+        }
+
+        return value >= reference
+            ? (value - reference) << 1
+            : ((reference - value) << 1) - 1;
+    }
+
     /// <summary>
     /// Appends one bit to the partially assembled output byte.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Av1Constants.cs

@@ -21,6 +21,41 @@ internal static class Av1Constants
     /// </summary>
     public const int LevelBits = 5;
 
+    /// <summary>
+    /// The number of bits used for the zero-based operating-point count.
+    /// </summary>
+    public const int OperatingPointCountBits = 5;
+
+    /// <summary>
+    /// The number of bits used for an operating-point layer-selection mask.
+    /// </summary>
+    public const int OperatingPointIdcBits = 12;
+
+    /// <summary>
+    /// The number of bits used for a frame type.
+    /// </summary>
+    public const int FrameTypeBits = 2;
+
+    /// <summary>
+    /// The first sequence-level index that carries an explicit tier bit.
+    /// </summary>
+    public const int SequenceTierMinimumLevelIndex = 8;
+
+    /// <summary>
+    /// The number of bits used to select a frame from the eight-slot reference map.
+    /// </summary>
+    public const int ReferenceFrameIndexBits = 3;
+
+    /// <summary>
+    /// The sequence-header value that lets each frame choose whether to use screen-content tools.
+    /// </summary>
+    public const int SelectScreenContentTools = 2;
+
+    /// <summary>
+    /// The sequence-header value that lets each applicable frame choose whether to require integer motion vectors.
+    /// </summary>
+    public const int SelectIntegerMotionVector = 2;
+
     /// <summary>
     /// The maximum number of operating points declared by one AV1 sequence header.
     /// </summary>
@@ -46,6 +81,11 @@ internal static class Av1Constants
     /// </summary>
     public const int ReferencesPerFrame = 7;
 
+    /// <summary>
+    /// The largest frame width or height representable by the 16-bit AV1 dimension syntax.
+    /// </summary>
+    public const int MaxFrameDimension = 1 << 16;
+
     /// <summary>
     /// The maximum area of a tile in units of luma samples.
     /// </summary>
@@ -66,6 +106,11 @@ internal static class Av1Constants
     /// </summary>
     public const int MaxTileRowCount = 64;
 
+    /// <summary>
+    /// The number of 64x64 CDEF filter units in a 128x128 superblock.
+    /// </summary>
+    public const int CdefUnitsPerSuperblock = 4;
+
     /// <summary>
     /// The number of frames that can be stored for future reference.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Av1Decoder.cs

@@ -261,6 +261,52 @@ internal sealed class Av1Decoder : IAv1TileReader, IDisposable
         return this.ConvertToFrame<TPixel>(outputFrame.FrameBuffer, outputFrame.FrameHeader, effectiveColorProfile);
     }
 
+    /// <summary>
+    /// Decodes the next visible sample in a bounded AV1 image sequence directly into a caller-owned frame.
+    /// </summary>
+    /// <typeparam name="TPixel">The destination pixel type.</typeparam>
+    /// <param name="buffer">The complete AV1 sample payload.</param>
+    /// <param name="containerColorProfile">The container color description.</param>
+    /// <param name="codecConfiguration">The AV1 sample-entry configuration.</param>
+    /// <param name="expectedCodedSize">The coded dimensions declared by the visual sample entry.</param>
+    /// <param name="sourceRectangle">The clean-aperture region mapped to the complete destination frame.</param>
+    /// <param name="destination">The caller-owned packed-pixel frame receiving the presented sample.</param>
+    public void DecodeSequenceFrame<TPixel>(
+        Span<byte> buffer,
+        CicpProfile? containerColorProfile,
+        Av1CodecConfiguration? codecConfiguration,
+        Size expectedCodedSize,
+        Rectangle sourceRectangle,
+        ImageFrame<TPixel> destination)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        CicpProfile effectiveColorProfile = this.DecodePayload(
+            buffer,
+            containerColorProfile,
+            codecConfiguration,
+            null,
+            requireShownFrame: true);
+
+        Av1ReferenceFrame outputFrame = this.referenceFrames.ResolveOutput();
+        Size codedSize = new(
+            outputFrame.FrameHeader.FrameSize.SuperResolutionUpscaledWidth,
+            outputFrame.FrameHeader.FrameSize.FrameHeight);
+
+        if (codedSize != expectedCodedSize)
+        {
+            throw new InvalidImageContentException(
+                "The decoded image-sequence sample dimensions do not match its visual sample entry.");
+        }
+
+        Av1YuvConverter.ConvertRegionToRgb(
+            this.configuration,
+            outputFrame.FrameBuffer,
+            sourceRectangle,
+            destination);
+
+        destination.Metadata.CicpProfile = effectiveColorProfile.DeepClone();
+    }
+
     /// <summary>
     /// Decodes one non-presented AV1 image-sequence sample while retaining its reference state.
     /// </summary>
@@ -286,6 +332,7 @@ internal sealed class Av1Decoder : IAv1TileReader, IDisposable
     /// <param name="containerColorProfile">The container color description.</param>
     /// <param name="codecConfiguration">The AV1 sample-entry configuration.</param>
     /// <param name="expectedCodedSize">The required coded dimensions.</param>
+    /// <param name="sourceRectangle">The clean-aperture luma region mapped to the destination.</param>
     /// <param name="destination">The packed color frame receiving alpha values.</param>
     /// <param name="outputSize">The complete presented size of the auxiliary image.</param>
     /// <param name="destinationRectangle">The destination region receiving the alpha image.</param>
@@ -295,6 +342,7 @@ internal sealed class Av1Decoder : IAv1TileReader, IDisposable
         CicpProfile? containerColorProfile,
         Av1CodecConfiguration? codecConfiguration,
         Size expectedCodedSize,
+        Rectangle sourceRectangle,
         ImageFrame<TPixel> destination,
         Size outputSize,
         Rectangle destinationRectangle,
@@ -312,6 +360,7 @@ internal sealed class Av1Decoder : IAv1TileReader, IDisposable
         this.ComposeAlpha(
             outputFrame.FrameBuffer,
             expectedCodedSize,
+            sourceRectangle,
             destination,
             outputSize,
             destinationRectangle,
@@ -424,6 +473,35 @@ internal sealed class Av1Decoder : IAv1TileReader, IDisposable
         Rectangle destinationRectangle,
         bool premultiplied)
         where TPixel : unmanaged, IPixel<TPixel>
+        => this.ComposeAlpha(
+            frameBuffer,
+            expectedCodedSize,
+            new Rectangle(0, 0, frameBuffer.Width, frameBuffer.Height),
+            destination,
+            outputSize,
+            destinationRectangle,
+            premultiplied);
+
+    /// <summary>
+    /// Composes one decoded monochrome region into a packed color frame.
+    /// </summary>
+    /// <typeparam name="TPixel">The destination color pixel type.</typeparam>
+    /// <param name="frameBuffer">The decoded monochrome planes.</param>
+    /// <param name="expectedCodedSize">The required coded dimensions.</param>
+    /// <param name="sourceRectangle">The luma region mapped to the destination rectangle.</param>
+    /// <param name="destination">The packed color frame receiving alpha values.</param>
+    /// <param name="outputSize">The complete presented size of the auxiliary image.</param>
+    /// <param name="destinationRectangle">The destination region receiving the alpha image.</param>
+    /// <param name="premultiplied">Whether stored color samples must be converted to unassociated alpha.</param>
+    private void ComposeAlpha<TPixel>(
+        Av1FrameBuffer<byte> frameBuffer,
+        Size expectedCodedSize,
+        Rectangle sourceRectangle,
+        ImageFrame<TPixel> destination,
+        Size outputSize,
+        Rectangle destinationRectangle,
+        bool premultiplied)
+        where TPixel : unmanaged, IPixel<TPixel>
     {
         if (expectedCodedSize != default && (frameBuffer.Width != expectedCodedSize.Width || frameBuffer.Height != expectedCodedSize.Height))
         {
@@ -440,6 +518,7 @@ internal sealed class Av1Decoder : IAv1TileReader, IDisposable
         Av1YuvConverter.ComposeAlpha(
             this.configuration,
             frameBuffer,
+            sourceRectangle,
             destination,
             outputSize,
             destinationRectangle,

src/ImageSharp/Formats/Heif/Av1/Color/Av1YuvConverter.cs

@@ -92,6 +92,48 @@ internal static class Av1YuvConverter
             mode);
     }
 
+    /// <summary>
+    /// Converts a rectangular region of reconstructed component planes directly to packed pixels.
+    /// </summary>
+    /// <typeparam name="TPixel">The destination pixel type.</typeparam>
+    /// <param name="configuration">The configuration used for allocation and pixel conversion.</param>
+    /// <param name="frameBuffer">The reconstructed AV1 frame.</param>
+    /// <param name="sourceRectangle">The luma-sample region mapped to the complete destination frame.</param>
+    /// <param name="image">The destination image frame.</param>
+    public static void ConvertRegionToRgb<TPixel>(
+        Configuration configuration,
+        Av1FrameBuffer<byte> frameBuffer,
+        Rectangle sourceRectangle,
+        ImageFrame<TPixel> image)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        HeifColorConversionParameters parameters = GetConversionParameters(frameBuffer.ColorConfig, out HeifColorConversionMode mode);
+        if (frameBuffer.BitDepth == Av1BitDepth.EightBit)
+        {
+            Av1PlanarSampleBuffer<byte> buffer = new(frameBuffer);
+            HeifPlanarColorConverter.ConvertToRgb<TPixel, Av1PlanarSampleBuffer<byte>, byte, HeifByteSampleConverter>(
+                configuration,
+                buffer,
+                image,
+                in parameters,
+                mode,
+                sourceRectangle.X,
+                sourceRectangle.Y);
+
+            return;
+        }
+
+        Av1PlanarSampleBuffer<ushort> highBitDepthBuffer = new(frameBuffer);
+        HeifPlanarColorConverter.ConvertToRgb<TPixel, Av1PlanarSampleBuffer<ushort>>(
+            configuration,
+            highBitDepthBuffer,
+            image,
+            in parameters,
+            mode,
+            sourceRectangle.X,
+            sourceRectangle.Y);
+    }
+
     /// <summary>
     /// Composes the reconstructed luma plane into a packed color frame as auxiliary alpha.
     /// </summary>
@@ -110,9 +152,37 @@ internal static class Av1YuvConverter
         Rectangle destinationRectangle,
         bool premultiplied)
         where TPixel : unmanaged, IPixel<TPixel>
+        => ComposeAlpha(
+            configuration,
+            frameBuffer,
+            new Rectangle(0, 0, frameBuffer.Width, frameBuffer.Height),
+            destination,
+            outputSize,
+            destinationRectangle,
+            premultiplied);
+
+    /// <summary>
+    /// Composes a rectangular reconstructed luma region into a packed color frame as auxiliary alpha.
+    /// </summary>
+    /// <typeparam name="TPixel">The destination color pixel type.</typeparam>
+    /// <param name="configuration">The configuration used for allocation and pixel conversion.</param>
+    /// <param name="frameBuffer">The reconstructed AV1 frame containing the alpha luma plane.</param>
+    /// <param name="sourceRectangle">The luma-sample region mapped to the destination rectangle.</param>
+    /// <param name="destination">The packed color frame receiving alpha values.</param>
+    /// <param name="outputSize">The complete presented size of the auxiliary image or grid tile.</param>
+    /// <param name="destinationRectangle">The destination region receiving the presented alpha image.</param>
+    /// <param name="premultiplied">Whether stored color samples must be converted to unassociated alpha.</param>
+    public static void ComposeAlpha<TPixel>(
+        Configuration configuration,
+        Av1FrameBuffer<byte> frameBuffer,
+        Rectangle sourceRectangle,
+        ImageFrame<TPixel> destination,
+        Size outputSize,
+        Rectangle destinationRectangle,
+        bool premultiplied)
+        where TPixel : unmanaged, IPixel<TPixel>
     {
         HeifColorConversionParameters parameters = GetConversionParameters(frameBuffer.ColorConfig, out _);
-        Rectangle sourceRectangle = new(0, 0, frameBuffer.Width, frameBuffer.Height);
         if (frameBuffer.BitDepth == Av1BitDepth.EightBit)
         {
             Av1PlanarSampleBuffer<byte> buffer = new(frameBuffer);

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1FrameEntropyContext.cs

@@ -512,8 +512,8 @@ internal sealed class Av1FrameEntropyContext
     {
         int qContext = GetQContext(qIndex);
 
-        // The prototypes are never exposed to a range reader. Copying their state lets a decoder session reuse the
-        // same three mutable object graphs even when successive frames select different coefficient-model bands.
+        // The prototypes are never exposed to a range reader or writer. Copying their state lets each codec session
+        // reuse its mutable object graphs even when successive frames select different coefficient-model bands.
         this.CopyFrom(DefaultPrototypes[qContext]);
     }
 

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1MotionVectorContext.cs

@@ -41,10 +41,7 @@ internal sealed class Av1MotionVectorContext
         /// <param name="symbol">The zero-based symbol.</param>
         /// <param name="distribution">The live symbol distribution.</param>
         /// <returns>The symbol cost in 1/512-bit units, or zero when writing.</returns>
-        public static abstract int ProcessSymbol(
-            Av1SymbolWriter writer,
-            int symbol,
-            Av1Distribution distribution);
+        public static abstract int ProcessSymbol(Av1SymbolWriter writer, int symbol, Av1Distribution distribution);
     }
 
     /// <summary>
@@ -103,13 +100,14 @@ internal sealed class Av1MotionVectorContext
     }
 
     /// <summary>
-    /// Writes an integer displacement vector relative to a spatially derived reference.
+    /// Writes a motion vector relative to a spatially derived reference.
     /// </summary>
     /// <param name="writer">The tile range encoder.</param>
     /// <param name="value">The displacement vector to encode.</param>
     /// <param name="reference">The spatially derived reference vector.</param>
-    public void Write(Av1SymbolWriter writer, Av1MotionVector value, Av1MotionVector reference)
-        => _ = this.Process<MotionVectorWriteOperation>(writer, value, reference);
+    /// <param name="precision">The fractional precision selected by the frame header.</param>
+    public void Write(Av1SymbolWriter writer, Av1MotionVector value, Av1MotionVector reference, Av1MotionVectorPrecision precision)
+        => _ = this.Process<MotionVectorWriteOperation>(writer, value, reference, precision);
 
     /// <summary>
     /// Measures a motion-vector delta against the live distributions without changing them.
@@ -117,17 +115,15 @@ internal sealed class Av1MotionVectorContext
     /// <param name="writer">The tile range encoder associated with the live context.</param>
     /// <param name="value">The motion vector to measure.</param>
     /// <param name="reference">The spatially derived reference vector.</param>
+    /// <param name="precision">The fractional precision selected by the frame header.</param>
     /// <returns>The syntax cost in 1/512-bit units.</returns>
-    public int GetCost(Av1SymbolWriter writer, Av1MotionVector value, Av1MotionVector reference)
-        => this.Process<MotionVectorCostOperation>(writer, value, reference);
+    public int GetCost(Av1SymbolWriter writer, Av1MotionVector value, Av1MotionVector reference, Av1MotionVectorPrecision precision)
+        => this.Process<MotionVectorCostOperation>(writer, value, reference, precision);
 
     /// <summary>
     /// Processes one complete motion-vector delta through a closed symbol operation.
     /// </summary>
-    private int Process<TOperation>(
-        Av1SymbolWriter writer,
-        Av1MotionVector value,
-        Av1MotionVector reference)
+    private int Process<TOperation>(Av1SymbolWriter writer, Av1MotionVector value, Av1MotionVector reference, Av1MotionVectorPrecision precision)
         where TOperation : struct, IMotionVectorSymbolOperation
     {
         int row = value.Row - reference.Row;
@@ -140,12 +136,12 @@ internal sealed class Av1MotionVectorContext
         int rate = TOperation.ProcessSymbol(writer, jointType, this.Joint);
         if (row != 0)
         {
-            rate += this.Vertical.Process<TOperation>(writer, row);
+            rate += this.Vertical.Process<TOperation>(writer, row, precision);
         }
 
         if (column != 0)
         {
-            rate += this.Horizontal.Process<TOperation>(writer, column);
+            rate += this.Horizontal.Process<TOperation>(writer, column, precision);
         }
 
         return rate;
@@ -157,10 +153,7 @@ internal sealed class Av1MotionVectorContext
     private readonly struct MotionVectorWriteOperation : IMotionVectorSymbolOperation
     {
         /// <inheritdoc/>
-        public static int ProcessSymbol(
-            Av1SymbolWriter writer,
-            int symbol,
-            Av1Distribution distribution)
+        public static int ProcessSymbol(Av1SymbolWriter writer, int symbol, Av1Distribution distribution)
         {
             writer.WriteSymbol(symbol, distribution);
             return 0;
@@ -173,10 +166,7 @@ internal sealed class Av1MotionVectorContext
     private readonly struct MotionVectorCostOperation : IMotionVectorSymbolOperation
     {
         /// <inheritdoc/>
-        public static int ProcessSymbol(
-            Av1SymbolWriter writer,
-            int symbol,
-            Av1Distribution distribution)
+        public static int ProcessSymbol(Av1SymbolWriter writer, int symbol, Av1Distribution distribution)
             => Av1ProbabilityCost.GetSymbolCost(distribution, symbol);
     }
 
@@ -350,48 +340,72 @@ internal sealed class Av1MotionVectorContext
         }
 
         /// <summary>
-        /// Writes one signed integer-precision component.
+        /// Writes one signed motion-vector component.
         /// </summary>
         /// <param name="writer">The tile range encoder.</param>
         /// <param name="value">The nonzero component in one-eighth-sample units.</param>
-        public void Write(Av1SymbolWriter writer, int value)
-            => _ = this.Process<MotionVectorWriteOperation>(writer, value);
+        /// <param name="precision">The fractional precision selected by the frame header.</param>
+        public void Write(Av1SymbolWriter writer, int value, Av1MotionVectorPrecision precision)
+            => _ = this.Process<MotionVectorWriteOperation>(writer, value, precision);
 
         /// <summary>
         /// Processes one nonzero signed component through the shared motion-vector symbol operation.
         /// </summary>
-        public int Process<TOperation>(Av1SymbolWriter writer, int value)
+        public int Process<TOperation>(Av1SymbolWriter writer, int value, Av1MotionVectorPrecision precision)
             where TOperation : struct, IMotionVectorSymbolOperation
         {
             int magnitude = Math.Abs(value);
-            DebugGuard.IsTrue(magnitude > 0 && (magnitude & 7) == 0, "Displacement-vector components must use whole-sample precision.");
-
-            // Class zero contains the two whole-sample magnitudes 8 and 16. Above it, the highest set bit of magnitude
-            // minus one selects the doubling range; subtracting three converts the eighth-sample bit index to the class.
-            int magnitudeClass = magnitude <= (ClassZeroSize << 3) ? 0 : Av1Math.MostSignificantBit((uint)(magnitude - 1)) - 3;
+            int precisionMask = precision == Av1MotionVectorPrecision.Integer
+                ? 7
+                : precision == Av1MotionVectorPrecision.QuarterSample ? 1 : 0;
+
+            DebugGuard.IsTrue(
+                magnitude > 0 && (magnitude & precisionMask) == 0,
+                "Motion-vector components must match the frame precision.");
+
+            // The coded value is magnitude minus one. Its whole-sample portion selects the doubling class,
+            // while the remainder carries integer offset, fractional phase, and the high-precision bit.
+            int codedMagnitude = magnitude - 1;
+            uint classValue = (uint)(codedMagnitude >> 3);
+            int magnitudeClass = classValue == 0 ? 0 : Av1Math.MostSignificantBit(classValue);
             DebugGuard.MustBeLessThan(magnitudeClass, MagnitudeClassCount, nameof(magnitudeClass));
+            int magnitudeBase = magnitudeClass == 0 ? 0 : ClassZeroSize << (magnitudeClass + 2);
+            int offset = codedMagnitude - magnitudeBase;
+            int integerOffset = offset >> 3;
+            int fractional = (offset >> 1) & 3;
+            int highPrecision = offset & 1;
             int rate = TOperation.ProcessSymbol(writer, value < 0 ? 1 : 0, this.Sign);
             rate += TOperation.ProcessSymbol(writer, magnitudeClass, this.MagnitudeClass);
 
             if (magnitudeClass == 0)
             {
-                rate += TOperation.ProcessSymbol(writer, (magnitude >> 3) - 1, this.ClassZero);
-                return rate;
+                rate += TOperation.ProcessSymbol(writer, integerOffset, this.ClassZero);
+            }
+            else
+            {
+                for (int bit = 0; bit < magnitudeClass; bit++)
+                {
+                    // Integer offsets are transmitted least-significant bit first through independent models.
+                    rate += TOperation.ProcessSymbol(writer, (integerOffset >> bit) & 1, this.OffsetBits[bit]);
+                }
             }
 
-            // Remove the class base and the implicit low-bit value 7 plus the final one before coding the remaining
-            // whole-sample offset least-significant bit first.
-            int magnitudeBase = ClassZeroSize << (magnitudeClass + 2);
-            int integerOffset = (magnitude - magnitudeBase - 8) >> 3;
+            if (precision != Av1MotionVectorPrecision.Integer)
+            {
+                Av1Distribution fractionalDistribution = magnitudeClass == 0
+                    ? this.ClassZeroFractional[integerOffset]
+                    : this.Fractional;
 
-            for (int bit = 0; bit < magnitudeClass; bit++)
+                rate += TOperation.ProcessSymbol(writer, fractional, fractionalDistribution);
+            }
+
+            if (precision == Av1MotionVectorPrecision.EighthSample)
             {
-                // The decoder reconstructs offsets least-significant bit first, so each adaptive bit model must be
-                // updated in the same order during encoding.
-                rate += TOperation.ProcessSymbol(
-                    writer,
-                    (integerOffset >> bit) & 1,
-                    this.OffsetBits[bit]);
+                Av1Distribution highPrecisionDistribution = magnitudeClass == 0
+                    ? this.ClassZeroHighPrecision
+                    : this.HighPrecision;
+
+                rate += TOperation.ProcessSymbol(writer, highPrecision, highPrecisionDistribution);
             }
 
             return rate;

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1RateDistortion.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using System.Runtime.Intrinsics;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline.Quantizers;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
@@ -10,6 +11,108 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
 /// </summary>
 internal static class Av1RateDistortion
 {
+    /// <summary>
+    /// Each fitted curve contains 65 equally spaced samples, including the cubic interpolation endpoints.
+    /// </summary>
+    private const int ModelCurveLength = 65;
+
+    /// <summary>
+    /// Gets the rate-curve category for each AV1 block geometry.
+    /// </summary>
+    private static ReadOnlySpan<byte> ModelRateCategories => [0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 1, 1, 2, 2, 3, 3];
+
+    /// <summary>
+    /// Gets the four block-size rate curves in fixed-point bit-cost units per sample.
+    /// </summary>
+    private static ReadOnlySpan<double> ModelRateCurves =>
+    [
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 118.257702, 120.210658, 121.434853, 122.100487,
+        122.377758, 122.436865, 72.290102, 96.974289, 101.652727,
+        126.830141, 140.417377, 157.644879, 184.315291, 215.823873,
+        262.300169, 335.919859, 420.624173, 519.185032, 619.854243,
+        726.053595, 827.663369, 933.127475, 1037.988755, 1138.839609,
+        1233.342933, 1333.508064, 1428.760126, 1533.396364, 1616.952052,
+        1744.539319, 1803.413586, 1951.466618, 1994.227838, 2086.031680,
+        2148.635443, 2239.068450, 2222.590637, 2338.859809, 2402.929011,
+        2418.727875, 2435.342670, 2471.159469, 2523.187446, 2591.183827,
+        2674.905840, 2774.110714, 2888.555675, 3017.997952, 3162.194773,
+        3320.903365, 3493.880956, 3680.884773, 3881.672045, 4096.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 13.087244, 15.919735, 25.930313, 24.412411,
+        28.567417, 29.924194, 30.857010, 32.742979, 36.382570,
+        39.210386, 42.265690, 47.378572, 57.014850, 82.740067,
+        137.346562, 219.968084, 316.781856, 415.643773, 516.706538,
+        614.914364, 714.303763, 815.512135, 911.210485, 1008.501528,
+        1109.787854, 1213.772279, 1322.922561, 1414.752579, 1510.505641,
+        1615.741888, 1697.989032, 1780.123933, 1847.453790, 1913.742309,
+        1960.828122, 2047.500168, 2085.454095, 2129.230668, 2158.171824,
+        2182.231724, 2217.684864, 2269.589211, 2337.264824, 2420.618694,
+        2519.557814, 2633.989178, 2763.819779, 2908.956609, 3069.306660,
+        3244.776927, 3435.274401, 3640.706076, 3860.978945, 4096.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 4.656893, 5.123633, 5.594132, 6.162376,
+        6.918433, 7.768444, 8.739415, 10.105862, 11.477328,
+        13.236604, 15.421030, 19.093623, 25.801871, 46.724612,
+        98.841054, 181.113466, 272.586364, 359.499769, 445.546343,
+        525.944439, 605.188743, 681.793483, 756.668359, 838.486885,
+        926.950356, 1015.482542, 1113.353926, 1204.897193, 1288.871992,
+        1373.464145, 1455.746628, 1527.796460, 1588.475066, 1658.144771,
+        1710.302500, 1807.563351, 1863.197608, 1927.281616, 1964.450872,
+        2022.719898, 2100.041145, 2185.205712, 2280.993936, 2387.616216,
+        2505.282950, 2634.204540, 2774.591385, 2926.653884, 3090.602436,
+        3266.647443, 3454.999303, 3655.868416, 3869.465182, 4096.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
+        0.000000, 0.337370, 0.391916, 0.468839, 0.566334,
+        0.762564, 1.069225, 1.384361, 1.787581, 2.293948,
+        3.251909, 4.412991, 8.050068, 11.606073, 27.668092,
+        65.227758, 128.463938, 202.097653, 262.715851, 312.464873,
+        355.601398, 400.609054, 447.201352, 495.761568, 552.871938,
+        619.067625, 691.984883, 773.753288, 860.628503, 946.262808,
+        1019.805896, 1106.061360, 1178.422145, 1244.852258, 1302.173987,
+        1399.650266, 1548.092912, 1545.928652, 1670.817500, 1694.523823,
+        1779.195362, 1882.155494, 1990.662097, 2108.325181, 2235.456119,
+        2372.366287, 2519.367059, 2676.769812, 2844.885918, 3024.026754,
+        3214.503695, 3416.628115, 3630.711389, 3857.064892, 4096.000000,
+    ];
+
+    /// <summary>
+    /// Gets the low- and high-error distortion curves in sixteenth-sample-error units.
+    /// </summary>
+    private static ReadOnlySpan<double> ModelDistortionCurves =>
+    [
+        16.000000, 15.962891, 15.925174, 15.886888, 15.848074,
+        15.808770, 15.769015, 15.728850, 15.688313, 15.647445,
+        15.606284, 15.564870, 15.525918, 15.483820, 15.373330,
+        15.126844, 14.637442, 14.184387, 13.560070, 12.880717,
+        12.165995, 11.378144, 10.438769, 9.130790, 7.487633,
+        5.688649, 4.267515, 3.196300, 2.434201, 1.834064,
+        1.369920, 1.035921, 0.775279, 0.574895, 0.427232,
+        0.314123, 0.233236, 0.171440, 0.128188, 0.092762,
+        0.067569, 0.049324, 0.036330, 0.027008, 0.019853,
+        0.015539, 0.011093, 0.008733, 0.007624, 0.008105,
+        0.005427, 0.004065, 0.003427, 0.002848, 0.002328,
+        0.001865, 0.001457, 0.001103, 0.000801, 0.000550,
+        0.000348, 0.000193, 0.000085, 0.000021, 0.000000,
+        16.000000, 15.996116, 15.984769, 15.966413, 15.941505,
+        15.910501, 15.873856, 15.832026, 15.785466, 15.734633,
+        15.679981, 15.621967, 15.560961, 15.460157, 15.288367,
+        15.052462, 14.466922, 13.921212, 13.073692, 12.222005,
+        11.237799, 9.985848, 8.898823, 7.423519, 5.995325,
+        4.773152, 3.744032, 2.938217, 2.294526, 1.762412,
+        1.327145, 1.020728, 0.765535, 0.570548, 0.425833,
+        0.313825, 0.232959, 0.171324, 0.128174, 0.092750,
+        0.067558, 0.049319, 0.036330, 0.027008, 0.019853,
+        0.015539, 0.011093, 0.008733, 0.007624, 0.008105,
+        0.005427, 0.004065, 0.003427, 0.002848, 0.002328,
+        0.001865, 0.001457, 0.001103, 0.000801, 0.000550,
+        0.000348, 0.000193, 0.000085, 0.000021, -0.000000,
+    ];
+
     /// <summary>
     /// Gets the key-frame rate multiplier for an AV1 quantizer and sample precision.
     /// </summary>
@@ -32,6 +135,28 @@ internal static class Av1RateDistortion
         return (int)Math.Max(multiplier, 1);
     }
 
+    /// <summary>
+    /// Gets the inter-frame rate multiplier for an AV1 quantizer and sample precision.
+    /// </summary>
+    /// <param name="qIndex">The segment quantizer index.</param>
+    /// <param name="bitDepth">The coded sample bit depth.</param>
+    /// <returns>The rate multiplier.</returns>
+    public static int GetInterFrameRateMultiplier(int qIndex, Av1BitDepth bitDepth)
+    {
+        int quantizer = Av1QuantizationLookup.GetDcQuant(qIndex, 0, bitDepth);
+
+        // Ordinary inter frames use a slightly lower rate weight than key frames, preserving more residual detail.
+        // Distortion remains normalized to the eight-bit domain before it is combined with this value.
+        long multiplier = (long)((quantizer * (long)quantizer) * (3.2 + (0.0015 * quantizer)));
+        int shift = (bitDepth.GetBitCount() - 8) * 2;
+        if (shift > 0)
+        {
+            multiplier = (multiplier + (1L << (shift - 1))) >> shift;
+        }
+
+        return (int)Math.Max(multiplier, 1);
+    }
+
     /// <summary>
     /// Gets a rate-distortion cost using the encoder probability-cost precision.
     /// </summary>
@@ -86,10 +211,7 @@ internal static class Av1RateDistortion
     /// <param name="motionVectorRate">The motion-vector syntax rate in 1/512-bit units.</param>
     /// <param name="sumOfAbsoluteDifferences">The unnormalized sample-domain absolute difference.</param>
     /// <returns>The absolute difference plus the motion-vector search cost.</returns>
-    public static int GetMotionSearchSadCost(
-        int sadPerBit,
-        int motionVectorRate,
-        int sumOfAbsoluteDifferences)
+    public static int GetMotionSearchSadCost(int sadPerBit, int motionVectorRate, int sumOfAbsoluteDifferences)
     {
         const int MotionRateShift = 9;
 
@@ -99,4 +221,99 @@ internal static class Av1RateDistortion
         int motionError = (int)((weightedRate + (1 << (MotionRateShift - 1))) >> MotionRateShift);
         return sumOfAbsoluteDifferences + motionError;
     }
+
+    /// <summary>
+    /// Estimates residual rate and distortion from prediction error without running transforms or quantization.
+    /// </summary>
+    /// <param name="blockSize">The plane block geometry selecting the fitted rate curve.</param>
+    /// <param name="squaredError">The visible prediction error normalized to eight-bit precision.</param>
+    /// <param name="sampleCount">The number of visible samples contributing to the error.</param>
+    /// <param name="acQuantizer">The plane AC dequantization step at native sample precision.</param>
+    /// <param name="bitDepth">The native sample precision.</param>
+    /// <param name="rateMultiplier">The block's rate-distortion multiplier.</param>
+    /// <param name="rate">The estimated residual rate in 1/512-bit units.</param>
+    /// <param name="distortion">The estimated residual distortion in sixteenth-sample-error units.</param>
+    public static void ModelPredictionError(
+        Av1BlockSize blockSize,
+        long squaredError,
+        int sampleCount,
+        int acQuantizer,
+        Av1BitDepth bitDepth,
+        int rateMultiplier,
+        out int rate,
+        out long distortion)
+    {
+        if (squaredError == 0)
+        {
+            rate = 0;
+            distortion = 0;
+            return;
+        }
+
+        const double CurveStart = -15.5;
+        const double CurveStep = 0.5;
+        const double EndpointMargin = 1E-6;
+        const double HighErrorThreshold = 16;
+        const int DistortionScaleShift = 4;
+
+        // Transform dequantizers are scaled by eight. Normalize both their precision and the prediction error
+        // before taking the logarithmic feature, so the same fitted curves serve eight-, ten-, and twelve-bit input.
+        int quantizerStep = Math.Max(acQuantizer >> (bitDepth.GetBitCount() - 5), 1);
+        double normalizedError = (double)squaredError / sampleCount;
+        double feature = Math.Log2(normalizedError / ((double)quantizerStep * quantizerStep));
+        double lastCurvePosition = CurveStart + ((ModelCurveLength - 1) * CurveStep);
+        feature = Math.Clamp(feature, CurveStart + CurveStep + EndpointMargin, lastCurvePosition - CurveStep - EndpointMargin);
+        double position = (feature - CurveStart) / CurveStep;
+        int index = (int)position;
+        double fraction = position - index;
+        int rateCategory = ModelRateCategories[(int)blockSize];
+        int distortionCategory = normalizedError > HighErrorThreshold ? 1 : 0;
+        ReadOnlySpan<double> ratePoints = ModelRateCurves.Slice((rateCategory * ModelCurveLength) + index - 1, 4);
+        ReadOnlySpan<double> distortionPoints = ModelDistortionCurves.Slice((distortionCategory * ModelCurveLength) + index - 1, 4);
+        double rateEstimate;
+        double distortionEstimate;
+        if (Vector128.IsHardwareAccelerated)
+        {
+            // The two lanes evaluate rate and distortion together. Keep the cubic polynomial's operation order,
+            // including its separate multiplies and adds, so vector and scalar rounding agree at decision boundaries.
+            Vector128<double> p0 = Vector128.Create(ratePoints[0], distortionPoints[0]);
+            Vector128<double> p1 = Vector128.Create(ratePoints[1], distortionPoints[1]);
+            Vector128<double> p2 = Vector128.Create(ratePoints[2], distortionPoints[2]);
+            Vector128<double> p3 = Vector128.Create(ratePoints[3], distortionPoints[3]);
+            Vector128<double> x = Vector128.Create(fraction);
+            Vector128<double> cubic = (Vector128.Create(3.0) * (p1 - p2)) + p3 - p0;
+            Vector128<double> quadratic = (Vector128.Create(2.0) * p0) - (Vector128.Create(5.0) * p1) + (Vector128.Create(4.0) * p2) - p3;
+            Vector128<double> result = p1 + (Vector128.Create(0.5) * x * (p2 - p0 + (x * (quadratic + (x * cubic)))));
+
+            rateEstimate = result.GetElement(0);
+            distortionEstimate = result.GetElement(1);
+        }
+        else
+        {
+            rateEstimate = InterpolateModelCurve(ratePoints, fraction);
+            distortionEstimate = InterpolateModelCurve(distortionPoints, fraction);
+        }
+
+        rate = (int)(Math.Max(0, rateEstimate * sampleCount) + 0.5);
+        distortion = (long)(Math.Max(0, (distortionEstimate * normalizedError) * sampleCount) + 0.5);
+        long skipDistortion = squaredError << DistortionScaleShift;
+
+        // A modeled coded residual is useful only if it beats leaving the prediction unchanged. Preserve the
+        // reference model's zero-rate rule instead of returning an artificially low distortion for a skipped block.
+        if (rate == 0 || GetCost(rateMultiplier, rate, distortion) >= GetCost(rateMultiplier, 0, skipDistortion))
+        {
+            rate = 0;
+            distortion = skipDistortion;
+        }
+    }
+
+    /// <summary>
+    /// Evaluates one fitted curve's cubic segment without fusing arithmetic operations.
+    /// </summary>
+    private static double InterpolateModelCurve(ReadOnlySpan<double> points, double fraction)
+    {
+        double cubic = (3.0 * (points[1] - points[2])) + points[3] - points[0];
+        double quadratic = (2.0 * points[0]) - (5.0 * points[1]) + (4.0 * points[2]) - points[3];
+        return points[1] + (0.5 * fraction * (points[2] - points[0] + (fraction * (quadratic + (fraction * cubic)))));
+    }
 }

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1SymbolContextHelper.cs

@@ -46,7 +46,7 @@ internal static class Av1SymbolContextHelper
     /// <summary>
     /// The number of interpolation filters selectable by per-block switchable syntax.
     /// </summary>
-    private const int SwitchableInterpolationFilterCount = 3;
+    public const int SwitchableInterpolationFilterCount = 3;
 
     /// <summary>
     /// The number of transform types represented by each flattened transform-set row.
@@ -961,25 +961,70 @@ internal static class Av1SymbolContextHelper
         Av1BlockModeInfo? left,
         int direction)
     {
-        const int filterContextCount = SwitchableInterpolationFilterCount + 1;
-        const int horizontalContextOffset = filterContextCount * 2;
         ReadOnlySpan<Av1ReferenceFrameType> referenceFrames = modeInfo.ReferenceFrames;
         Av1ReferenceFrameType primaryReference = referenceFrames[0];
         bool isCompound = referenceFrames[1] > Av1ReferenceFrameType.Intra;
 
-        // The sixteen rows are laid out as single vertical, compound vertical, single horizontal, then compound
-        // horizontal, with four neighbor states in each group.
-        int context = (isCompound ? filterContextCount : 0) + (direction * horizontalContextOffset);
         int leftFilter = GetReferenceInterpolationFilterContext(left, primaryReference, direction);
         int aboveFilter = GetReferenceInterpolationFilterContext(above, primaryReference, direction);
 
+        return GetSwitchableInterpolationContext(aboveFilter, leftFilter, isCompound, direction);
+    }
+
+    /// <summary>
+    /// Gets the switchable interpolation-filter context from the encoder's packed single-reference neighbors.
+    /// </summary>
+    /// <param name="modeInfo">The current inter block.</param>
+    /// <param name="macroBlock">The current block's available spatial neighbors.</param>
+    /// <param name="direction">Zero for the vertical filter or one for the horizontal filter.</param>
+    /// <returns>The single-reference context for the selected direction.</returns>
+    public static int GetSwitchableInterpolationContext(
+        Av1EncoderBlockModeInfo modeInfo,
+        Av1MacroBlockD macroBlock,
+        int direction)
+    {
+        int aboveFilter = SwitchableInterpolationFilterCount;
+        int leftFilter = SwitchableInterpolationFilterCount;
+        if (macroBlock.IsUpAvailable)
+        {
+            Av1EncoderBlockModeInfo above = macroBlock.GetRelativeModeInfo(-macroBlock.ModeInfoStride).Block;
+            if (above.ReferenceFrame == modeInfo.ReferenceFrame)
+            {
+                aboveFilter = (int)(direction == 0 ? above.VerticalInterpolationFilter : above.HorizontalInterpolationFilter);
+            }
+        }
+
+        if (macroBlock.IsLeftAvailable)
+        {
+            Av1EncoderBlockModeInfo left = macroBlock.GetRelativeModeInfo(-1).Block;
+            if (left.ReferenceFrame == modeInfo.ReferenceFrame)
+            {
+                leftFilter = (int)(direction == 0 ? left.VerticalInterpolationFilter : left.HorizontalInterpolationFilter);
+            }
+        }
+
+        return GetSwitchableInterpolationContext(aboveFilter, leftFilter, isCompound: false, direction);
+    }
+
+    /// <summary>
+    /// Combines the two neighboring filter states into the shared encoder and decoder context layout.
+    /// </summary>
+    private static int GetSwitchableInterpolationContext(int aboveFilter, int leftFilter, bool isCompound, int direction)
+    {
+        const int filterContextCount = SwitchableInterpolationFilterCount + 1;
+        const int horizontalContextOffset = filterContextCount * 2;
+
+        // The sixteen rows are single vertical, compound vertical, single horizontal, then compound horizontal,
+        // with four neighbor states in each group. Both storage representations must use this same mapping.
+        int context = (isCompound ? filterContextCount : 0) + (direction * horizontalContextOffset);
+
         if (leftFilter == aboveFilter)
         {
             return context + leftFilter;
         }
 
-        // The fourth neighbor state is not a selectable Bilinear filter. It is the value the reference decoder uses when a neighbor
-        // does not share the current primary reference, and when two contributing neighbors selected different filters.
+        // The fourth neighbor state means no matching primary reference or disagreement between contributing
+        // neighbors. It is not the Bilinear filter, which is absent from the switchable alphabet.
         if (leftFilter == SwitchableInterpolationFilterCount)
         {
             return context + aboveFilter;

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1SymbolEncoder.cs

@@ -6,6 +6,7 @@ using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -22,6 +23,11 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// </summary>
     private const int MaximumCoefficientContextCount = (Av1Constants.MaxTransformSize / 2) * (Av1Constants.MaxTransformSize / 2);
 
+    /// <summary>
+    /// Owns every mutable tile distribution and restores normative defaults without rebuilding the object graph.
+    /// </summary>
+    private readonly Av1FrameEntropyContext entropyContext;
+
     /// <summary>
     /// The tile-adaptive intra-block-copy distribution.
     /// </summary>
@@ -30,7 +36,32 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <summary>
     /// The tile-adaptive integer displacement-vector context.
     /// </summary>
-    private readonly Av1MotionVectorContext displacementVector = new();
+    private readonly Av1MotionVectorContext displacementVector;
+
+    /// <summary>
+    /// The tile-adaptive normal inter motion-vector context.
+    /// </summary>
+    private readonly Av1MotionVectorContext motionVector;
+
+    /// <summary>
+    /// The tile-adaptive NEWMV branch distributions.
+    /// </summary>
+    private readonly Av1Distribution[] newMotionVector;
+
+    /// <summary>
+    /// The tile-adaptive GLOBALMV branch distributions.
+    /// </summary>
+    private readonly Av1Distribution[] zeroMotionVector;
+
+    /// <summary>
+    /// The tile-adaptive NEARESTMV branch distributions.
+    /// </summary>
+    private readonly Av1Distribution[] referenceMotionVector;
+
+    /// <summary>
+    /// The tile-adaptive dynamic-reference-list distributions.
+    /// </summary>
+    private readonly Av1Distribution[] dynamicReferenceList;
 
     /// <summary>
     /// The tile-adaptive partition-type distributions.
@@ -42,6 +73,21 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// </summary>
     private readonly Av1Distribution[][] keyFrameYMode;
 
+    /// <summary>
+    /// The tile-adaptive inter-frame intra luma-mode distributions.
+    /// </summary>
+    private readonly Av1Distribution[] frameYMode;
+
+    /// <summary>
+    /// The tile-adaptive intra-versus-inter distributions.
+    /// </summary>
+    private readonly Av1Distribution[] intraInter;
+
+    /// <summary>
+    /// The tile-adaptive single-reference branch distributions.
+    /// </summary>
+    private readonly Av1Distribution[][] singleReference;
+
     /// <summary>
     /// The tile-adaptive chroma intra-mode distributions.
     /// </summary>
@@ -82,11 +128,6 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// </summary>
     private readonly Av1Distribution filterIntraMode;
 
-    /// <summary>
-    /// The palette probability state, created only when screen-content coding uses it.
-    /// </summary>
-    private PaletteEntropyContext? paletteEntropyContext;
-
     /// <summary>
     /// The tile-adaptive absolute quantizer delta distribution.
     /// </summary>
@@ -157,20 +198,15 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// </summary>
     private bool isDisposed;
 
-    /// <summary>
-    /// The configuration providing lazily allocated coefficient scratch.
-    /// </summary>
-    private readonly Configuration configuration;
-
     /// <summary>
     /// The reusable padded coefficient levels used to derive entropy contexts.
     /// </summary>
-    private Av1LevelBuffer? levels;
+    private readonly Av1LevelBuffer levels;
 
     /// <summary>
     /// The reusable raster-order coefficient contexts for one transform.
     /// </summary>
-    private IMemoryOwner<sbyte>? coefficientContexts;
+    private readonly IMemoryOwner<sbyte> coefficientContexts;
 
     /// <summary>
     /// The range writer producing the current tile payload.
@@ -183,7 +219,7 @@ internal sealed class Av1SymbolEncoder : IDisposable
     private readonly int baseQIndex;
 
     /// <summary>
-    /// Initializes a new instance of the <see cref="Av1SymbolEncoder"/> class for one AV1 tile.
+    /// Initializes a new instance of the <see cref="Av1SymbolEncoder"/> class with reusable tile state.
     /// </summary>
     /// <param name="configuration">The configuration providing output and temporary memory.</param>
     /// <param name="bufferLength">The complete fixed output allocation length in bytes.</param>
@@ -191,34 +227,50 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <param name="updateCdf">A value indicating whether encoded symbols adapt their tile distributions.</param>
     public Av1SymbolEncoder(Configuration configuration, int bufferLength, int qIndex, bool updateCdf)
     {
-        this.configuration = configuration;
-
-        // Every default accessor creates independently mutable state. Encoding and decoding therefore begin from
-        // equivalent tile-local models without constructing and immediately deep-copying a second object graph.
-        this.tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy;
-        this.tilePartitionTypes = Av1DefaultDistributions.PartitionTypes;
-        this.keyFrameYMode = Av1DefaultDistributions.KeyFrameYMode;
-        this.uvMode = Av1DefaultDistributions.UvMode;
-        this.filterIntra = Av1DefaultDistributions.FilterIntra;
-        this.filterIntraMode = Av1DefaultDistributions.FilterIntraMode;
-        this.deltaQuantizerAbsolute = Av1DefaultDistributions.DeltaQuantizerAbsolute;
-        this.intraExtendedTransform = Av1DefaultDistributions.IntraExtendedTransform;
-        this.interExtendedTransform = Av1DefaultDistributions.InterExtendedTransform;
-        this.transformSize = Av1DefaultDistributions.TransformSize;
-        this.transformPartition = Av1DefaultDistributions.TransformPartition;
-        this.segmentId = Av1DefaultDistributions.SegmentId;
-        this.angleDelta = Av1DefaultDistributions.AngleDelta;
-        this.skip = Av1DefaultDistributions.Skip;
-        this.skipMode = Av1DefaultDistributions.SkipMode;
-        this.chromaFromLumaSign = Av1DefaultDistributions.ChromaFromLumaSign;
-        this.chromaFromLumaAlpha = Av1DefaultDistributions.ChromaFromLumaAlpha;
-        this.transformBlockSkip = Av1DefaultDistributions.GetTransformBlockSkip(qIndex);
-        this.endOfBlockFlag = Av1DefaultDistributions.GetEndOfBlockFlag(qIndex);
-        this.coefficientsBaseRange = Av1DefaultDistributions.GetCoefficientsBaseRange(qIndex);
-        this.coefficientsBase = Av1DefaultDistributions.GetCoefficientsBase(qIndex);
-        this.coefficientsBaseEndOfBlock = Av1DefaultDistributions.GetBaseEndOfBlock(qIndex);
-        this.dcSign = Av1DefaultDistributions.GetDcSign(qIndex);
-        this.endOfBlockExtra = Av1DefaultDistributions.GetEndOfBlockExtra(qIndex);
+        this.entropyContext = new Av1FrameEntropyContext(qIndex);
+
+        // Encoder and decoder now share the same mutable context shape. Every field aliases that single graph so
+        // sequence samples can restore normative defaults without replacing any distribution or array.
+        this.tileIntraBlockCopy = this.entropyContext.IntraBlockCopy;
+        this.motionVector = this.entropyContext.MotionVector;
+        this.displacementVector = this.entropyContext.DisplacementVector;
+        this.tilePartitionTypes = this.entropyContext.PartitionTypes;
+        this.keyFrameYMode = this.entropyContext.KeyFrameYMode;
+        this.frameYMode = this.entropyContext.FrameYMode;
+        this.intraInter = this.entropyContext.IntraInter;
+        this.singleReference = this.entropyContext.SingleReference;
+        this.newMotionVector = this.entropyContext.NewMv;
+        this.zeroMotionVector = this.entropyContext.ZeroMv;
+        this.referenceMotionVector = this.entropyContext.RefMv;
+        this.dynamicReferenceList = this.entropyContext.Drl;
+        this.uvMode = this.entropyContext.UvMode;
+        this.filterIntra = this.entropyContext.FilterIntra;
+        this.filterIntraMode = this.entropyContext.FilterIntraMode;
+        this.deltaQuantizerAbsolute = this.entropyContext.DeltaQuantizerAbsolute;
+        this.intraExtendedTransform = this.entropyContext.IntraExtendedTransform;
+        this.interExtendedTransform = this.entropyContext.InterExtendedTransform;
+        this.transformSize = this.entropyContext.TransformSize;
+        this.transformPartition = this.entropyContext.TransformPartition;
+        this.segmentId = this.entropyContext.SegmentId;
+        this.angleDelta = this.entropyContext.AngleDelta;
+        this.skip = this.entropyContext.Skip;
+        this.skipMode = this.entropyContext.SkipMode;
+        this.chromaFromLumaSign = this.entropyContext.ChromaFromLumaSign;
+        this.chromaFromLumaAlpha = this.entropyContext.ChromaFromLumaAlpha;
+        this.transformBlockSkip = this.entropyContext.TransformBlockSkip;
+        this.endOfBlockFlag = this.entropyContext.EndOfBlockFlag;
+        this.coefficientsBaseRange = this.entropyContext.CoefficientsBaseRange;
+        this.coefficientsBase = this.entropyContext.CoefficientsBase;
+        this.coefficientsBaseEndOfBlock = this.entropyContext.BaseEndOfBlock;
+        this.dcSign = this.entropyContext.DcSign;
+        this.endOfBlockExtra = this.entropyContext.EndOfBlockExtra;
+
+        // Transform dimensions are bounded by the AV1 coefficient-coding rules, so the complete entropy scratch
+        // is known with the tile output capacity and remains valid for every transform in every sequence sample.
+        this.levels = new Av1LevelBuffer(configuration);
+        this.coefficientContexts =
+            configuration.MemoryAllocator.Allocate<sbyte>(MaximumCoefficientContextCount);
+
         this.writer = new(configuration, bufferLength, updateCdf);
         this.baseQIndex = qIndex;
     }
@@ -287,6 +339,25 @@ internal sealed class Av1SymbolEncoder : IDisposable
             int colorOrderIndex);
     }
 
+    /// <summary>
+    /// Restores the initial tile distributions and range coder while retaining their complete object graph and buffers.
+    /// </summary>
+    public void Reset()
+    {
+        this.entropyContext.ResetToDefaults(this.baseQIndex);
+        this.writer.Reset();
+    }
+
+    /// <summary>
+    /// Restores the initial tile distributions and begins the next tile at an offset in the retained output buffer.
+    /// </summary>
+    /// <param name="outputOffset">The first output byte available to the next tile.</param>
+    public void Reset(int outputOffset)
+    {
+        this.entropyContext.ResetToDefaults(this.baseQIndex);
+        this.writer.Reset(outputOffset);
+    }
+
     /// <summary>
     /// Writes an unsigned fixed-width literal to the tile entropy stream.
     /// </summary>
@@ -342,9 +413,8 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <returns>The rate cost in 1/512-bit units.</returns>
     public int GetPaletteYModeCost(bool usePalette, int blockSizeContext, int neighborContext)
     {
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
         return Av1ProbabilityCost.GetSymbolCost(
-            context.YMode[blockSizeContext][neighborContext],
+            this.entropyContext.PaletteYMode[blockSizeContext][neighborContext],
             usePalette ? 1 : 0);
     }
 
@@ -357,8 +427,7 @@ internal sealed class Av1SymbolEncoder : IDisposable
     public void WritePaletteYMode(bool usePalette, int blockSizeContext, int neighborContext)
     {
         ref Av1SymbolWriter w = ref this.writer;
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
-        w.WriteSymbol(usePalette, context.YMode[blockSizeContext][neighborContext]);
+        w.WriteSymbol(usePalette, this.entropyContext.PaletteYMode[blockSizeContext][neighborContext]);
     }
 
     /// <summary>
@@ -369,9 +438,8 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <returns>The rate cost in 1/512-bit units.</returns>
     public int GetPaletteUvModeCost(bool usePalette, bool hasLumaPalette)
     {
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
         return Av1ProbabilityCost.GetSymbolCost(
-            context.UvMode[hasLumaPalette ? 1 : 0],
+            this.entropyContext.PaletteUvMode[hasLumaPalette ? 1 : 0],
             usePalette ? 1 : 0);
     }
 
@@ -383,8 +451,7 @@ internal sealed class Av1SymbolEncoder : IDisposable
     public void WritePaletteUvMode(bool usePalette, bool hasLumaPalette)
     {
         ref Av1SymbolWriter w = ref this.writer;
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
-        w.WriteSymbol(usePalette, context.UvMode[hasLumaPalette ? 1 : 0]);
+        w.WriteSymbol(usePalette, this.entropyContext.PaletteUvMode[hasLumaPalette ? 1 : 0]);
     }
 
     /// <summary>
@@ -396,10 +463,9 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <returns>The rate cost in 1/512-bit units.</returns>
     public int GetPaletteSizeCost(int paletteSize, int blockSizeContext, Av1PlaneType planeType)
     {
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
         Av1Distribution distribution = planeType == Av1PlaneType.Y
-            ? context.YSize[blockSizeContext]
-            : context.UvSize[blockSizeContext];
+            ? this.entropyContext.PaletteYSize[blockSizeContext]
+            : this.entropyContext.PaletteUvSize[blockSizeContext];
 
         return Av1ProbabilityCost.GetSymbolCost(distribution, paletteSize - 2);
     }
@@ -413,10 +479,9 @@ internal sealed class Av1SymbolEncoder : IDisposable
     public void WritePaletteSize(int paletteSize, int blockSizeContext, Av1PlaneType planeType)
     {
         ref Av1SymbolWriter w = ref this.writer;
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
         Av1Distribution distribution = planeType == Av1PlaneType.Y
-            ? context.YSize[blockSizeContext]
-            : context.UvSize[blockSizeContext];
+            ? this.entropyContext.PaletteYSize[blockSizeContext]
+            : this.entropyContext.PaletteUvSize[blockSizeContext];
 
         w.WriteSymbol(paletteSize - 2, distribution);
     }
@@ -435,10 +500,9 @@ internal sealed class Av1SymbolEncoder : IDisposable
         int colorContext,
         Av1PlaneType planeType)
     {
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
         Av1Distribution distribution = planeType == Av1PlaneType.Y
-            ? context.YColorIndex[paletteSize - 2][colorContext]
-            : context.UvColorIndex[paletteSize - 2][colorContext];
+            ? this.entropyContext.PaletteYColorIndex[paletteSize - 2][colorContext]
+            : this.entropyContext.PaletteUvColorIndex[paletteSize - 2][colorContext];
 
         return Av1ProbabilityCost.GetSymbolCost(distribution, colorOrderIndex);
     }
@@ -457,10 +521,9 @@ internal sealed class Av1SymbolEncoder : IDisposable
         Av1PlaneType planeType)
     {
         ref Av1SymbolWriter w = ref this.writer;
-        PaletteEntropyContext context = this.paletteEntropyContext ??= new();
         Av1Distribution distribution = planeType == Av1PlaneType.Y
-            ? context.YColorIndex[paletteSize - 2][colorContext]
-            : context.UvColorIndex[paletteSize - 2][colorContext];
+            ? this.entropyContext.PaletteYColorIndex[paletteSize - 2][colorContext]
+            : this.entropyContext.PaletteUvColorIndex[paletteSize - 2][colorContext];
 
         w.WriteSymbol(colorOrderIndex, distribution);
     }
@@ -695,7 +758,7 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <param name="value">The displacement vector to encode.</param>
     /// <param name="reference">The spatially derived reference vector.</param>
     public void WriteDisplacementVector(Av1MotionVector value, Av1MotionVector reference)
-        => this.displacementVector.Write(this.writer, value, reference);
+        => this.displacementVector.Write(this.writer, value, reference, Av1MotionVectorPrecision.Integer);
 
     /// <summary>
     /// Measures an integer intra-block-copy displacement vector against the live distributions.
@@ -707,7 +770,11 @@ internal sealed class Av1SymbolEncoder : IDisposable
     {
         const int DisplacementVectorCostWeight = 120;
         const int WeightShift = 7;
-        int rate = this.displacementVector.GetCost(this.writer, value, reference);
+        int rate = this.displacementVector.GetCost(
+            this.writer,
+            value,
+            reference,
+            Av1MotionVectorPrecision.Integer);
 
         // Displacement syntax uses a 120/128 discount during mode search; adding half the divisor rounds to nearest.
         return ((rate * DisplacementVectorCostWeight) + (1 << (WeightShift - 1))) >> WeightShift;
@@ -720,7 +787,133 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// <param name="reference">The spatially derived reference vector.</param>
     /// <returns>The syntax cost in 1/512-bit units.</returns>
     public int GetDisplacementVectorSearchCost(Av1MotionVector value, Av1MotionVector reference)
-        => this.displacementVector.GetCost(this.writer, value, reference);
+        => this.displacementVector.GetCost(
+            this.writer,
+            value,
+            reference,
+            Av1MotionVectorPrecision.Integer);
+
+    /// <summary>
+    /// Measures one switchable interpolation filter against its live tile distribution.
+    /// </summary>
+    /// <param name="filter">The regular, smooth, or sharp filter.</param>
+    /// <param name="context">The spatial filter context for the selected direction.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetSwitchableInterpolationFilterCost(Av1InterpolationFilter filter, int context)
+        => Av1ProbabilityCost.GetSymbolCost(this.entropyContext.SwitchableInterpolation[context], (int)filter);
+
+    /// <summary>
+    /// Writes one switchable interpolation filter and updates its live tile distribution.
+    /// </summary>
+    /// <param name="filter">The regular, smooth, or sharp filter.</param>
+    /// <param name="context">The spatial filter context for the selected direction.</param>
+    public void WriteSwitchableInterpolationFilter(Av1InterpolationFilter filter, int context)
+        => this.writer.WriteSymbol((int)filter, this.entropyContext.SwitchableInterpolation[context]);
+
+    /// <summary>
+    /// Measures a single-reference inter mode against the live branch distributions.
+    /// </summary>
+    /// <param name="mode">The new, global, nearest, or near motion-vector mode.</param>
+    /// <param name="modeContext">The packed context derived from the reference-vector stack.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetInterModeCost(Av1PredictionMode mode, int modeContext)
+    {
+        bool isNotNew = mode != Av1PredictionMode.NewMotionVector;
+        int rate = Av1ProbabilityCost.GetSymbolCost(
+            this.newMotionVector[Av1SymbolContextHelper.GetNewMvContext(modeContext)],
+            isNotNew ? 1 : 0);
+
+        if (!isNotNew)
+        {
+            return rate;
+        }
+
+        bool isNotGlobal = mode != Av1PredictionMode.GlobalMotionVector;
+        rate += Av1ProbabilityCost.GetSymbolCost(
+            this.zeroMotionVector[Av1SymbolContextHelper.GetZeroMvContext(modeContext)],
+            isNotGlobal ? 1 : 0);
+
+        if (!isNotGlobal)
+        {
+            return rate;
+        }
+
+        return rate + Av1ProbabilityCost.GetSymbolCost(
+            this.referenceMotionVector[Av1SymbolContextHelper.GetRefMvContext(modeContext)],
+            mode == Av1PredictionMode.NearMotionVector ? 1 : 0);
+    }
+
+    /// <summary>
+    /// Writes a single-reference inter mode through the NEWMV, GLOBALMV, and NEARESTMV branch tree.
+    /// </summary>
+    /// <param name="mode">The new, global, nearest, or near motion-vector mode.</param>
+    /// <param name="modeContext">The packed context derived from the reference-vector stack.</param>
+    public void WriteInterMode(Av1PredictionMode mode, int modeContext)
+    {
+        ref Av1SymbolWriter w = ref this.writer;
+        bool isNotNew = mode != Av1PredictionMode.NewMotionVector;
+        w.WriteSymbol(isNotNew, this.newMotionVector[Av1SymbolContextHelper.GetNewMvContext(modeContext)]);
+        if (!isNotNew)
+        {
+            return;
+        }
+
+        bool isNotGlobal = mode != Av1PredictionMode.GlobalMotionVector;
+        w.WriteSymbol(isNotGlobal, this.zeroMotionVector[Av1SymbolContextHelper.GetZeroMvContext(modeContext)]);
+        if (!isNotGlobal)
+        {
+            return;
+        }
+
+        w.WriteSymbol(
+            mode == Av1PredictionMode.NearMotionVector,
+            this.referenceMotionVector[Av1SymbolContextHelper.GetRefMvContext(modeContext)]);
+    }
+
+    /// <summary>
+    /// Measures one dynamic-reference-list advance decision.
+    /// </summary>
+    /// <param name="advance">Whether selection advances to the next candidate.</param>
+    /// <param name="context">The candidate-weight context.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetDynamicReferenceListCost(bool advance, int context)
+        => Av1ProbabilityCost.GetSymbolCost(this.dynamicReferenceList[context], advance ? 1 : 0);
+
+    /// <summary>
+    /// Writes one dynamic-reference-list advance decision.
+    /// </summary>
+    /// <param name="advance">Whether selection advances to the next candidate.</param>
+    /// <param name="context">The candidate-weight context.</param>
+    public void WriteDynamicReferenceList(bool advance, int context)
+    {
+        ref Av1SymbolWriter w = ref this.writer;
+        w.WriteSymbol(advance, this.dynamicReferenceList[context]);
+    }
+
+    /// <summary>
+    /// Measures an inter motion vector relative to its selected stack reference.
+    /// </summary>
+    /// <param name="value">The selected motion vector.</param>
+    /// <param name="reference">The differential reference from the candidate stack.</param>
+    /// <param name="precision">The fractional precision selected by the frame header.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetMotionVectorCost(
+        Av1MotionVector value,
+        Av1MotionVector reference,
+        Av1MotionVectorPrecision precision)
+        => this.motionVector.GetCost(this.writer, value, reference, precision);
+
+    /// <summary>
+    /// Writes an inter motion vector relative to its selected stack reference.
+    /// </summary>
+    /// <param name="value">The selected motion vector.</param>
+    /// <param name="reference">The differential reference from the candidate stack.</param>
+    /// <param name="precision">The fractional precision selected by the frame header.</param>
+    public void WriteMotionVector(
+        Av1MotionVector value,
+        Av1MotionVector reference,
+        Av1MotionVectorPrecision precision)
+        => this.motionVector.Write(this.writer, value, reference, precision);
 
     /// <summary>
     /// Gets the current fixed-point cost of a complete block partition symbol.
@@ -1136,15 +1329,11 @@ internal sealed class Av1SymbolEncoder : IDisposable
         bool clearLevels,
         out Span<sbyte> coefficientContexts)
     {
-        Av1LevelBuffer levels = this.levels ??= new(this.configuration);
-        IMemoryOwner<sbyte> coefficientContextOwner = this.coefficientContexts ??=
-            this.configuration.MemoryAllocator.Allocate<sbyte>(MaximumCoefficientContextCount);
-
         // AV1 omits high-frequency coefficients beyond 32 samples on every 64-point transform dimension. The tile
         // creates maximum-sized workspaces once, then changes only the active views for subsequent transform blocks.
-        levels.Reset(new Size(width, height), clearLevels);
-        coefficientContexts = coefficientContextOwner.Memory.Span[..(width * height)];
-        return levels;
+        this.levels.Reset(new Size(width, height), clearLevels);
+        coefficientContexts = this.coefficientContexts.Memory.Span[..(width * height)];
+        return this.levels;
     }
 
     /// <summary>
@@ -1314,21 +1503,23 @@ internal sealed class Av1SymbolEncoder : IDisposable
     /// </summary>
     /// <returns>The memory owner containing the encoded tile bytes.</returns>
     public IMemoryOwner<byte> Exit()
-    {
-        ref Av1SymbolWriter w = ref this.writer;
-        return w.Exit();
-    }
+        => this.writer.Exit();
 
     /// <summary>
     /// Finalizes the range-coded tile payload and exposes its encoded prefix without copying.
     /// </summary>
     /// <param name="length">The number of encoded bytes in the returned memory.</param>
-    /// <returns>The encoded prefix, valid until this encoder is disposed.</returns>
+    /// <returns>The encoded prefix, valid until this encoder is reset or disposed.</returns>
     public ReadOnlyMemory<byte> Exit(out int length)
-    {
-        ref Av1SymbolWriter w = ref this.writer;
-        return w.Exit(out length);
-    }
+        => this.writer.Exit(out length);
+
+    /// <summary>
+    /// Exposes a prefix containing every consecutively encoded tile without copying their bytes.
+    /// </summary>
+    /// <param name="length">The number of bytes in the prefix.</param>
+    /// <returns>The encoded prefix, valid until this encoder is reset to offset zero or disposed.</returns>
+    public ReadOnlyMemory<byte> GetOutput(int length)
+        => this.writer.GetOutput(length);
 
     /// <summary>
     /// Releases the range-coder output buffer and coefficient scratch memory.
@@ -1337,8 +1528,8 @@ internal sealed class Av1SymbolEncoder : IDisposable
     {
         if (!this.isDisposed)
         {
-            this.coefficientContexts?.Dispose();
-            this.levels?.Dispose();
+            this.coefficientContexts.Dispose();
+            this.levels.Dispose();
             this.writer.Dispose();
             this.isDisposed = true;
         }
@@ -1657,6 +1848,142 @@ internal sealed class Av1SymbolEncoder : IDisposable
         w.WriteSymbol((int)lumaMode, this.keyFrameYMode[topContext][leftContext]);
     }
 
+    /// <summary>
+    /// Gets the cost of an intra luma mode coded inside an inter frame.
+    /// </summary>
+    /// <param name="lumaMode">The intra luma mode.</param>
+    /// <param name="blockSize">The coding block size selecting the size group.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetInterFrameLumaModeCost(Av1PredictionMode lumaMode, Av1BlockSize blockSize)
+        => Av1ProbabilityCost.GetSymbolCost(this.frameYMode[blockSize.GetSizeGroup()], (int)lumaMode);
+
+    /// <summary>
+    /// Writes an intra luma mode coded inside an inter frame.
+    /// </summary>
+    /// <param name="lumaMode">The intra luma mode.</param>
+    /// <param name="blockSize">The coding block size selecting the size group.</param>
+    public void WriteInterFrameLumaMode(Av1PredictionMode lumaMode, Av1BlockSize blockSize)
+    {
+        ref Av1SymbolWriter w = ref this.writer;
+        w.WriteSymbol((int)lumaMode, this.frameYMode[blockSize.GetSizeGroup()]);
+    }
+
+    /// <summary>
+    /// Gets the cost of the prediction-domain decision for an inter-frame block.
+    /// </summary>
+    /// <param name="isInter">Whether the block uses a retained reference frame.</param>
+    /// <param name="context">The neighboring prediction-domain context.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetIsInterCost(bool isInter, int context)
+        => Av1ProbabilityCost.GetSymbolCost(this.intraInter[context], isInter ? 1 : 0);
+
+    /// <summary>
+    /// Writes the prediction-domain decision for an inter-frame block.
+    /// </summary>
+    /// <param name="isInter">Whether the block uses a retained reference frame.</param>
+    /// <param name="context">The neighboring prediction-domain context.</param>
+    public void WriteIsInter(bool isInter, int context)
+    {
+        ref Av1SymbolWriter w = ref this.writer;
+        w.WriteSymbol(isInter, this.intraInter[context]);
+    }
+
+    /// <summary>
+    /// Gets the cost of selecting one reference from the single-reference branch tree.
+    /// </summary>
+    /// <param name="referenceFrame">The selected reference-frame label.</param>
+    /// <param name="referenceCounts">The neighboring reference counts indexed by reference-frame label.</param>
+    /// <returns>The syntax cost in 1/512-bit units.</returns>
+    public int GetSingleReferenceCost(
+        Av1ReferenceFrameType referenceFrame,
+        ReadOnlySpan<byte> referenceCounts)
+    {
+        bool isBackward = referenceFrame >= Av1ReferenceFrameType.Backward;
+        int context = Av1SymbolContextHelper.GetSingleReferenceBackwardContext(referenceCounts);
+        int rate = Av1ProbabilityCost.GetSymbolCost(this.singleReference[context][0], isBackward ? 1 : 0);
+        if (isBackward)
+        {
+            bool isAlternate = referenceFrame == Av1ReferenceFrameType.Alternate;
+            context = Av1SymbolContextHelper.GetSingleReferenceAlternateContext(referenceCounts);
+            rate += Av1ProbabilityCost.GetSymbolCost(this.singleReference[context][1], isAlternate ? 1 : 0);
+            if (isAlternate)
+            {
+                return rate;
+            }
+
+            context = Av1SymbolContextHelper.GetSingleReferenceAlternate2Context(referenceCounts);
+            return rate + Av1ProbabilityCost.GetSymbolCost(
+                this.singleReference[context][5],
+                referenceFrame == Av1ReferenceFrameType.Alternate2 ? 1 : 0);
+        }
+
+        bool isLast3OrGolden = referenceFrame is Av1ReferenceFrameType.Last3 or Av1ReferenceFrameType.Golden;
+        context = Av1SymbolContextHelper.GetSingleReferenceLast3OrGoldenContext(referenceCounts);
+        rate += Av1ProbabilityCost.GetSymbolCost(this.singleReference[context][2], isLast3OrGolden ? 1 : 0);
+        if (isLast3OrGolden)
+        {
+            context = Av1SymbolContextHelper.GetSingleReferenceGoldenContext(referenceCounts);
+            return rate + Av1ProbabilityCost.GetSymbolCost(
+                this.singleReference[context][4],
+                referenceFrame == Av1ReferenceFrameType.Golden ? 1 : 0);
+        }
+
+        context = Av1SymbolContextHelper.GetSingleReferenceLast2Context(referenceCounts);
+        return rate + Av1ProbabilityCost.GetSymbolCost(
+            this.singleReference[context][3],
+            referenceFrame == Av1ReferenceFrameType.Last2 ? 1 : 0);
+    }
+
+    /// <summary>
+    /// Writes one reference through the single-reference branch tree.
+    /// </summary>
+    /// <param name="referenceFrame">The selected reference-frame label.</param>
+    /// <param name="referenceCounts">The neighboring reference counts indexed by reference-frame label.</param>
+    public void WriteSingleReference(
+        Av1ReferenceFrameType referenceFrame,
+        ReadOnlySpan<byte> referenceCounts)
+    {
+        ref Av1SymbolWriter w = ref this.writer;
+        bool isBackward = referenceFrame >= Av1ReferenceFrameType.Backward;
+        int context = Av1SymbolContextHelper.GetSingleReferenceBackwardContext(referenceCounts);
+        w.WriteSymbol(isBackward, this.singleReference[context][0]);
+        if (isBackward)
+        {
+            bool isAlternate = referenceFrame == Av1ReferenceFrameType.Alternate;
+            context = Av1SymbolContextHelper.GetSingleReferenceAlternateContext(referenceCounts);
+            w.WriteSymbol(isAlternate, this.singleReference[context][1]);
+            if (isAlternate)
+            {
+                return;
+            }
+
+            context = Av1SymbolContextHelper.GetSingleReferenceAlternate2Context(referenceCounts);
+            w.WriteSymbol(
+                referenceFrame == Av1ReferenceFrameType.Alternate2,
+                this.singleReference[context][5]);
+
+            return;
+        }
+
+        bool isLast3OrGolden = referenceFrame is Av1ReferenceFrameType.Last3 or Av1ReferenceFrameType.Golden;
+        context = Av1SymbolContextHelper.GetSingleReferenceLast3OrGoldenContext(referenceCounts);
+        w.WriteSymbol(isLast3OrGolden, this.singleReference[context][2]);
+        if (isLast3OrGolden)
+        {
+            context = Av1SymbolContextHelper.GetSingleReferenceGoldenContext(referenceCounts);
+            w.WriteSymbol(
+                referenceFrame == Av1ReferenceFrameType.Golden,
+                this.singleReference[context][4]);
+
+            return;
+        }
+
+        context = Av1SymbolContextHelper.GetSingleReferenceLast2Context(referenceCounts);
+        w.WriteSymbol(
+            referenceFrame == Av1ReferenceFrameType.Last2,
+            this.singleReference[context][3]);
+    }
+
     /// <summary>
     /// Gets the current fixed-point cost of a directional angle-delta symbol.
     /// </summary>
@@ -2087,40 +2414,4 @@ internal sealed class Av1SymbolEncoder : IDisposable
                 colorContext,
                 planeType);
     }
-
-    /// <summary>
-    /// Owns the adaptive distributions used only by AV1 palette syntax.
-    /// </summary>
-    private sealed class PaletteEntropyContext
-    {
-        /// <summary>
-        /// Gets the luma palette-mode distributions.
-        /// </summary>
-        public Av1Distribution[][] YMode { get; } = Av1DefaultDistributions.PaletteYMode;
-
-        /// <summary>
-        /// Gets the chroma palette-mode distributions.
-        /// </summary>
-        public Av1Distribution[] UvMode { get; } = Av1DefaultDistributions.PaletteUvMode;
-
-        /// <summary>
-        /// Gets the luma palette-size distributions.
-        /// </summary>
-        public Av1Distribution[] YSize { get; } = Av1DefaultDistributions.PaletteYSize;
-
-        /// <summary>
-        /// Gets the chroma palette-size distributions.
-        /// </summary>
-        public Av1Distribution[] UvSize { get; } = Av1DefaultDistributions.PaletteUvSize;
-
-        /// <summary>
-        /// Gets the luma palette color-index distributions.
-        /// </summary>
-        public Av1Distribution[][] YColorIndex { get; } = Av1DefaultDistributions.PaletteYColorIndex;
-
-        /// <summary>
-        /// Gets the chroma palette color-index distributions.
-        /// </summary>
-        public Av1Distribution[][] UvColorIndex { get; } = Av1DefaultDistributions.PaletteUvColorIndex;
-    }
 }

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1SymbolWriter.cs

@@ -12,6 +12,16 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
 /// </summary>
 internal sealed class Av1SymbolWriter : IDisposable
 {
+    /// <summary>
+    /// The normalized range before the first symbol narrows the coding interval.
+    /// </summary>
+    private const uint InitialRange = 0x8000U;
+
+    /// <summary>
+    /// The initial bit count that crosses the first byte-and-carry flush boundary after one output byte.
+    /// </summary>
+    private const int InitialCount = -9;
+
     /// <summary>
     /// The lower endpoint of the current coding interval.
     /// </summary>
@@ -20,7 +30,7 @@ internal sealed class Av1SymbolWriter : IDisposable
     /// <summary>
     /// The width of the current normalized coding interval.
     /// </summary>
-    private uint rng = 0x8000U;
+    private uint rng = InitialRange;
 
     /// <summary>
     /// The number of accumulated bits relative to the next byte-and-carry flush boundary.
@@ -28,7 +38,7 @@ internal sealed class Av1SymbolWriter : IDisposable
     /// <remarks>
     /// The initial value of -9 crosses zero after one output byte and its carry bit have accumulated.
     /// </remarks>
-    private int cnt = -9;
+    private int cnt = InitialCount;
 
     /// <summary>
     /// The configuration that supplies output allocation.
@@ -36,14 +46,19 @@ internal sealed class Av1SymbolWriter : IDisposable
     private readonly Configuration configuration;
 
     /// <summary>
-    /// The owner of the fixed output buffer supplied for this tile.
+    /// The owner of the fixed output buffer shared by consecutively encoded tiles.
     /// </summary>
     private readonly IMemoryOwner<byte> bufferOwner;
 
+    /// <summary>
+    /// The complete requested output allocation, including every consecutively encoded tile.
+    /// </summary>
+    private readonly Memory<byte> outputBuffer;
+
     /// <summary>
     /// The requested output range, excluding any excess capacity returned by a pooling allocator.
     /// </summary>
-    private readonly Memory<byte> buffer;
+    private Memory<byte> buffer;
 
     /// <summary>
     /// Indicates whether encoded symbols adapt their distributions.
@@ -65,10 +80,29 @@ internal sealed class Av1SymbolWriter : IDisposable
     {
         this.configuration = configuration;
         this.bufferOwner = configuration.MemoryAllocator.Allocate<byte>(bufferLength);
-        this.buffer = this.bufferOwner.Memory[..bufferLength];
+        this.outputBuffer = this.bufferOwner.Memory[..bufferLength];
+        this.buffer = this.outputBuffer;
         this.updateCdf = updateCdf;
     }
 
+    /// <summary>
+    /// Restores the initial range-coder state while retaining the bounded output allocation.
+    /// </summary>
+    public void Reset() => this.Reset(0);
+
+    /// <summary>
+    /// Restores the initial range-coder state and begins writing at an offset in the retained output allocation.
+    /// </summary>
+    /// <param name="outputOffset">The first byte available to the next range-coded tile.</param>
+    public void Reset(int outputOffset)
+    {
+        this.buffer = this.outputBuffer[outputOffset..];
+        this.low = 0;
+        this.rng = InitialRange;
+        this.cnt = InitialCount;
+        this.position = 0;
+    }
+
     /// <summary>
     /// Releases the tile output buffer.
     /// </summary>
@@ -147,13 +181,20 @@ internal sealed class Av1SymbolWriter : IDisposable
     /// Finalizes the range-coded sequence and exposes its encoded prefix without copying.
     /// </summary>
     /// <param name="length">The number of encoded bytes in the returned memory.</param>
-    /// <returns>The encoded prefix, valid until this writer is disposed.</returns>
+    /// <returns>The encoded prefix, valid until this writer is reset or disposed.</returns>
     public ReadOnlyMemory<byte> Exit(out int length)
     {
         length = this.FinalizeRange();
         return this.buffer[..length];
     }
 
+    /// <summary>
+    /// Exposes a prefix containing consecutively encoded tiles without copying their bytes.
+    /// </summary>
+    /// <param name="length">The number of bytes in the prefix.</param>
+    /// <returns>The encoded prefix, valid until this writer is reset to offset zero or disposed.</returns>
+    public ReadOnlyMemory<byte> GetOutput(int length) => this.outputBuffer[..length];
+
     /// <summary>
     /// Terminates the range-coded sequence in the current output allocation.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Motion/Av1GlobalMotionParameters.cs

@@ -20,6 +20,51 @@ internal struct Av1GlobalMotionParameters
     /// </summary>
     public const int ModelScale = 1 << ModelPrecisionBits;
 
+    /// <summary>
+    /// The initial finite-subexponential group width used by every global-motion parameter.
+    /// </summary>
+    public const int SubexponentialGroupBitCount = 3;
+
+    /// <summary>
+    /// The finite signed-domain size parameter for coded affine coefficients.
+    /// </summary>
+    public const int AlphaValueMagnitude = (1 << 12) + 1;
+
+    /// <summary>
+    /// The number of fractional bits carried by coded affine coefficients.
+    /// </summary>
+    public const int AlphaPrecisionBits = 15;
+
+    /// <summary>
+    /// The precision increase from a coded affine coefficient to the stored matrix.
+    /// </summary>
+    public const int AlphaPrecisionDifference = ModelPrecisionBits - AlphaPrecisionBits;
+
+    /// <summary>
+    /// The scale factor that restores a coded affine coefficient to the stored matrix precision.
+    /// </summary>
+    public const int AlphaDecodeFactor = 1 << AlphaPrecisionDifference;
+
+    /// <summary>
+    /// The signed magnitude bit count of a general affine model's translation components.
+    /// </summary>
+    public const int AbsoluteTranslationBits = 12;
+
+    /// <summary>
+    /// The signed magnitude bit count of a translation-only model before precision adjustment.
+    /// </summary>
+    public const int AbsoluteTranslationOnlyBits = 9;
+
+    /// <summary>
+    /// The number of fractional bits carried by general affine translation components.
+    /// </summary>
+    public const int TranslationPrecisionBits = 6;
+
+    /// <summary>
+    /// The number of fractional bits carried by translation-only components.
+    /// </summary>
+    public const int TranslationOnlyPrecisionBits = 3;
+
     /// <summary>
     /// The number of low-order bits removed from the derived shear parameters.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Motion/Av1MotionVector.cs

@@ -13,6 +13,16 @@ internal readonly struct Av1MotionVector : IEquatable<Av1MotionVector>
     /// </summary>
     public const int MaximumTemporalDistance = 31;
 
+    /// <summary>
+    /// The number of fractional bits used by AV1 motion-vector components.
+    /// </summary>
+    public const int SubpixelBits = 3;
+
+    /// <summary>
+    /// The number of motion-vector units in one full pixel.
+    /// </summary>
+    public const int SubpixelScale = 1 << SubpixelBits;
+
     /// <summary>
     /// The reserved lower endpoint of the signed AV1 motion-vector domain.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Motion/Av1ReferenceMotionVectors.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using System.Diagnostics.CodeAnalysis;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
@@ -10,7 +11,7 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 /// <summary>
 /// Derives the weighted AV1 reference-motion-vector candidates for one inter block.
 /// </summary>
-internal sealed class Av1ReferenceMotionVectors
+internal struct Av1ReferenceMotionVectors
 {
     /// <summary>
     /// The number of surrounding mode-information rows and columns examined by the spatial search.
@@ -80,16 +81,19 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Gets the derived candidates in normative nearest-region then outer-region order.
     /// </summary>
+    [UnscopedRef]
     public ReadOnlySpan<Av1MotionVector> Candidates => this.candidates[..this.Count];
 
     /// <summary>
     /// Gets the secondary vectors corresponding to <see cref="Candidates"/> for a compound block.
     /// </summary>
+    [UnscopedRef]
     public ReadOnlySpan<Av1MotionVector> CompoundCandidates => this.compoundCandidates[..this.Count];
 
     /// <summary>
     /// Gets the accumulated weight corresponding to each entry in <see cref="Candidates"/>.
     /// </summary>
+    [UnscopedRef]
     public ReadOnlySpan<ushort> Weights => this.weights[..this.Count];
 
     /// <summary>
@@ -116,11 +120,67 @@ internal sealed class Av1ReferenceMotionVectors
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame = Av1ReferenceFrameType.None)
     {
-        Av1BlockSize blockSize = partitionInfo.ModeInfo.BlockSize;
+        ReferenceContext context = new(ref partitionInfo, sequenceHeader.SuperblockModeInfoSize, frameInfo);
+        this.Build(
+            in context,
+            tileInfo,
+            sequenceHeader,
+            frameHeader,
+            referenceFrame,
+            secondaryReferenceFrame);
+    }
+
+    /// <summary>
+    /// Derives the spatial single-reference motion-vector candidates for an encoder block.
+    /// </summary>
+    /// <param name="picture">The encoder picture state containing previously coded neighbors.</param>
+    /// <param name="macroBlock">The current block geometry and tile availability.</param>
+    /// <param name="modeInfoPosition">The current block origin in 4x4 mode-information units.</param>
+    /// <param name="blockSize">The current coding-block size.</param>
+    /// <param name="partitionType">The partition that produced the current block.</param>
+    /// <param name="sequenceHeader">The sequence-level superblock configuration.</param>
+    /// <param name="frameHeader">The frame-level global-motion and motion-vector precision configuration.</param>
+    /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
+    public void Build(
+        Av1PictureControlSet picture,
+        Av1MacroBlockD macroBlock,
+        Point modeInfoPosition,
+        Av1BlockSize blockSize,
+        Av1PartitionType partitionType,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader,
+        Av1ReferenceFrameType referenceFrame)
+    {
+        ReferenceContext context = new(
+            picture,
+            macroBlock,
+            modeInfoPosition,
+            blockSize,
+            partitionType,
+            sequenceHeader.SuperblockModeInfoSize);
+
+        this.Build(
+            in context,
+            macroBlock.Tile,
+            sequenceHeader,
+            frameHeader,
+            referenceFrame,
+            Av1ReferenceFrameType.None);
+    }
+
+    private void Build(
+        in ReferenceContext context,
+        Av1TileInfo tileInfo,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader,
+        Av1ReferenceFrameType referenceFrame,
+        Av1ReferenceFrameType secondaryReferenceFrame)
+    {
+        Av1BlockSize blockSize = context.BlockSize;
         int width = blockSize.Get4x4WideCount();
         int height = blockSize.Get4x4HighCount();
-        int row = partitionInfo.RowIndex;
-        int column = partitionInfo.ColumnIndex;
+        int row = context.RowIndex;
+        int column = context.ColumnIndex;
         int rowAdjustment = height < 2 && (row & 1) != 0 ? 1 : 0;
         int columnAdjustment = width < 2 && (column & 1) != 0 ? 1 : 0;
         int maximumRowOffset = 0;
@@ -129,13 +189,13 @@ internal sealed class Av1ReferenceMotionVectors
         this.Count = 0;
         this.ModeContext = 0;
 
-        if (partitionInfo.AvailableAbove)
+        if (context.AvailableAbove)
         {
             maximumRowOffset = height < 2 ? -4 + rowAdjustment : -(ReferenceSearchDistance << 1) + rowAdjustment;
             maximumRowOffset = Math.Clamp(maximumRowOffset, tileInfo.ModeInfoRowStart - row, tileInfo.ModeInfoRowEnd - row - 1);
         }
 
-        if (partitionInfo.AvailableLeft)
+        if (context.AvailableLeft)
         {
             maximumColumnOffset = width < 2 ? -4 + columnAdjustment : -(ReferenceSearchDistance << 1) + columnAdjustment;
             maximumColumnOffset = Math.Clamp(maximumColumnOffset, tileInfo.ModeInfoColumnStart - column, tileInfo.ModeInfoColumnEnd - column - 1);
@@ -171,7 +231,7 @@ internal sealed class Av1ReferenceMotionVectors
         if (Math.Abs(maximumRowOffset) >= 1)
         {
             this.ScanRow(
-                ref partitionInfo,
+                in context,
                 referenceFrame,
                 secondaryReferenceFrame,
                 in globalMotion,
@@ -188,7 +248,7 @@ internal sealed class Av1ReferenceMotionVectors
         if (Math.Abs(maximumColumnOffset) >= 1)
         {
             this.ScanColumn(
-                ref partitionInfo,
+                in context,
                 referenceFrame,
                 secondaryReferenceFrame,
                 in globalMotion,
@@ -202,10 +262,10 @@ internal sealed class Av1ReferenceMotionVectors
                 ref processedColumns);
         }
 
-        if (partitionInfo.HasTopRight(sequenceHeader.SuperblockModeInfoSize))
+        if (context.HasTopRight)
         {
             this.AddSpatialBlock(
-                ref partitionInfo,
+                in context,
                 tileInfo,
                 referenceFrame,
                 secondaryReferenceFrame,
@@ -229,9 +289,8 @@ internal sealed class Av1ReferenceMotionVectors
         if (frameHeader.UseReferenceFrameMotionVectors)
         {
             this.AddTemporalCandidates(
-                ref partitionInfo,
+                in context,
                 tileInfo,
-                frameInfo,
                 sequenceHeader.OrderHintInfo,
                 frameHeader,
                 referenceFrame,
@@ -245,7 +304,7 @@ internal sealed class Av1ReferenceMotionVectors
         // The top-left block begins the lower-priority outer region. Candidate deduplication still spans both
         // regions, while the two independent stable sorts below preserve the normative nearest-before-outer order.
         this.AddSpatialBlock(
-            ref partitionInfo,
+            in context,
             tileInfo,
             referenceFrame,
             secondaryReferenceFrame,
@@ -265,7 +324,7 @@ internal sealed class Av1ReferenceMotionVectors
             if (Math.Abs(rowOffset) <= Math.Abs(maximumRowOffset) && Math.Abs(rowOffset) > processedRows)
             {
                 this.ScanRow(
-                    ref partitionInfo,
+                    in context,
                     referenceFrame,
                     secondaryReferenceFrame,
                     in globalMotion,
@@ -282,7 +341,7 @@ internal sealed class Av1ReferenceMotionVectors
             if (Math.Abs(columnOffset) <= Math.Abs(maximumColumnOffset) && Math.Abs(columnOffset) > processedColumns)
             {
                 this.ScanColumn(
-                    ref partitionInfo,
+                    in context,
                     referenceFrame,
                     secondaryReferenceFrame,
                     in globalMotion,
@@ -323,8 +382,7 @@ internal sealed class Av1ReferenceMotionVectors
             if (this.Count < 2)
             {
                 this.ExtendCompoundStack(
-                    ref partitionInfo,
-                    frameInfo,
+                    in context,
                     referenceFrame,
                     secondaryReferenceFrame,
                     globalMotionVector,
@@ -340,15 +398,15 @@ internal sealed class Av1ReferenceMotionVectors
             // Differing reference sign biases are reversed before either candidate enters that pair.
             for (int index = 0; Math.Abs(maximumRowOffset) >= 1 && index < extensionLength && this.Count < 2;)
             {
-                Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(new Point(column + index, row - 1));
-                this.AddExtensionCandidate(candidate, frameInfo, referenceFrame);
+                ReferenceBlock candidate = context.GetModeInfoAt(new Point(column + index, row - 1));
+                this.AddExtensionCandidate(candidate, in context, referenceFrame);
                 index += candidate.BlockSize.Get4x4WideCount();
             }
 
             for (int index = 0; Math.Abs(maximumColumnOffset) >= 1 && index < extensionLength && this.Count < 2;)
             {
-                Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(new Point(column - 1, row + index));
-                this.AddExtensionCandidate(candidate, frameInfo, referenceFrame);
+                ReferenceBlock candidate = context.GetModeInfoAt(new Point(column - 1, row + index));
+                this.AddExtensionCandidate(candidate, in context, referenceFrame);
                 index += candidate.BlockSize.Get4x4HighCount();
             }
         }
@@ -358,20 +416,20 @@ internal sealed class Av1ReferenceMotionVectors
             this.candidates[index] = this.candidates[index].ClampReference(
                 blockSize.GetWidth(),
                 blockSize.GetHeight(),
-                partitionInfo.ModeBlockToLeftEdge,
-                partitionInfo.ModeBlockToRightEdge,
-                partitionInfo.ModeBlockToTopEdge,
-                partitionInfo.ModeBlockToBottomEdge);
+                context.ModeBlockToLeftEdge,
+                context.ModeBlockToRightEdge,
+                context.ModeBlockToTopEdge,
+                context.ModeBlockToBottomEdge);
 
             if (secondaryReferenceFrame > Av1ReferenceFrameType.Intra)
             {
                 this.compoundCandidates[index] = this.compoundCandidates[index].ClampReference(
                     blockSize.GetWidth(),
                     blockSize.GetHeight(),
-                    partitionInfo.ModeBlockToLeftEdge,
-                    partitionInfo.ModeBlockToRightEdge,
-                    partitionInfo.ModeBlockToTopEdge,
-                    partitionInfo.ModeBlockToBottomEdge);
+                    context.ModeBlockToLeftEdge,
+                    context.ModeBlockToRightEdge,
+                    context.ModeBlockToTopEdge,
+                    context.ModeBlockToBottomEdge);
             }
         }
 
@@ -452,7 +510,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Scans one spatial row using AV1's block-size-dependent steps and weights.
     /// </summary>
-    /// <param name="partitionInfo">The current block geometry and frame-wide mode map.</param>
+    /// <param name="context">The current block geometry and frame-wide mode map.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
     /// <param name="secondaryReferenceFrame">The secondary compound reference, or <see cref="Av1ReferenceFrameType.None"/>.</param>
     /// <param name="globalMotion">The selected reference's global-motion model.</param>
@@ -465,7 +523,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="newMotionVectorCount">Accumulates matching neighbors whose inter mode contains a new vector.</param>
     /// <param name="processedRows">Receives the spatial depth covered by block-height weighting.</param>
     private void ScanRow(
-        ref Av1PartitionInfo partitionInfo,
+        in ReferenceContext context,
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame,
         in Av1GlobalMotionParameters globalMotion,
@@ -478,13 +536,13 @@ internal sealed class Av1ReferenceMotionVectors
         ref int newMotionVectorCount,
         ref int processedRows)
     {
-        int width = partitionInfo.ModeInfo.BlockSize.Get4x4WideCount();
-        int end = Math.Min(partitionInfo.GetMaxBlockWide(partitionInfo.ModeInfo.BlockSize, false), MaximumSearchBlockSize);
+        int width = context.BlockSize.Get4x4WideCount();
+        int end = Math.Min(context.GetMaxBlockWide(), MaximumSearchBlockSize);
         int columnOffset = 0;
         if (Math.Abs(rowOffset) > 1)
         {
             columnOffset = 1;
-            if ((partitionInfo.ColumnIndex & 1) != 0 && width < 2)
+            if ((context.ColumnIndex & 1) != 0 && width < 2)
             {
                 columnOffset--;
             }
@@ -495,8 +553,8 @@ internal sealed class Av1ReferenceMotionVectors
         bool useFourUnitStep = width >= MaximumSearchBlockSize;
         for (int index = 0; index < end;)
         {
-            Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(
-                new Point(partitionInfo.ColumnIndex + columnOffset + index, partitionInfo.RowIndex + rowOffset));
+            ReferenceBlock candidate = context.GetModeInfoAt(
+                new Point(context.ColumnIndex + columnOffset + index, context.RowIndex + rowOffset));
 
             int candidateWidth = candidate.BlockSize.Get4x4WideCount();
             int length = Math.Min(width, candidateWidth);
@@ -536,7 +594,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Scans one spatial column using AV1's block-size-dependent steps and weights.
     /// </summary>
-    /// <param name="partitionInfo">The current block geometry and frame-wide mode map.</param>
+    /// <param name="context">The current block geometry and frame-wide mode map.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
     /// <param name="secondaryReferenceFrame">The secondary compound reference, or <see cref="Av1ReferenceFrameType.None"/>.</param>
     /// <param name="globalMotion">The selected reference's global-motion model.</param>
@@ -549,7 +607,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="newMotionVectorCount">Accumulates matching neighbors whose inter mode contains a new vector.</param>
     /// <param name="processedColumns">Receives the spatial depth covered by block-width weighting.</param>
     private void ScanColumn(
-        ref Av1PartitionInfo partitionInfo,
+        in ReferenceContext context,
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame,
         in Av1GlobalMotionParameters globalMotion,
@@ -562,13 +620,13 @@ internal sealed class Av1ReferenceMotionVectors
         ref int newMotionVectorCount,
         ref int processedColumns)
     {
-        int height = partitionInfo.ModeInfo.BlockSize.Get4x4HighCount();
-        int end = Math.Min(partitionInfo.GetMaxBlockHigh(partitionInfo.ModeInfo.BlockSize, false), MaximumSearchBlockSize);
+        int height = context.BlockSize.Get4x4HighCount();
+        int end = Math.Min(context.GetMaxBlockHigh(), MaximumSearchBlockSize);
         int rowOffset = 0;
         if (Math.Abs(columnOffset) > 1)
         {
             rowOffset = 1;
-            if ((partitionInfo.RowIndex & 1) != 0 && height < 2)
+            if ((context.RowIndex & 1) != 0 && height < 2)
             {
                 rowOffset--;
             }
@@ -579,8 +637,8 @@ internal sealed class Av1ReferenceMotionVectors
         bool useFourUnitStep = height >= MaximumSearchBlockSize;
         for (int index = 0; index < end;)
         {
-            Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(
-                new Point(partitionInfo.ColumnIndex + columnOffset, partitionInfo.RowIndex + rowOffset + index));
+            ReferenceBlock candidate = context.GetModeInfoAt(
+                new Point(context.ColumnIndex + columnOffset, context.RowIndex + rowOffset + index));
 
             int candidateHeight = candidate.BlockSize.Get4x4HighCount();
             int length = Math.Min(height, candidateHeight);
@@ -620,7 +678,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Adds the candidate at one tile-relative spatial search position.
     /// </summary>
-    /// <param name="partitionInfo">The current block geometry and frame-wide mode map.</param>
+    /// <param name="context">The current block geometry and frame-wide mode map.</param>
     /// <param name="tileInfo">The active tile boundaries.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
     /// <param name="secondaryReferenceFrame">The secondary compound reference, or <see cref="Av1ReferenceFrameType.None"/>.</param>
@@ -633,7 +691,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="referenceMatchCount">Accumulates matching reference labels at the search position.</param>
     /// <param name="newMotionVectorCount">Accumulates matching neighbors whose inter mode contains a new vector.</param>
     private void AddSpatialBlock(
-        ref Av1PartitionInfo partitionInfo,
+        in ReferenceContext context,
         Av1TileInfo tileInfo,
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame,
@@ -646,15 +704,15 @@ internal sealed class Av1ReferenceMotionVectors
         ref int referenceMatchCount,
         ref int newMotionVectorCount)
     {
-        int row = partitionInfo.RowIndex + rowOffset;
-        int column = partitionInfo.ColumnIndex + columnOffset;
+        int row = context.RowIndex + rowOffset;
+        int column = context.ColumnIndex + columnOffset;
         if (row < tileInfo.ModeInfoRowStart || row >= tileInfo.ModeInfoRowEnd ||
             column < tileInfo.ModeInfoColumnStart || column >= tileInfo.ModeInfoColumnEnd)
         {
             return;
         }
 
-        Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(new Point(column, row));
+        ReferenceBlock candidate = context.GetModeInfoAt(new Point(column, row));
         this.AddCandidate(
             candidate,
             referenceFrame,
@@ -682,7 +740,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="referenceMatchCount">Accumulates matching reference labels in the active scan direction.</param>
     /// <param name="newMotionVectorCount">Accumulates matching neighbors whose inter mode contains a new vector.</param>
     private void AddCandidate(
-        Av1BlockModeInfo candidate,
+        ReferenceBlock candidate,
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame,
         in Av1GlobalMotionParameters globalMotion,
@@ -693,16 +751,15 @@ internal sealed class Av1ReferenceMotionVectors
         ref int referenceMatchCount,
         ref int newMotionVectorCount)
     {
-        Span<Av1ReferenceFrameType> candidateReferences = candidate.ReferenceFrames;
-        if (candidateReferences[0] <= Av1ReferenceFrameType.Intra)
+        if (candidate.GetReferenceFrame(0) <= Av1ReferenceFrameType.Intra)
         {
             return;
         }
 
-        Span<Av1MotionVector> candidateMotionVectors = candidate.MotionVectors;
         if (secondaryReferenceFrame > Av1ReferenceFrameType.Intra)
         {
-            if (candidateReferences[0] != referenceFrame || candidateReferences[1] != secondaryReferenceFrame)
+            if (candidate.GetReferenceFrame(0) != referenceFrame ||
+                candidate.GetReferenceFrame(1) != secondaryReferenceFrame)
             {
                 return;
             }
@@ -719,11 +776,11 @@ internal sealed class Av1ReferenceMotionVectors
 
             Av1MotionVector primaryMotionVector = usePrimaryGlobalMotion
                 ? globalMotionVector
-                : candidateMotionVectors[0];
+                : candidate.GetMotionVector(0);
 
             Av1MotionVector secondaryMotionVector = useSecondaryGlobalMotion
                 ? secondaryGlobalMotionVector
-                : candidateMotionVectors[1];
+                : candidate.GetMotionVector(1);
 
             this.AddUnique(primaryMotionVector, secondaryMotionVector, weight);
             if (UsesNewMotionVector(candidate.YMode))
@@ -737,7 +794,7 @@ internal sealed class Av1ReferenceMotionVectors
 
         for (int referenceIndex = 0; referenceIndex < 2; referenceIndex++)
         {
-            if (candidateReferences[referenceIndex] != referenceFrame)
+            if (candidate.GetReferenceFrame(referenceIndex) != referenceFrame)
             {
                 continue;
             }
@@ -752,7 +809,7 @@ internal sealed class Av1ReferenceMotionVectors
 
             Av1MotionVector motionVector = useGlobalMotion
                 ? globalMotionVector
-                : candidateMotionVectors[referenceIndex];
+                : candidate.GetMotionVector(referenceIndex);
 
             this.AddUnique(motionVector, weight);
 
@@ -770,9 +827,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Adds projected temporal candidates over the current block and its permitted extension positions.
     /// </summary>
-    /// <param name="partitionInfo">The current block geometry.</param>
+    /// <param name="context">The current block geometry and decoder temporal state.</param>
     /// <param name="tileInfo">The active tile boundaries.</param>
-    /// <param name="frameInfo">The projected temporal motion field.</param>
     /// <param name="orderHintInfo">The sequence modulo order-hint configuration.</param>
     /// <param name="frameHeader">The frame-level motion-vector precision configuration.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
@@ -780,9 +836,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="globalMotionVector">The selected reference's global-motion vector at the current block.</param>
     /// <param name="secondaryGlobalMotionVector">The secondary reference's global-motion vector at the current block.</param>
     private void AddTemporalCandidates(
-        ref Av1PartitionInfo partitionInfo,
+        in ReferenceContext context,
         Av1TileInfo tileInfo,
-        Av1FrameInfo frameInfo,
         ObuOrderHintInfo orderHintInfo,
         ObuFrameHeader frameHeader,
         Av1ReferenceFrameType referenceFrame,
@@ -790,8 +845,8 @@ internal sealed class Av1ReferenceMotionVectors
         Av1MotionVector globalMotionVector,
         Av1MotionVector secondaryGlobalMotionVector)
     {
-        int width = partitionInfo.ModeInfo.BlockSize.Get4x4WideCount();
-        int height = partitionInfo.ModeInfo.BlockSize.Get4x4HighCount();
+        int width = context.BlockSize.Get4x4WideCount();
+        int height = context.BlockSize.Get4x4HighCount();
         int verticalOffset = Math.Max(2, height);
         int horizontalOffset = Math.Max(2, width);
         int blockRowEnd = Math.Min(height, MaximumSearchBlockSize);
@@ -805,9 +860,8 @@ internal sealed class Av1ReferenceMotionVectors
             for (int blockColumn = 0; blockColumn < blockColumnEnd; blockColumn += columnStep)
             {
                 bool available = this.AddTemporalCandidate(
-                    ref partitionInfo,
+                    in context,
                     tileInfo,
-                    frameInfo,
                     orderHintInfo,
                     frameHeader,
                     referenceFrame,
@@ -838,9 +892,8 @@ internal sealed class Av1ReferenceMotionVectors
         // These three positions extend the temporal search below-left, below-right, and above-right. The 64x64
         // boundary test is normative even when the sequence uses 128x128 superblocks.
         this.AddTemporalExtension(
-            ref partitionInfo,
+            in context,
             tileInfo,
-            frameInfo,
             orderHintInfo,
             frameHeader,
             referenceFrame,
@@ -851,9 +904,8 @@ internal sealed class Av1ReferenceMotionVectors
             -2);
 
         this.AddTemporalExtension(
-            ref partitionInfo,
+            in context,
             tileInfo,
-            frameInfo,
             orderHintInfo,
             frameHeader,
             referenceFrame,
@@ -864,9 +916,8 @@ internal sealed class Av1ReferenceMotionVectors
             horizontalOffset);
 
         this.AddTemporalExtension(
-            ref partitionInfo,
+            in context,
             tileInfo,
-            frameInfo,
             orderHintInfo,
             frameHeader,
             referenceFrame,
@@ -880,9 +931,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Adds one optional temporal extension candidate after applying the normative 64x64 boundary rule.
     /// </summary>
-    /// <param name="partitionInfo">The current block geometry.</param>
+    /// <param name="context">The current block geometry and decoder temporal state.</param>
     /// <param name="tileInfo">The active tile boundaries.</param>
-    /// <param name="frameInfo">The projected temporal motion field.</param>
     /// <param name="orderHintInfo">The sequence modulo order-hint configuration.</param>
     /// <param name="frameHeader">The frame-level motion-vector precision configuration.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
@@ -892,9 +942,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="blockRow">The temporal sample row relative to the current block in 4x4 units.</param>
     /// <param name="blockColumn">The temporal sample column relative to the current block in 4x4 units.</param>
     private void AddTemporalExtension(
-        ref Av1PartitionInfo partitionInfo,
+        in ReferenceContext context,
         Av1TileInfo tileInfo,
-        Av1FrameInfo frameInfo,
         ObuOrderHintInfo orderHintInfo,
         ObuFrameHeader frameHeader,
         Av1ReferenceFrameType referenceFrame,
@@ -904,8 +953,8 @@ internal sealed class Av1ReferenceMotionVectors
         int blockRow,
         int blockColumn)
     {
-        int rowWithinBlock64 = partitionInfo.RowIndex & (MaximumSearchBlockSize - 1);
-        int columnWithinBlock64 = partitionInfo.ColumnIndex & (MaximumSearchBlockSize - 1);
+        int rowWithinBlock64 = context.RowIndex & (MaximumSearchBlockSize - 1);
+        int columnWithinBlock64 = context.ColumnIndex & (MaximumSearchBlockSize - 1);
         if (rowWithinBlock64 + blockRow < 0 || rowWithinBlock64 + blockRow >= MaximumSearchBlockSize ||
             columnWithinBlock64 + blockColumn < 0 || columnWithinBlock64 + blockColumn >= MaximumSearchBlockSize)
         {
@@ -913,9 +962,8 @@ internal sealed class Av1ReferenceMotionVectors
         }
 
         _ = this.AddTemporalCandidate(
-            ref partitionInfo,
+            in context,
             tileInfo,
-            frameInfo,
             orderHintInfo,
             frameHeader,
             referenceFrame,
@@ -929,9 +977,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// <summary>
     /// Projects and accumulates one temporal motion-field sample.
     /// </summary>
-    /// <param name="partitionInfo">The current block geometry.</param>
+    /// <param name="context">The current block geometry and decoder temporal state.</param>
     /// <param name="tileInfo">The active tile boundaries.</param>
-    /// <param name="frameInfo">The projected temporal motion field.</param>
     /// <param name="orderHintInfo">The sequence modulo order-hint configuration.</param>
     /// <param name="frameHeader">The frame-level motion-vector precision configuration.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
@@ -942,9 +989,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// <param name="blockColumn">The temporal sample column relative to the current block in 4x4 units.</param>
     /// <returns><see langword="true"/> when the projected motion field covers the requested position.</returns>
     private bool AddTemporalCandidate(
-        ref Av1PartitionInfo partitionInfo,
+        in ReferenceContext context,
         Av1TileInfo tileInfo,
-        Av1FrameInfo frameInfo,
         ObuOrderHintInfo orderHintInfo,
         ObuFrameHeader frameHeader,
         Av1ReferenceFrameType referenceFrame,
@@ -954,17 +1000,17 @@ internal sealed class Av1ReferenceMotionVectors
         int blockRow,
         int blockColumn)
     {
-        int rowOffset = (partitionInfo.RowIndex & 1) != 0 ? blockRow : blockRow + 1;
-        int columnOffset = (partitionInfo.ColumnIndex & 1) != 0 ? blockColumn : blockColumn + 1;
-        int row = partitionInfo.RowIndex + rowOffset;
-        int column = partitionInfo.ColumnIndex + columnOffset;
+        int rowOffset = (context.RowIndex & 1) != 0 ? blockRow : blockRow + 1;
+        int columnOffset = (context.ColumnIndex & 1) != 0 ? blockColumn : blockColumn + 1;
+        int row = context.RowIndex + rowOffset;
+        int column = context.ColumnIndex + columnOffset;
         if (row < tileInfo.ModeInfoRowStart || row >= tileInfo.ModeInfoRowEnd ||
             column < tileInfo.ModeInfoColumnStart || column >= tileInfo.ModeInfoColumnEnd)
         {
             return false;
         }
 
-        if (!frameInfo.TryGetProjectedTemporalMotionVector(
+        if (!context.TryGetProjectedTemporalMotionVector(
             row,
             column,
             referenceFrame,
@@ -978,7 +1024,7 @@ internal sealed class Av1ReferenceMotionVectors
 
         Av1MotionVector secondaryMotionVector = default;
         if (secondaryReferenceFrame > Av1ReferenceFrameType.Intra &&
-            !frameInfo.TryGetProjectedTemporalMotionVector(
+            !context.TryGetProjectedTemporalMotionVector(
                 row,
                 column,
                 secondaryReferenceFrame,
@@ -1018,27 +1064,25 @@ internal sealed class Av1ReferenceMotionVectors
     /// Extends a short stack with inter vectors from a neighboring block, correcting their temporal direction.
     /// </summary>
     /// <param name="candidate">The decoded neighboring block.</param>
-    /// <param name="frameInfo">The reference-side classification for the current frame.</param>
+    /// <param name="context">The current block geometry and decoder reference classification.</param>
     /// <param name="referenceFrame">The canonical inter reference selected for the current block.</param>
     private void AddExtensionCandidate(
-        Av1BlockModeInfo candidate,
-        Av1FrameInfo frameInfo,
+        ReferenceBlock candidate,
+        in ReferenceContext context,
         Av1ReferenceFrameType referenceFrame)
     {
-        Span<Av1ReferenceFrameType> candidateReferences = candidate.ReferenceFrames;
-        Span<Av1MotionVector> candidateMotionVectors = candidate.MotionVectors;
-        bool targetSignBias = frameInfo.IsReferenceSignBiased(referenceFrame);
+        bool targetSignBias = context.IsReferenceSignBiased(referenceFrame);
 
         for (int referenceIndex = 0; referenceIndex < 2; referenceIndex++)
         {
-            Av1ReferenceFrameType candidateReference = candidateReferences[referenceIndex];
+            Av1ReferenceFrameType candidateReference = candidate.GetReferenceFrame(referenceIndex);
             if (candidateReference <= Av1ReferenceFrameType.Intra)
             {
                 continue;
             }
 
-            Av1MotionVector motionVector = candidateMotionVectors[referenceIndex];
-            if (frameInfo.IsReferenceSignBiased(candidateReference) != targetSignBias)
+            Av1MotionVector motionVector = candidate.GetMotionVector(referenceIndex);
+            if (context.IsReferenceSignBiased(candidateReference) != targetSignBias)
             {
                 motionVector = new Av1MotionVector(-motionVector.Row, -motionVector.Column);
             }
@@ -1067,8 +1111,7 @@ internal sealed class Av1ReferenceMotionVectors
     /// Extends a short compound stack from the immediate above and left blocks.
     /// </summary>
     private void ExtendCompoundStack(
-        ref Av1PartitionInfo partitionInfo,
-        Av1FrameInfo frameInfo,
+        in ReferenceContext context,
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame,
         Av1MotionVector globalMotionVector,
@@ -1085,15 +1128,15 @@ internal sealed class Av1ReferenceMotionVectors
         int secondaryExactCount = 0;
         int primaryDifferentCount = 0;
         int secondaryDifferentCount = 0;
-        int row = partitionInfo.RowIndex;
-        int column = partitionInfo.ColumnIndex;
+        int row = context.RowIndex;
+        int column = context.ColumnIndex;
 
         for (int index = 0; hasAbove && index < extensionLength;)
         {
-            Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(new Point(column + index, row - 1));
+            ReferenceBlock candidate = context.GetModeInfoAt(new Point(column + index, row - 1));
             CollectCompoundExtensionCandidate(
                 candidate,
-                frameInfo,
+                in context,
                 referenceFrame,
                 secondaryReferenceFrame,
                 ref primaryExact,
@@ -1110,10 +1153,10 @@ internal sealed class Av1ReferenceMotionVectors
 
         for (int index = 0; hasLeft && index < extensionLength;)
         {
-            Av1BlockModeInfo candidate = partitionInfo.SuperblockInfo.GetModeInfoAt(new Point(column - 1, row + index));
+            ReferenceBlock candidate = context.GetModeInfoAt(new Point(column - 1, row + index));
             CollectCompoundExtensionCandidate(
                 candidate,
-                frameInfo,
+                in context,
                 referenceFrame,
                 secondaryReferenceFrame,
                 ref primaryExact,
@@ -1168,8 +1211,8 @@ internal sealed class Av1ReferenceMotionVectors
     /// Collects exact-reference and temporal-direction-corrected fallback vectors from one neighboring block.
     /// </summary>
     private static void CollectCompoundExtensionCandidate(
-        Av1BlockModeInfo candidate,
-        Av1FrameInfo frameInfo,
+        ReferenceBlock candidate,
+        in ReferenceContext context,
         Av1ReferenceFrameType referenceFrame,
         Av1ReferenceFrameType secondaryReferenceFrame,
         ref InlineArray2<Av1MotionVector> primaryExact,
@@ -1181,13 +1224,10 @@ internal sealed class Av1ReferenceMotionVectors
         ref InlineArray2<Av1MotionVector> secondaryDifferent,
         ref int secondaryDifferentCount)
     {
-        Span<Av1ReferenceFrameType> candidateReferences = candidate.ReferenceFrames;
-        Span<Av1MotionVector> candidateMotionVectors = candidate.MotionVectors;
-
         for (int candidateIndex = 0; candidateIndex < 2; candidateIndex++)
         {
-            Av1ReferenceFrameType candidateReference = candidateReferences[candidateIndex];
-            Av1MotionVector candidateMotionVector = candidateMotionVectors[candidateIndex];
+            Av1ReferenceFrameType candidateReference = candidate.GetReferenceFrame(candidateIndex);
+            Av1MotionVector candidateMotionVector = candidate.GetMotionVector(candidateIndex);
 
             for (int targetIndex = 0; targetIndex < 2; targetIndex++)
             {
@@ -1223,7 +1263,7 @@ internal sealed class Av1ReferenceMotionVectors
                 }
 
                 Av1MotionVector differentMotionVector = candidateMotionVector;
-                if (frameInfo.IsReferenceSignBiased(candidateReference) != frameInfo.IsReferenceSignBiased(targetReference))
+                if (context.IsReferenceSignBiased(candidateReference) != context.IsReferenceSignBiased(targetReference))
                 {
                     differentMotionVector = new Av1MotionVector(-differentMotionVector.Row, -differentMotionVector.Column);
                 }
@@ -1364,4 +1404,195 @@ internal sealed class Av1ReferenceMotionVectors
             Av1PredictionMode.NewNearestMotionVector or
             Av1PredictionMode.NearNewMotionVector or
             Av1PredictionMode.NewNearMotionVector;
+
+    /// <summary>
+    /// Provides one allocation-free view over decoder or encoder mode-information storage.
+    /// </summary>
+    private readonly struct ReferenceContext
+    {
+        private readonly Av1SuperblockInfo decodedSuperblock;
+        private readonly Av1PictureControlSet? encodedPicture;
+        private readonly Av1FrameInfo? decodedFrame;
+
+        public ReferenceContext(ref Av1PartitionInfo partitionInfo, int superblockModeInfoSize, Av1FrameInfo frameInfo)
+        {
+            this.decodedSuperblock = partitionInfo.SuperblockInfo;
+            this.encodedPicture = null;
+            this.decodedFrame = frameInfo;
+            this.BlockSize = partitionInfo.ModeInfo.BlockSize;
+            this.RowIndex = partitionInfo.RowIndex;
+            this.ColumnIndex = partitionInfo.ColumnIndex;
+            this.AvailableAbove = partitionInfo.AvailableAbove;
+            this.AvailableLeft = partitionInfo.AvailableLeft;
+            this.ModeBlockToLeftEdge = partitionInfo.ModeBlockToLeftEdge;
+            this.ModeBlockToRightEdge = partitionInfo.ModeBlockToRightEdge;
+            this.ModeBlockToTopEdge = partitionInfo.ModeBlockToTopEdge;
+            this.ModeBlockToBottomEdge = partitionInfo.ModeBlockToBottomEdge;
+            this.HasTopRight = partitionInfo.HasTopRight(superblockModeInfoSize);
+        }
+
+        public ReferenceContext(
+            Av1PictureControlSet picture,
+            Av1MacroBlockD macroBlock,
+            Point modeInfoPosition,
+            Av1BlockSize blockSize,
+            Av1PartitionType partitionType,
+            int superblockModeInfoSize)
+        {
+            this.decodedSuperblock = default;
+            this.encodedPicture = picture;
+            this.decodedFrame = null;
+            this.BlockSize = blockSize;
+            this.RowIndex = modeInfoPosition.Y;
+            this.ColumnIndex = modeInfoPosition.X;
+            this.AvailableAbove = macroBlock.IsUpAvailable;
+            this.AvailableLeft = macroBlock.IsLeftAvailable;
+            this.ModeBlockToLeftEdge = macroBlock.ToLeftEdge;
+            this.ModeBlockToRightEdge = macroBlock.ToRightEdge;
+            this.ModeBlockToTopEdge = macroBlock.ToTopEdge;
+            this.ModeBlockToBottomEdge = macroBlock.ToBottomEdge;
+            this.HasTopRight = Av1PartitionInfo.HasTopRight(
+                blockSize,
+                partitionType,
+                modeInfoPosition.Y,
+                modeInfoPosition.X,
+                superblockModeInfoSize);
+        }
+
+        public Av1BlockSize BlockSize { get; }
+
+        public int RowIndex { get; }
+
+        public int ColumnIndex { get; }
+
+        public bool AvailableAbove { get; }
+
+        public bool AvailableLeft { get; }
+
+        public int ModeBlockToLeftEdge { get; }
+
+        public int ModeBlockToRightEdge { get; }
+
+        public int ModeBlockToTopEdge { get; }
+
+        public int ModeBlockToBottomEdge { get; }
+
+        public bool HasTopRight { get; }
+
+        public int GetMaxBlockWide()
+        {
+            int width = this.BlockSize.GetWidth();
+            if (this.ModeBlockToRightEdge < 0)
+            {
+                width += this.ModeBlockToRightEdge >> 3;
+            }
+
+            return width >> Av1Constants.ModeInfoSizeLog2;
+        }
+
+        public int GetMaxBlockHigh()
+        {
+            int height = this.BlockSize.GetHeight();
+            if (this.ModeBlockToBottomEdge < 0)
+            {
+                height += this.ModeBlockToBottomEdge >> 3;
+            }
+
+            return height >> Av1Constants.ModeInfoSizeLog2;
+        }
+
+        public ReferenceBlock GetModeInfoAt(Point position)
+        {
+            Av1PictureControlSet? picture = this.encodedPicture;
+            if (picture is not null)
+            {
+                Av1MacroBlockModeInfo encodedModeInfo = picture.GetFromModeInfoGrid(position);
+                return new ReferenceBlock(
+                    encodedModeInfo.Block.BlockSize,
+                    encodedModeInfo.Block.Mode,
+                    encodedModeInfo.Block.ReferenceFrame,
+                    Av1ReferenceFrameType.None,
+                    picture.GetDisplacementVector(position),
+                    default);
+            }
+
+            Av1BlockModeInfo decodedModeInfo = this.decodedSuperblock.GetModeInfoAt(position);
+            return new ReferenceBlock(
+                decodedModeInfo.BlockSize,
+                decodedModeInfo.YMode,
+                decodedModeInfo.ReferenceFrames[0],
+                decodedModeInfo.ReferenceFrames[1],
+                decodedModeInfo.MotionVectors[0],
+                decodedModeInfo.MotionVectors[1]);
+        }
+
+        public bool IsReferenceSignBiased(Av1ReferenceFrameType referenceFrame)
+        {
+            Av1FrameInfo? frameInfo = this.decodedFrame;
+            return frameInfo is not null && frameInfo.IsReferenceSignBiased(referenceFrame);
+        }
+
+        public bool TryGetProjectedTemporalMotionVector(
+            int row,
+            int column,
+            Av1ReferenceFrameType referenceFrame,
+            ObuOrderHintInfo orderHintInfo,
+            bool allowHighPrecisionMotionVector,
+            bool forceIntegerMotionVector,
+            out Av1MotionVector motionVector)
+        {
+            Av1FrameInfo? frameInfo = this.decodedFrame;
+            if (frameInfo is null)
+            {
+                motionVector = default;
+                return false;
+            }
+
+            return frameInfo.TryGetProjectedTemporalMotionVector(
+                row,
+                column,
+                referenceFrame,
+                orderHintInfo,
+                allowHighPrecisionMotionVector,
+                forceIntegerMotionVector,
+                out motionVector);
+        }
+    }
+
+    /// <summary>
+    /// Carries the neighboring mode fields consumed by reference-vector ranking.
+    /// </summary>
+    private readonly struct ReferenceBlock
+    {
+        private readonly Av1ReferenceFrameType primaryReferenceFrame;
+        private readonly Av1ReferenceFrameType secondaryReferenceFrame;
+        private readonly Av1MotionVector primaryMotionVector;
+        private readonly Av1MotionVector secondaryMotionVector;
+
+        public ReferenceBlock(
+            Av1BlockSize blockSize,
+            Av1PredictionMode mode,
+            Av1ReferenceFrameType primaryReferenceFrame,
+            Av1ReferenceFrameType secondaryReferenceFrame,
+            Av1MotionVector primaryMotionVector,
+            Av1MotionVector secondaryMotionVector)
+        {
+            this.BlockSize = blockSize;
+            this.YMode = mode;
+            this.primaryReferenceFrame = primaryReferenceFrame;
+            this.secondaryReferenceFrame = secondaryReferenceFrame;
+            this.primaryMotionVector = primaryMotionVector;
+            this.secondaryMotionVector = secondaryMotionVector;
+        }
+
+        public Av1BlockSize BlockSize { get; }
+
+        public Av1PredictionMode YMode { get; }
+
+        public Av1ReferenceFrameType GetReferenceFrame(int index)
+            => index == 0 ? this.primaryReferenceFrame : this.secondaryReferenceFrame;
+
+        public Av1MotionVector GetMotionVector(int index)
+            => index == 0 ? this.primaryMotionVector : this.secondaryMotionVector;
+    }
 }

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuFrameHeader.cs

@@ -72,6 +72,15 @@ internal sealed class ObuFrameHeader
     /// </summary>
     public bool AllowHighPrecisionMotionVector { get; set; }
 
+    /// <summary>
+    /// Gets the component precision selected by the integer and high-precision frame flags.
+    /// </summary>
+    public Av1MotionVectorPrecision MotionVectorPrecision => this.ForceIntegerMotionVector
+        ? Av1MotionVectorPrecision.Integer
+        : this.AllowHighPrecisionMotionVector
+            ? Av1MotionVectorPrecision.EighthSample
+            : Av1MotionVectorPrecision.QuarterSample;
+
     /// <summary>
     /// Gets or sets the frame-level interpolation filter used for inter prediction.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuOperatingPoint.cs

@@ -28,6 +28,21 @@ internal sealed class ObuOperatingPoint
     /// </summary>
     public bool IsDecoderModelInfoPresent { get; set; }
 
+    /// <summary>
+    /// Gets or sets the decoder-buffer delay measured in decoding ticks.
+    /// </summary>
+    public uint DecoderBufferDelay { get; set; }
+
+    /// <summary>
+    /// Gets or sets the encoder-buffer delay measured in decoding ticks.
+    /// </summary>
+    public uint EncoderBufferDelay { get; set; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether the operating point uses the low-delay decoding model.
+    /// </summary>
+    public bool LowDelayMode { get; set; }
+
     /// <summary>
     /// Gets or sets a value indicating whether an initial display delay is present for this operating point.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuReader.cs

@@ -14,57 +14,6 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 /// </summary>
 internal sealed class ObuReader
 {
-    /// <summary>
-    /// The number of bits used to address one of AV1's eight reference-map slots.
-    /// </summary>
-    private const int ReferenceFrameIndexBits = 3;
-
-    /// <summary>
-    /// The initial finite-subexponential group width used by every global-motion parameter.
-    /// </summary>
-    private const int GlobalMotionSubexponentialGroupBitCount = 3;
-
-    /// <summary>
-    /// The finite signed-domain size parameter for coded global-motion affine coefficients.
-    /// </summary>
-    private const int GlobalMotionAlphaValueMagnitude = (1 << 12) + 1;
-
-    /// <summary>
-    /// The number of fractional bits carried by coded global-motion affine coefficients.
-    /// </summary>
-    private const int GlobalMotionAlphaPrecisionBits = 15;
-
-    /// <summary>
-    /// The precision increase from a coded affine coefficient to the stored global-motion matrix.
-    /// </summary>
-    private const int GlobalMotionAlphaPrecisionDifference =
-        Av1GlobalMotionParameters.ModelPrecisionBits - GlobalMotionAlphaPrecisionBits;
-
-    /// <summary>
-    /// The scale factor that restores a coded affine coefficient to the global-motion matrix precision.
-    /// </summary>
-    private const int GlobalMotionAlphaDecodeFactor = 1 << GlobalMotionAlphaPrecisionDifference;
-
-    /// <summary>
-    /// The signed magnitude bit count of a general affine model's translation components.
-    /// </summary>
-    private const int GlobalMotionAbsoluteTranslationBits = 12;
-
-    /// <summary>
-    /// The signed magnitude bit count of a translation-only model before precision adjustment.
-    /// </summary>
-    private const int GlobalMotionAbsoluteTranslationOnlyBits = 9;
-
-    /// <summary>
-    /// The number of fractional bits carried by general affine translation components.
-    /// </summary>
-    private const int GlobalMotionTranslationPrecisionBits = 6;
-
-    /// <summary>
-    /// The number of fractional bits carried by translation-only components.
-    /// </summary>
-    private const int GlobalMotionTranslationOnlyPrecisionBits = 3;
-
     /// <summary>
     /// The zero-based sequence-header operating-point index selected by the container.
     /// </summary>
@@ -951,7 +900,7 @@ internal sealed class ObuReader
             }
 
             sequenceHeader.InitialDisplayDelayPresentFlag = reader.ReadBoolean();
-            int operatingPointsCnt = (int)reader.ReadLiteral(5) + 1;
+            int operatingPointsCnt = (int)reader.ReadLiteral(Av1Constants.OperatingPointCountBits) + 1;
             if (sequenceHeader.OperatingPoint.Length != operatingPointsCnt)
             {
                 sequenceHeader.OperatingPoint = new ObuOperatingPoint[operatingPointsCnt];
@@ -961,15 +910,15 @@ internal sealed class ObuReader
             {
                 sequenceHeader.OperatingPoint[i] = new ObuOperatingPoint
                 {
-                    Idc = reader.ReadLiteral(12),
-                    SequenceLevelIndex = (int)reader.ReadLiteral(5)
+                    Idc = reader.ReadLiteral(Av1Constants.OperatingPointIdcBits),
+                    SequenceLevelIndex = (int)reader.ReadLiteral(Av1Constants.LevelBits)
                 };
                 if (!IsValidSequenceLevel(sequenceHeader.OperatingPoint[i].SequenceLevelIndex))
                 {
                     throw new InvalidImageContentException("The AV1 sequence header contains an undefined sequence-level index.");
                 }
 
-                if (sequenceHeader.OperatingPoint[i].SequenceLevelIndex > 7)
+                if (sequenceHeader.OperatingPoint[i].SequenceLevelIndex >= Av1Constants.SequenceTierMinimumLevelIndex)
                 {
                     sequenceHeader.OperatingPoint[i].SequenceTier = (int)reader.ReadLiteral(1);
                 }
@@ -983,10 +932,13 @@ internal sealed class ObuReader
                     sequenceHeader.OperatingPoint[i].IsDecoderModelInfoPresent = reader.ReadBoolean();
                     if (sequenceHeader.OperatingPoint[i].IsDecoderModelInfoPresent)
                     {
-                        // Operating-point delays affect scheduling rather than still-image reconstruction, but their
-                        // syntax must be consumed so the following image dimensions remain bit aligned.
+                        // Retain the scheduling values so the parsed sequence header can be written again without
+                        // losing decoder-model state that is independent from pixel reconstruction.
                         ObuDecoderModelInfo decoderModelInfo = sequenceHeader.GetDecoderModelInfo();
-                        ReadOperatingParametersInfo(ref reader, (int)decoderModelInfo.BufferDelayLength);
+                        ReadOperatingParametersInfo(
+                            ref reader,
+                            (int)decoderModelInfo.BufferDelayLength,
+                            sequenceHeader.OperatingPoint[i]);
                     }
                 }
                 else
@@ -1047,8 +999,8 @@ internal sealed class ObuReader
             sequenceHeader.EnableDualFilter = false;
             sequenceHeader.OrderHintInfo.EnableJointCompound = false;
             sequenceHeader.OrderHintInfo.EnableReferenceFrameMotionVectors = false;
-            sequenceHeader.ForceScreenContentTools = 2; // SELECT_SCREEN_CONTENT_TOOLS
-            sequenceHeader.ForceIntegerMotionVector = 2; // SELECT_INTEGER_MV
+            sequenceHeader.ForceScreenContentTools = Av1Constants.SelectScreenContentTools;
+            sequenceHeader.ForceIntegerMotionVector = Av1Constants.SelectIntegerMotionVector;
             sequenceHeader.OrderHintInfo.OrderHintBits = 0;
         }
         else
@@ -1072,7 +1024,7 @@ internal sealed class ObuReader
             bool seqChooseScreenContentTools = reader.ReadBoolean();
             if (seqChooseScreenContentTools)
             {
-                sequenceHeader.ForceScreenContentTools = 2; // SELECT_SCREEN_CONTENT_TOOLS
+                sequenceHeader.ForceScreenContentTools = Av1Constants.SelectScreenContentTools;
             }
             else
             {
@@ -1084,7 +1036,7 @@ internal sealed class ObuReader
                 bool seqChooseIntegerMv = reader.ReadBoolean();
                 if (seqChooseIntegerMv)
                 {
-                    sequenceHeader.ForceIntegerMotionVector = 2; // SELECT_INTEGER_MV
+                    sequenceHeader.ForceIntegerMotionVector = Av1Constants.SelectIntegerMotionVector;
                 }
                 else
                 {
@@ -1093,7 +1045,7 @@ internal sealed class ObuReader
             }
             else
             {
-                sequenceHeader.ForceIntegerMotionVector = 2; // SELECT_INTEGER_MV
+                sequenceHeader.ForceIntegerMotionVector = Av1Constants.SelectIntegerMotionVector;
             }
 
             if (sequenceHeader.EnableOrderHint)
@@ -1233,15 +1185,19 @@ internal sealed class ObuReader
     };
 
     /// <summary>
-    /// Consumes operating-point buffer parameters that do not affect still-image reconstruction.
+    /// Reads the decoder-model parameters for one operating point.
     /// </summary>
     /// <param name="reader">The reader positioned at the operating-point parameters.</param>
     /// <param name="bufferDelayLength">The bit width of each encoded buffer delay.</param>
-    private static void ReadOperatingParametersInfo(ref Av1BitStreamReader reader, int bufferDelayLength)
+    /// <param name="operatingPoint">The operating point that receives the decoded parameters.</param>
+    private static void ReadOperatingParametersInfo(
+        ref Av1BitStreamReader reader,
+        int bufferDelayLength,
+        ObuOperatingPoint operatingPoint)
     {
-        _ = reader.ReadLiteral(bufferDelayLength);
-        _ = reader.ReadLiteral(bufferDelayLength);
-        _ = reader.ReadBoolean();
+        operatingPoint.DecoderBufferDelay = reader.ReadLiteral(bufferDelayLength);
+        operatingPoint.EncoderBufferDelay = reader.ReadLiteral(bufferDelayLength);
+        operatingPoint.LowDelayMode = reader.ReadBoolean();
     }
 
     /// <summary>
@@ -1716,7 +1672,7 @@ internal sealed class ObuReader
                     throw new InvalidImageContentException("An AV1 still picture cannot display a previously decoded frame.");
                 }
 
-                frameHeader.FrameToShowMapIdx = reader.ReadLiteral(3);
+                frameHeader.FrameToShowMapIdx = reader.ReadLiteral(Av1Constants.ReferenceFrameIndexBits);
 
                 if (sequenceHeader.DecoderModelInfoPresentFlag && sequenceHeader.TimingInfo?.EqualPictureInterval == false)
                 {
@@ -1781,7 +1737,7 @@ internal sealed class ObuReader
                 return;
             }
 
-            frameHeader.FrameType = (ObuFrameType)reader.ReadLiteral(2);
+            frameHeader.FrameType = (ObuFrameType)reader.ReadLiteral(Av1Constants.FrameTypeBits);
             frameHeader.ShowFrame = reader.ReadBoolean();
             if (sequenceHeader.IsStillPicture && (frameHeader.FrameType != ObuFrameType.KeyFrame || !frameHeader.ShowFrame))
             {
@@ -2109,8 +2065,8 @@ internal sealed class ObuReader
 
         if (usesShortSignaling)
         {
-            uint lastFrameIndex = reader.ReadLiteral(ReferenceFrameIndexBits);
-            uint goldenFrameIndex = reader.ReadLiteral(ReferenceFrameIndexBits);
+            uint lastFrameIndex = reader.ReadLiteral(Av1Constants.ReferenceFrameIndexBits);
+            uint goldenFrameIndex = reader.ReadLiteral(Av1Constants.ReferenceFrameIndexBits);
             InlineArray8<bool> slotOccupancyStorage = default;
             Span<bool> slotOccupancy = slotOccupancyStorage;
 
@@ -2136,7 +2092,7 @@ internal sealed class ObuReader
             uint slot = referenceFrameIndices[reference];
             if (!usesShortSignaling)
             {
-                slot = reader.ReadLiteral(ReferenceFrameIndexBits);
+                slot = reader.ReadLiteral(Av1Constants.ReferenceFrameIndexBits);
                 referenceFrameIndices[reference] = slot;
             }
 
@@ -2778,31 +2734,43 @@ internal sealed class ObuReader
         {
             // Diagonal terms are coded as a delta from the identity scale, whereas off-diagonal terms are centered
             // directly around zero. Both are restored to the common sixteen-bit matrix precision after decoding.
+            int referenceHorizontalScale =
+                (referenceParameters[2] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) -
+                (1 << Av1GlobalMotionParameters.AlphaPrecisionBits);
+
             parameters[2] =
                 (reader.ReadSignedReferenceSubexponential(
-                    GlobalMotionAlphaValueMagnitude,
-                    GlobalMotionSubexponentialGroupBitCount,
-                    (referenceParameters[2] >> GlobalMotionAlphaPrecisionDifference) - (1 << GlobalMotionAlphaPrecisionBits)) * GlobalMotionAlphaDecodeFactor) +
+                    Av1GlobalMotionParameters.AlphaValueMagnitude,
+                    Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                    referenceHorizontalScale) *
+                    Av1GlobalMotionParameters.AlphaDecodeFactor) +
                 Av1GlobalMotionParameters.ModelScale;
 
             parameters[3] = reader.ReadSignedReferenceSubexponential(
-                GlobalMotionAlphaValueMagnitude,
-                GlobalMotionSubexponentialGroupBitCount,
-                referenceParameters[3] >> GlobalMotionAlphaPrecisionDifference) * GlobalMotionAlphaDecodeFactor;
+                Av1GlobalMotionParameters.AlphaValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceParameters[3] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) *
+                Av1GlobalMotionParameters.AlphaDecodeFactor;
         }
 
         if (type >= Av1GlobalMotionType.Affine)
         {
+            int referenceVerticalScale =
+                (referenceParameters[5] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) -
+                (1 << Av1GlobalMotionParameters.AlphaPrecisionBits);
+
             parameters[4] = reader.ReadSignedReferenceSubexponential(
-                GlobalMotionAlphaValueMagnitude,
-                GlobalMotionSubexponentialGroupBitCount,
-                referenceParameters[4] >> GlobalMotionAlphaPrecisionDifference) * GlobalMotionAlphaDecodeFactor;
+                Av1GlobalMotionParameters.AlphaValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceParameters[4] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) *
+                Av1GlobalMotionParameters.AlphaDecodeFactor;
 
             parameters[5] =
                 (reader.ReadSignedReferenceSubexponential(
-                    GlobalMotionAlphaValueMagnitude,
-                    GlobalMotionSubexponentialGroupBitCount,
-                    (referenceParameters[5] >> GlobalMotionAlphaPrecisionDifference) - (1 << GlobalMotionAlphaPrecisionBits)) * GlobalMotionAlphaDecodeFactor) +
+                    Av1GlobalMotionParameters.AlphaValueMagnitude,
+                    Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                    referenceVerticalScale) *
+                    Av1GlobalMotionParameters.AlphaDecodeFactor) +
                 Av1GlobalMotionParameters.ModelScale;
         }
         else
@@ -2820,23 +2788,26 @@ internal sealed class ObuReader
             // remains in quarter-sample units. Affine translation retains the fixed model-to-translation precision gap.
             int precisionAdjustment = type == Av1GlobalMotionType.Translation && !allowHighPrecisionMotionVector ? 1 : 0;
             int translationBits = type == Av1GlobalMotionType.Translation
-                ? GlobalMotionAbsoluteTranslationOnlyBits - precisionAdjustment
-                : GlobalMotionAbsoluteTranslationBits;
+                ? Av1GlobalMotionParameters.AbsoluteTranslationOnlyBits - precisionAdjustment
+                : Av1GlobalMotionParameters.AbsoluteTranslationBits;
 
             int translationPrecisionDifference = type == Av1GlobalMotionType.Translation
-                ? Av1GlobalMotionParameters.ModelPrecisionBits - GlobalMotionTranslationOnlyPrecisionBits + precisionAdjustment
-                : Av1GlobalMotionParameters.ModelPrecisionBits - GlobalMotionTranslationPrecisionBits;
+                ? Av1GlobalMotionParameters.ModelPrecisionBits -
+                    Av1GlobalMotionParameters.TranslationOnlyPrecisionBits +
+                    precisionAdjustment
+                : Av1GlobalMotionParameters.ModelPrecisionBits -
+                    Av1GlobalMotionParameters.TranslationPrecisionBits;
 
             int translationDecodeFactor = 1 << translationPrecisionDifference;
             int translationValueMagnitude = (1 << translationBits) + 1;
             parameters[0] = reader.ReadSignedReferenceSubexponential(
                 translationValueMagnitude,
-                GlobalMotionSubexponentialGroupBitCount,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
                 referenceParameters[0] >> translationPrecisionDifference) * translationDecodeFactor;
 
             parameters[1] = reader.ReadSignedReferenceSubexponential(
                 translationValueMagnitude,
-                GlobalMotionSubexponentialGroupBitCount,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
                 referenceParameters[1] >> translationPrecisionDifference) * translationDecodeFactor;
         }
 

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuSequenceHeader.cs

@@ -219,6 +219,14 @@ internal sealed class ObuSequenceHeader
     /// </summary>
     public uint AdditionalFrameIdLength { get; set; }
 
+    /// <summary>
+    /// Gets the timing information required by syntax whose presence flag is set.
+    /// </summary>
+    /// <returns>The sequence timing information.</returns>
+    public ObuTimingInfo GetTimingInfo() =>
+        this.TimingInfo
+        ?? throw new InvalidOperationException("The AV1 sequence has no timing information.");
+
     /// <summary>
     /// Gets the decoder-buffer model information required by syntax whose presence flag is set.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuWriter.cs

@@ -3,82 +3,142 @@
 
 using System.Buffers;
 using System.Buffers.Binary;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline.Quantizers;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 
 /// <summary>
-/// Writes the AV1 open bitstream units required for a single still-image frame.
+/// Writes the AV1 open bitstream units for one coded frame.
 /// </summary>
-internal sealed class ObuWriter
+internal sealed class ObuWriter : IDisposable
 {
     // Sequence and uncompressed-frame syntax have fixed field and array limits. A 512-byte owner covers their
     // maximum supported representation without retaining any entropy-coded tile bytes in the header scratch.
     private const int MaximumHeaderLength = 512;
 
+    private readonly IMemoryOwner<byte> headerOwner;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ObuWriter"/> class.
+    /// </summary>
+    /// <param name="configuration">The configuration providing reusable header memory.</param>
+    public ObuWriter(Configuration configuration)
+        => this.headerOwner = configuration.MemoryAllocator.Allocate<byte>(MaximumHeaderLength);
+
+    /// <summary>
+    /// Writes a temporal delimiter, sequence header, and coded frame for the first sample in a sequence.
+    /// </summary>
+    /// <typeparam name="TTileWriter">The non-boxed tile source type.</typeparam>
+    /// <param name="stream">The destination stream.</param>
+    /// <param name="sequenceHeader">The sequence header.</param>
+    /// <param name="frameHeader">The frame header.</param>
+    /// <param name="tileWriter">The encoded tile source.</param>
+    public void WriteSequenceFrame<TTileWriter>(
+        Stream stream,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader,
+        TTileWriter tileWriter)
+        where TTileWriter : IAv1TileWriter
+    {
+        Span<byte> headerBuffer = this.headerOwner.Memory.Span[..MaximumHeaderLength];
+        Av1BitStreamWriter writer = new(headerBuffer);
+        WriteObuHeaderAndSize(stream, ObuType.TemporalDelimiter, []);
+        WriteSequenceHeader(ref writer, sequenceHeader);
+        int bytesWritten = (writer.BitPosition + 7) >> 3;
+        writer.Flush();
+        WriteObuHeaderAndSize(stream, ObuType.SequenceHeader, headerBuffer[..bytesWritten]);
+        WriteFrameObu(stream, sequenceHeader, frameHeader, tileWriter, headerBuffer, ref writer);
+    }
+
     /// <summary>
-    /// Writes a temporal delimiter and the supplied sequence and frame OBUs.
+    /// Writes an empty temporal-delimiter OBU.
+    /// </summary>
+    /// <param name="stream">The destination stream.</param>
+    public static void WriteTemporalDelimiter(Stream stream)
+        => WriteObuHeaderAndSize(stream, ObuType.TemporalDelimiter, []);
+
+    /// <summary>
+    /// Writes a temporal delimiter followed by one sequence-header OBU.
     /// </summary>
     /// <param name="configuration">The configuration used to allocate temporary encoding memory.</param>
     /// <param name="stream">The destination stream.</param>
-    /// <param name="sequenceHeader">The optional still-picture sequence header.</param>
-    /// <param name="frameHeader">The optional intra-frame header.</param>
-    /// <param name="tileWriter">The tile writer used when a frame header is supplied.</param>
-    [System.Diagnostics.CodeAnalysis.SuppressMessage(
-        "Performance",
-        "CA1822:Mark members as static",
-        Justification = "Preserves the existing writer instance contract.")]
-    public void WriteAll(Configuration configuration, Stream stream, ObuSequenceHeader sequenceHeader, ObuFrameHeader frameHeader, IAv1TileWriter tileWriter)
-    {
-        // The reusable scratch only contains headers. Entropy-coded tiles remain in their owning buffers and are
-        // streamed directly so the complete compressed frame is never duplicated.
+    /// <param name="sequenceHeader">The sequence header.</param>
+    public static void WriteSequenceHeader(Configuration configuration, Stream stream, ObuSequenceHeader sequenceHeader)
+    {
         using IMemoryOwner<byte> headerOwner = configuration.MemoryAllocator.Allocate<byte>(MaximumHeaderLength);
         Span<byte> headerBuffer = headerOwner.Memory.Span[..MaximumHeaderLength];
         Av1BitStreamWriter writer = new(headerBuffer);
         WriteObuHeaderAndSize(stream, ObuType.TemporalDelimiter, []);
+        WriteSequenceHeader(ref writer, sequenceHeader);
+        int bytesWritten = (writer.BitPosition + 7) >> 3;
+        writer.Flush();
+        WriteObuHeaderAndSize(stream, ObuType.SequenceHeader, headerBuffer[..bytesWritten]);
+    }
 
-        if (sequenceHeader != null)
-        {
-            WriteSequenceHeader(ref writer, sequenceHeader);
-            int bytesWritten = (writer.BitPosition + 7) >> 3;
-            writer.Flush();
-            WriteObuHeaderAndSize(stream, ObuType.SequenceHeader, headerBuffer[..bytesWritten]);
-        }
-
-        if (frameHeader != null && sequenceHeader != null)
-        {
-            WriteFrameHeader(ref writer, sequenceHeader, frameHeader);
-            ObuTileGroupHeader tileInfo = frameHeader.TilesInfo;
-            if (tileInfo != null)
-            {
-                WriteTileGroupHeader(ref writer, tileInfo);
-            }
-
-            int frameHeaderBytes = (writer.BitPosition + 7) >> 3;
-            writer.Flush();
+    /// <summary>
+    /// Writes a temporal delimiter and coded frame that continues an established sequence.
+    /// </summary>
+    /// <typeparam name="TTileWriter">The non-boxed tile source type.</typeparam>
+    /// <param name="stream">The destination stream.</param>
+    /// <param name="sequenceHeader">The sequence header established by an earlier sample.</param>
+    /// <param name="frameHeader">The frame header.</param>
+    /// <param name="tileWriter">The encoded tile source.</param>
+    public void WriteFrame<TTileWriter>(
+        Stream stream,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader,
+        TTileWriter tileWriter)
+        where TTileWriter : IAv1TileWriter
+    {
+        Span<byte> headerBuffer = this.headerOwner.Memory.Span[..MaximumHeaderLength];
+        Av1BitStreamWriter writer = new(headerBuffer);
+        WriteObuHeaderAndSize(stream, ObuType.TemporalDelimiter, []);
+        WriteFrameObu(stream, sequenceHeader, frameHeader, tileWriter, headerBuffer, ref writer);
+    }
 
-            uint framePayloadSize = (uint)frameHeaderBytes;
-            if (tileInfo != null)
-            {
-                int tileCount = tileInfo.TileColumnCount * tileInfo.TileRowCount;
-                framePayloadSize += (uint)((tileCount - 1) * tileInfo.TileSizeBytes);
+    /// <inheritdoc/>
+    public void Dispose() => this.headerOwner.Dispose();
 
-                for (int tileNum = 0; tileNum < tileCount; tileNum++)
-                {
-                    framePayloadSize += (uint)tileWriter.GetTileData(tileNum).Length;
-                }
-            }
+    /// <summary>
+    /// Writes the combined frame OBU header followed by each retained tile payload.
+    /// </summary>
+    /// <typeparam name="TTileWriter">The non-boxed tile source type.</typeparam>
+    /// <param name="stream">The destination stream.</param>
+    /// <param name="sequenceHeader">The sequence header governing frame syntax.</param>
+    /// <param name="frameHeader">The uncompressed frame header.</param>
+    /// <param name="tileWriter">The encoded tile source.</param>
+    /// <param name="headerBuffer">The reusable OBU header scratch.</param>
+    /// <param name="writer">The bit writer over <paramref name="headerBuffer"/>.</param>
+    private static void WriteFrameObu<TTileWriter>(
+        Stream stream,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader,
+        TTileWriter tileWriter,
+        Span<byte> headerBuffer,
+        ref Av1BitStreamWriter writer)
+        where TTileWriter : IAv1TileWriter
+    {
+        WriteFrameHeader(ref writer, sequenceHeader, frameHeader);
+        ObuTileGroupHeader tileInfo = frameHeader.TilesInfo;
+        WriteTileGroupHeader(ref writer, tileInfo);
 
-            WriteObuHeaderAndSize(stream, ObuType.Frame, framePayloadSize);
-            stream.Write(headerBuffer[..frameHeaderBytes]);
+        int frameHeaderBytes = (writer.BitPosition + 7) >> 3;
+        writer.Flush();
 
-            if (tileInfo != null)
-            {
-                WriteTileData(stream, tileInfo, tileWriter);
-            }
+        int tileCount = tileInfo.TileColumnCount * tileInfo.TileRowCount;
+        uint framePayloadSize = (uint)(frameHeaderBytes + ((tileCount - 1) * tileInfo.TileSizeBytes));
+        for (int tileNum = 0; tileNum < tileCount; tileNum++)
+        {
+            framePayloadSize += (uint)tileWriter.GetTileData(tileNum).Length;
         }
+
+        WriteObuHeaderAndSize(stream, ObuType.Frame, framePayloadSize);
+        stream.Write(headerBuffer[..frameHeaderBytes]);
+        WriteTileData(stream, tileInfo, tileWriter);
     }
 
     /// <summary>
@@ -145,29 +205,123 @@ internal sealed class ObuWriter
     }
 
     /// <summary>
-    /// Writes a reduced still-picture sequence header.
+    /// Writes an AV1 sequence header.
     /// </summary>
     /// <param name="writer">The bit writer receiving the sequence header.</param>
     /// <param name="sequenceHeader">The sequence header to encode.</param>
     private static void WriteSequenceHeader(ref Av1BitStreamWriter writer, ObuSequenceHeader sequenceHeader)
     {
         writer.WriteLiteral((uint)sequenceHeader.SequenceProfile, 3);
-        writer.WriteBoolean(true); // IsStillPicture
-        writer.WriteBoolean(true); // IsReducedStillPicture
-        writer.WriteLiteral((uint)sequenceHeader.OperatingPoint[0].SequenceLevelIndex, Av1Constants.LevelBits);
+        writer.WriteBoolean(sequenceHeader.IsStillPicture);
+        writer.WriteBoolean(sequenceHeader.IsReducedStillPictureHeader);
+        if (sequenceHeader.IsReducedStillPictureHeader)
+        {
+            writer.WriteLiteral((uint)sequenceHeader.OperatingPoint[0].SequenceLevelIndex, Av1Constants.LevelBits);
+        }
+        else
+        {
+            writer.WriteBoolean(sequenceHeader.TimingInfoPresentFlag);
+            if (sequenceHeader.TimingInfoPresentFlag)
+            {
+                WriteTimingInfo(ref writer, sequenceHeader.GetTimingInfo());
+                writer.WriteBoolean(sequenceHeader.DecoderModelInfoPresentFlag);
+                if (sequenceHeader.DecoderModelInfoPresentFlag)
+                {
+                    WriteDecoderModelInfo(ref writer, sequenceHeader.GetDecoderModelInfo());
+                }
+            }
+
+            writer.WriteBoolean(sequenceHeader.InitialDisplayDelayPresentFlag);
+            writer.WriteLiteral(
+                (uint)(sequenceHeader.OperatingPoint.Length - 1),
+                Av1Constants.OperatingPointCountBits);
+
+            foreach (ObuOperatingPoint operatingPoint in sequenceHeader.OperatingPoint)
+            {
+                writer.WriteLiteral(operatingPoint.Idc, Av1Constants.OperatingPointIdcBits);
+                writer.WriteLiteral((uint)operatingPoint.SequenceLevelIndex, Av1Constants.LevelBits);
+                if (operatingPoint.SequenceLevelIndex >= Av1Constants.SequenceTierMinimumLevelIndex)
+                {
+                    writer.WriteBoolean(operatingPoint.SequenceTier != 0);
+                }
+
+                if (sequenceHeader.DecoderModelInfoPresentFlag)
+                {
+                    writer.WriteBoolean(operatingPoint.IsDecoderModelInfoPresent);
+                    if (operatingPoint.IsDecoderModelInfoPresent)
+                    {
+                        WriteOperatingParametersInfo(
+                            ref writer,
+                            sequenceHeader.GetDecoderModelInfo(),
+                            operatingPoint);
+                    }
+                }
+
+                if (sequenceHeader.InitialDisplayDelayPresentFlag)
+                {
+                    writer.WriteBoolean(operatingPoint.IsInitialDisplayDelayPresent);
+                    if (operatingPoint.IsInitialDisplayDelayPresent)
+                    {
+                        writer.WriteLiteral(operatingPoint.InitialDisplayDelay - 1, 4);
+                    }
+                }
+            }
+        }
 
-        // Frame width and Height
+        // The maximum dimensions determine the fixed-width fields used by every frame in the sequence.
         writer.WriteLiteral((uint)sequenceHeader.FrameWidthBits - 1, 4);
         writer.WriteLiteral((uint)sequenceHeader.FrameHeightBits - 1, 4);
         writer.WriteLiteral((uint)sequenceHeader.MaxFrameWidth - 1, sequenceHeader.FrameWidthBits);
         writer.WriteLiteral((uint)sequenceHeader.MaxFrameHeight - 1, sequenceHeader.FrameHeightBits);
+        if (!sequenceHeader.IsReducedStillPictureHeader)
+        {
+            writer.WriteBoolean(sequenceHeader.IsFrameIdNumbersPresent);
+            if (sequenceHeader.IsFrameIdNumbersPresent)
+            {
+                writer.WriteLiteral((uint)sequenceHeader.DeltaFrameIdLength - 2, 4);
+                writer.WriteLiteral(sequenceHeader.AdditionalFrameIdLength - 1, 3);
+            }
+        }
 
-        // Video related flags removed
         writer.WriteBoolean(sequenceHeader.Use128x128Superblock);
         writer.WriteBoolean(sequenceHeader.EnableFilterIntra);
         writer.WriteBoolean(sequenceHeader.EnableIntraEdgeFilter);
+        if (!sequenceHeader.IsReducedStillPictureHeader)
+        {
+            writer.WriteBoolean(sequenceHeader.EnableInterIntraCompound);
+            writer.WriteBoolean(sequenceHeader.EnableMaskedCompound);
+            writer.WriteBoolean(sequenceHeader.EnableWarpedMotion);
+            writer.WriteBoolean(sequenceHeader.EnableDualFilter);
+            writer.WriteBoolean(sequenceHeader.EnableOrderHint);
+            if (sequenceHeader.EnableOrderHint)
+            {
+                writer.WriteBoolean(sequenceHeader.OrderHintInfo.EnableJointCompound);
+                writer.WriteBoolean(sequenceHeader.OrderHintInfo.EnableReferenceFrameMotionVectors);
+            }
+
+            bool chooseScreenContentTools = sequenceHeader.ForceScreenContentTools == Av1Constants.SelectScreenContentTools;
+            writer.WriteBoolean(chooseScreenContentTools);
+            if (!chooseScreenContentTools)
+            {
+                writer.WriteBoolean(sequenceHeader.ForceScreenContentTools != 0);
+            }
+
+            if (sequenceHeader.ForceScreenContentTools > 0)
+            {
+                bool chooseIntegerMotionVector = sequenceHeader.ForceIntegerMotionVector == Av1Constants.SelectIntegerMotionVector;
+                writer.WriteBoolean(chooseIntegerMotionVector);
+                if (!chooseIntegerMotionVector)
+                {
+                    writer.WriteBoolean(sequenceHeader.ForceIntegerMotionVector != 0);
+                }
+            }
+
+            if (sequenceHeader.EnableOrderHint)
+            {
+                writer.WriteLiteral((uint)sequenceHeader.OrderHintInfo.OrderHintBits - 1, 3);
+            }
+        }
 
-        // Video related flags removed
         writer.WriteBoolean(sequenceHeader.EnableSuperResolution);
         writer.WriteBoolean(sequenceHeader.EnableCdef);
         writer.WriteBoolean(sequenceHeader.EnableRestoration);
@@ -176,6 +330,65 @@ internal sealed class ObuWriter
         WriteTrailingBits(ref writer);
     }
 
+    /// <summary>
+    /// Writes sequence timing in the fixed-width and unsigned-variable-length forms required by AV1.
+    /// </summary>
+    /// <param name="writer">The bit writer receiving the timing information.</param>
+    /// <param name="timingInfo">The timing values to encode.</param>
+    private static void WriteTimingInfo(ref Av1BitStreamWriter writer, ObuTimingInfo timingInfo)
+    {
+        writer.WriteLiteral(timingInfo.NumUnitsInDisplayTick, 32);
+        writer.WriteLiteral(timingInfo.TimeScale, 32);
+        writer.WriteBoolean(timingInfo.EqualPictureInterval);
+        if (timingInfo.EqualPictureInterval)
+        {
+            WriteUnsignedVariableLength(ref writer, timingInfo.NumTicksPerPicture - 1);
+        }
+    }
+
+    /// <summary>
+    /// Writes decoder-buffer field widths and decoding-clock units.
+    /// </summary>
+    /// <param name="writer">The bit writer receiving the decoder-model information.</param>
+    /// <param name="decoderModelInfo">The decoder-model values to encode.</param>
+    private static void WriteDecoderModelInfo(ref Av1BitStreamWriter writer, ObuDecoderModelInfo decoderModelInfo)
+    {
+        writer.WriteLiteral(decoderModelInfo.BufferDelayLength - 1, 5);
+        writer.WriteLiteral(decoderModelInfo.NumUnitsInDecodingTick, 32);
+        writer.WriteLiteral(decoderModelInfo.BufferRemovalTimeLength - 1, 5);
+        writer.WriteLiteral(decoderModelInfo.FramePresentationTimeLength - 1, 5);
+    }
+
+    /// <summary>
+    /// Writes the decoder-model parameters for one operating point.
+    /// </summary>
+    /// <param name="writer">The bit writer receiving the operating-point parameters.</param>
+    /// <param name="decoderModelInfo">The decoder model defining the delay field width.</param>
+    /// <param name="operatingPoint">The operating-point values to encode.</param>
+    private static void WriteOperatingParametersInfo(
+        ref Av1BitStreamWriter writer,
+        ObuDecoderModelInfo decoderModelInfo,
+        ObuOperatingPoint operatingPoint)
+    {
+        int bufferDelayLength = (int)decoderModelInfo.BufferDelayLength;
+        writer.WriteLiteral(operatingPoint.DecoderBufferDelay, bufferDelayLength);
+        writer.WriteLiteral(operatingPoint.EncoderBufferDelay, bufferDelayLength);
+        writer.WriteBoolean(operatingPoint.LowDelayMode);
+    }
+
+    /// <summary>
+    /// Writes an AV1 unsigned variable-length value.
+    /// </summary>
+    /// <param name="writer">The bit writer receiving the value.</param>
+    /// <param name="value">The value to encode.</param>
+    private static void WriteUnsignedVariableLength(ref Av1BitStreamWriter writer, uint value)
+    {
+        uint encodedValue = value + 1;
+        int leadingZeroCount = Av1Math.MostSignificantBit(encodedValue);
+        writer.WriteLiteral(0, leadingZeroCount);
+        writer.WriteLiteral(encodedValue, leadingZeroCount + 1);
+    }
+
     /// <summary>
     /// Writes the sequence color configuration.
     /// </summary>
@@ -418,15 +631,60 @@ internal sealed class ObuWriter
     }
 
     /// <summary>
-    /// Writes the reduced uncompressed header for an intra still-image frame.
+    /// Writes the uncompressed header for an AV1 frame.
     /// </summary>
     /// <param name="writer">The bit writer receiving the uncompressed frame header.</param>
     /// <param name="sequenceHeader">The sequence header controlling available coding tools.</param>
     /// <param name="frameHeader">The frame header to encode.</param>
     private static void WriteUncompressedFrameHeader(ref Av1BitStreamWriter writer, ObuSequenceHeader sequenceHeader, ObuFrameHeader frameHeader)
     {
+        bool frameSizeOverrideFlag = false;
+        if (!sequenceHeader.IsReducedStillPictureHeader)
+        {
+            writer.WriteBoolean(frameHeader.ShowExistingFrame);
+            if (frameHeader.ShowExistingFrame)
+            {
+                writer.WriteLiteral(frameHeader.FrameToShowMapIdx, Av1Constants.ReferenceFrameIndexBits);
+                if (sequenceHeader.DecoderModelInfoPresentFlag && !sequenceHeader.GetTimingInfo().EqualPictureInterval)
+                {
+                    writer.WriteLiteral(
+                        frameHeader.FramePresentationTime,
+                        (int)sequenceHeader.GetDecoderModelInfo().FramePresentationTimeLength);
+                }
+
+                if (sequenceHeader.IsFrameIdNumbersPresent)
+                {
+                    writer.WriteLiteral(frameHeader.DisplayFrameId, sequenceHeader.FrameIdLength);
+                }
+
+                return;
+            }
+
+            writer.WriteLiteral((uint)frameHeader.FrameType, Av1Constants.FrameTypeBits);
+            writer.WriteBoolean(frameHeader.ShowFrame);
+            if (frameHeader.ShowFrame &&
+                sequenceHeader.DecoderModelInfoPresentFlag &&
+                !sequenceHeader.GetTimingInfo().EqualPictureInterval)
+            {
+                writer.WriteLiteral(
+                    frameHeader.FramePresentationTime,
+                    (int)sequenceHeader.GetDecoderModelInfo().FramePresentationTimeLength);
+            }
+
+            if (!frameHeader.ShowFrame)
+            {
+                writer.WriteBoolean(frameHeader.ShowableFrame);
+            }
+
+            if (frameHeader.FrameType != ObuFrameType.SwitchFrame &&
+                (frameHeader.FrameType != ObuFrameType.KeyFrame || !frameHeader.ShowFrame))
+            {
+                writer.WriteBoolean(frameHeader.ErrorResilientMode);
+            }
+        }
+
         writer.WriteBoolean(frameHeader.DisableCdfUpdate);
-        if (sequenceHeader.ForceScreenContentTools == 2)
+        if (sequenceHeader.ForceScreenContentTools == Av1Constants.SelectScreenContentTools)
         {
             writer.WriteBoolean(frameHeader.AllowScreenContentTools);
         }
@@ -437,7 +695,7 @@ internal sealed class ObuWriter
 
         if (frameHeader.AllowScreenContentTools)
         {
-            if (sequenceHeader.ForceIntegerMotionVector == 2)
+            if (sequenceHeader.ForceIntegerMotionVector == Av1Constants.SelectIntegerMotionVector)
             {
                 writer.WriteBoolean(frameHeader.ForceIntegerMotionVector);
             }
@@ -447,6 +705,41 @@ internal sealed class ObuWriter
             }
         }
 
+        if (!sequenceHeader.IsReducedStillPictureHeader)
+        {
+            if (sequenceHeader.IsFrameIdNumbersPresent)
+            {
+                writer.WriteLiteral(frameHeader.CurrentFrameId, sequenceHeader.FrameIdLength);
+            }
+
+            frameSizeOverrideFlag = frameHeader.FrameType == ObuFrameType.SwitchFrame ||
+                frameHeader.FrameSize.SuperResolutionUpscaledWidth != sequenceHeader.MaxFrameWidth ||
+                frameHeader.FrameSize.FrameHeight != sequenceHeader.MaxFrameHeight;
+
+            if (frameHeader.FrameType != ObuFrameType.SwitchFrame)
+            {
+                writer.WriteBoolean(frameSizeOverrideFlag);
+            }
+
+            writer.WriteLiteral(frameHeader.OrderHint, sequenceHeader.OrderHintInfo.OrderHintBits);
+            if (!frameHeader.ErrorResilientMode && !frameHeader.IsIntra)
+            {
+                writer.WriteLiteral(frameHeader.PrimaryReferenceFrame, Av1Constants.PrimaryReferenceBits);
+            }
+        }
+
+        if (sequenceHeader.DecoderModelInfoPresentFlag)
+        {
+            // Image-sequence timing is carried by the container track, so encoded samples do not signal decoder-buffer removal times.
+            writer.WriteBoolean(false);
+        }
+
+        if ((frameHeader.FrameType == ObuFrameType.KeyFrame && !frameHeader.ShowFrame) ||
+            frameHeader.FrameType is ObuFrameType.InterFrame or ObuFrameType.IntraOnlyFrame)
+        {
+            writer.WriteLiteral(frameHeader.RefreshFrameFlags, Av1Constants.ReferenceFrameCount);
+        }
+
         if (frameHeader.FrameType == ObuFrameType.KeyFrame)
         {
             if (!frameHeader.ShowFrame)
@@ -461,7 +754,7 @@ internal sealed class ObuWriter
 
         if (frameHeader.FrameType == ObuFrameType.KeyFrame)
         {
-            WriteFrameSize(ref writer, sequenceHeader, frameHeader, false);
+            WriteFrameSize(ref writer, sequenceHeader, frameHeader, frameSizeOverrideFlag);
             WriteRenderSize(ref writer, frameHeader);
             if (frameHeader.AllowScreenContentTools)
             {
@@ -470,7 +763,7 @@ internal sealed class ObuWriter
         }
         else if (frameHeader.FrameType == ObuFrameType.IntraOnlyFrame)
         {
-            WriteFrameSize(ref writer, sequenceHeader, frameHeader, false);
+            WriteFrameSize(ref writer, sequenceHeader, frameHeader, frameSizeOverrideFlag);
             WriteRenderSize(ref writer, frameHeader);
             if (frameHeader.AllowScreenContentTools)
             {
@@ -479,7 +772,32 @@ internal sealed class ObuWriter
         }
         else
         {
-            throw new NotImplementedException("Inter frames not applicable for AVIF.");
+            WriteReferenceFrameIndices(ref writer, sequenceHeader, frameHeader);
+            WriteFrameSize(ref writer, sequenceHeader, frameHeader, frameSizeOverrideFlag);
+            WriteRenderSize(ref writer, frameHeader);
+            if (!frameHeader.ForceIntegerMotionVector)
+            {
+                writer.WriteBoolean(frameHeader.AllowHighPrecisionMotionVector);
+            }
+
+            WriteFrameInterpolationFilter(ref writer, frameHeader.InterpolationFilter);
+            writer.WriteBoolean(frameHeader.IsMotionModeSwitchable);
+        }
+
+        bool mightAllowReferenceFrameMotionVectors =
+            !frameHeader.ErrorResilientMode &&
+            sequenceHeader.OrderHintInfo.EnableReferenceFrameMotionVectors &&
+            sequenceHeader.EnableOrderHint &&
+            !frameHeader.IsIntra;
+
+        if (mightAllowReferenceFrameMotionVectors)
+        {
+            writer.WriteBoolean(frameHeader.UseReferenceFrameMotionVectors);
+        }
+
+        if (!sequenceHeader.IsReducedStillPictureHeader && !frameHeader.DisableCdfUpdate)
+        {
+            writer.WriteBoolean(frameHeader.DisableFrameEndUpdateCdf);
         }
 
         WriteTileInfo(ref writer, sequenceHeader, frameHeader);
@@ -531,20 +849,68 @@ internal sealed class ObuWriter
             }
         }
 
-        // No Frame Reference mode selection for AVIF
         WriteTransformMode(ref writer, frameHeader);
 
-        // No compound INTER-INTER for AVIF.
         WriteFrameReferenceMode(ref writer, frameHeader);
         WriteSkipModeParameters(ref writer, frameHeader);
+        if (!frameHeader.IsIntra && !frameHeader.ErrorResilientMode && sequenceHeader.EnableWarpedMotion)
+        {
+            writer.WriteBoolean(frameHeader.AllowWarpedMotion);
+        }
 
-        // No warp motion for AVIF.
         writer.WriteBoolean(frameHeader.UseReducedTransformSet);
 
         WriteGlobalMotionParameters(ref writer, frameHeader);
         WriteFilmGrainFilterParameters(ref writer, sequenceHeader, frameHeader);
     }
 
+    /// <summary>
+    /// Writes the seven reference-map slots selected by an inter frame.
+    /// </summary>
+    /// <param name="writer">The bit writer receiving the reference indices.</param>
+    /// <param name="sequenceHeader">The sequence header defining frame-ID and order-hint syntax.</param>
+    /// <param name="frameHeader">The frame header containing the selected reference slots.</param>
+    private static void WriteReferenceFrameIndices(
+        ref Av1BitStreamWriter writer,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader)
+    {
+        // Long signaling is deterministic and permits every reference role to select the same retained slot.
+        if (sequenceHeader.EnableOrderHint)
+        {
+            writer.WriteBoolean(false);
+        }
+
+        Span<uint> referenceFrameIndices = frameHeader.GetReferenceFrameIndices();
+        Span<uint> referenceFrameIds = frameHeader.GetReferenceFrameIds();
+        uint frameIdModulus = 1U << sequenceHeader.FrameIdLength;
+        for (int reference = 0; reference < Av1Constants.ReferencesPerFrame; reference++)
+        {
+            uint slot = referenceFrameIndices[reference];
+            writer.WriteLiteral(slot, Av1Constants.ReferenceFrameIndexBits);
+            if (sequenceHeader.IsFrameIdNumbersPresent)
+            {
+                uint deltaFrameId = (frameHeader.CurrentFrameId + frameIdModulus - referenceFrameIds[(int)slot]) % frameIdModulus;
+                writer.WriteLiteral(deltaFrameId - 1, sequenceHeader.DeltaFrameIdLength);
+            }
+        }
+    }
+
+    /// <summary>
+    /// Writes the frame-level interpolation-filter selection.
+    /// </summary>
+    /// <param name="writer">The bit writer receiving the filter selection.</param>
+    /// <param name="filter">The fixed filter family or per-block selection.</param>
+    private static void WriteFrameInterpolationFilter(ref Av1BitStreamWriter writer, Av1InterpolationFilter filter)
+    {
+        bool isSwitchable = filter == Av1InterpolationFilter.Switchable;
+        writer.WriteBoolean(isSwitchable);
+        if (!isSwitchable)
+        {
+            writer.WriteLiteral((uint)filter, 2);
+        }
+    }
+
     /// <summary>
     /// Writes the frame-header portion of a combined frame OBU.
     /// </summary>
@@ -571,8 +937,8 @@ internal sealed class ObuWriter
 
         if (tileCount > 1)
         {
-            // A combined OBU_FRAME has implicit complete-frame tile bounds. The reference decoder still
-            // writes the presence bit for a multi-tile frame, but requires that bit to remain zero.
+            // A combined frame always carries the complete raster tile group. The zero bit selects those implicit
+            // full-frame bounds instead of adding explicit start and end tile indices.
             writer.WriteBoolean(false);
         }
 
@@ -582,10 +948,15 @@ internal sealed class ObuWriter
     /// <summary>
     /// Writes the size-prefixed tile payloads in raster order.
     /// </summary>
+    /// <typeparam name="TTileWriter">The non-boxed tile source type.</typeparam>
     /// <param name="stream">The destination stream receiving tile data.</param>
     /// <param name="tileInfo">The frame tile layout and tile-size field width.</param>
     /// <param name="tileWriter">The writer that produces each tile payload.</param>
-    private static void WriteTileData(Stream stream, ObuTileGroupHeader tileInfo, IAv1TileWriter tileWriter)
+    private static void WriteTileData<TTileWriter>(
+        Stream stream,
+        ObuTileGroupHeader tileInfo,
+        TTileWriter tileWriter)
+        where TTileWriter : IAv1TileWriter
     {
         int tileCount = tileInfo.TileColumnCount * tileInfo.TileRowCount;
         Span<byte> tileSizeBuffer = stackalloc byte[sizeof(uint)];
@@ -659,7 +1030,7 @@ internal sealed class ObuWriter
     }
 
     /// <summary>
-    /// Writes segmentation feature data for an independently decoded still-image frame.
+    /// Writes segmentation feature data for one coded frame.
     /// </summary>
     /// <param name="writer">The bit writer receiving the segmentation parameters.</param>
     /// <param name="frameHeader">The frame header containing segmentation feature data.</param>
@@ -672,9 +1043,8 @@ internal sealed class ObuWriter
             return;
         }
 
-        // The still-image writer emits independent intra frames with no primary reference.
-        // AV1 therefore infers update-map and update-data as enabled and carries feature data
-        // directly, without the inter-frame update flags.
+        // A frame with no primary reference starts a new segmentation domain. AV1 therefore infers
+        // update-map and update-data as enabled and carries the complete feature state directly.
         for (int segmentId = 0; segmentId < Av1Constants.MaxSegmentCount; segmentId++)
         {
             for (int featureId = 0; featureId < Av1Constants.SegmentationLevelMax; featureId++)
@@ -839,15 +1209,140 @@ internal sealed class ObuWriter
     /// <param name="frameHeader">The current frame header.</param>
     private static void WriteGlobalMotionParameters(ref Av1BitStreamWriter writer, ObuFrameHeader frameHeader)
     {
-        _ = writer;
-
         if (frameHeader.IsIntra)
         {
-            // Nothing to be written for INTRA frames.
             return;
         }
 
-        throw new InvalidImageContentException("AVIF files can only contain INTRA frames.");
+        ReadOnlySpan<Av1GlobalMotionParameters> parameters = frameHeader.GetGlobalMotionParameters();
+        Av1GlobalMotionParameters referenceParameters = Av1GlobalMotionParameters.Identity;
+        for (int reference = 0; reference < Av1Constants.ReferencesPerFrame; reference++)
+        {
+            WriteGlobalMotionModel(
+                ref writer,
+                parameters[reference],
+                referenceParameters,
+                frameHeader.AllowHighPrecisionMotionVector);
+        }
+    }
+
+    /// <summary>
+    /// Writes one global-motion model relative to the same-role model in the primary reference frame.
+    /// </summary>
+    private static void WriteGlobalMotionModel(
+        ref Av1BitStreamWriter writer,
+        Av1GlobalMotionParameters parameters,
+        Av1GlobalMotionParameters referenceParameters,
+        bool allowHighPrecisionMotionVector)
+    {
+        Av1GlobalMotionType type = parameters.Type;
+        writer.WriteBoolean(type != Av1GlobalMotionType.Identity);
+        if (type != Av1GlobalMotionType.Identity)
+        {
+            writer.WriteBoolean(type == Av1GlobalMotionType.RotationZoom);
+            if (type != Av1GlobalMotionType.RotationZoom)
+            {
+                writer.WriteBoolean(type == Av1GlobalMotionType.Translation);
+            }
+        }
+
+        if (type >= Av1GlobalMotionType.RotationZoom)
+        {
+            int horizontalScale =
+                (parameters[2] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) -
+                (1 << Av1GlobalMotionParameters.AlphaPrecisionBits);
+
+            int referenceHorizontalScale =
+                (referenceParameters[2] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) -
+                (1 << Av1GlobalMotionParameters.AlphaPrecisionBits);
+
+            writer.WriteSignedReferenceSubexponential(
+                horizontalScale,
+                Av1GlobalMotionParameters.AlphaValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceHorizontalScale);
+
+            writer.WriteSignedReferenceSubexponential(
+                parameters[3] >> Av1GlobalMotionParameters.AlphaPrecisionDifference,
+                Av1GlobalMotionParameters.AlphaValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceParameters[3] >> Av1GlobalMotionParameters.AlphaPrecisionDifference);
+        }
+
+        if (type >= Av1GlobalMotionType.Affine)
+        {
+            int verticalScale =
+                (parameters[5] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) -
+                (1 << Av1GlobalMotionParameters.AlphaPrecisionBits);
+
+            int referenceVerticalScale =
+                (referenceParameters[5] >> Av1GlobalMotionParameters.AlphaPrecisionDifference) -
+                (1 << Av1GlobalMotionParameters.AlphaPrecisionBits);
+
+            writer.WriteSignedReferenceSubexponential(
+                parameters[4] >> Av1GlobalMotionParameters.AlphaPrecisionDifference,
+                Av1GlobalMotionParameters.AlphaValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceParameters[4] >> Av1GlobalMotionParameters.AlphaPrecisionDifference);
+
+            writer.WriteSignedReferenceSubexponential(
+                verticalScale,
+                Av1GlobalMotionParameters.AlphaValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceVerticalScale);
+        }
+
+        if (type >= Av1GlobalMotionType.Translation)
+        {
+            int precisionAdjustment =
+                type == Av1GlobalMotionType.Translation && !allowHighPrecisionMotionVector ? 1 : 0;
+
+            int translationBits = type == Av1GlobalMotionType.Translation
+                ? Av1GlobalMotionParameters.AbsoluteTranslationOnlyBits - precisionAdjustment
+                : Av1GlobalMotionParameters.AbsoluteTranslationBits;
+
+            int translationPrecisionDifference = type == Av1GlobalMotionType.Translation
+                ? Av1GlobalMotionParameters.ModelPrecisionBits -
+                    Av1GlobalMotionParameters.TranslationOnlyPrecisionBits +
+                    precisionAdjustment
+                : Av1GlobalMotionParameters.ModelPrecisionBits -
+                    Av1GlobalMotionParameters.TranslationPrecisionBits;
+
+            int translationValueMagnitude = (1 << translationBits) + 1;
+            writer.WriteSignedReferenceSubexponential(
+                parameters[0] >> translationPrecisionDifference,
+                translationValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceParameters[0] >> translationPrecisionDifference);
+
+            writer.WriteSignedReferenceSubexponential(
+                parameters[1] >> translationPrecisionDifference,
+                translationValueMagnitude,
+                Av1GlobalMotionParameters.SubexponentialGroupBitCount,
+                referenceParameters[1] >> translationPrecisionDifference);
+        }
+    }
+
+    /// <summary>
+    /// Gets the exact number of uncompressed-header bits required by one global-motion model.
+    /// </summary>
+    /// <param name="parameters">The model to measure.</param>
+    /// <param name="allowHighPrecisionMotionVector">Whether translation may retain one-eighth-sample precision.</param>
+    /// <returns>The encoded model length in bits.</returns>
+    internal static int GetGlobalMotionModelBitCount(
+        Av1GlobalMotionParameters parameters,
+        bool allowHighPrecisionMotionVector)
+    {
+        InlineArray16<byte> storage = default;
+        Span<byte> buffer = storage;
+        Av1BitStreamWriter writer = new(buffer);
+        WriteGlobalMotionModel(
+            ref writer,
+            parameters,
+            Av1GlobalMotionParameters.Identity,
+            allowHighPrecisionMotionVector);
+
+        return writer.BitPosition;
     }
 
     /// <summary>
@@ -857,15 +1352,12 @@ internal sealed class ObuWriter
     /// <param name="frameHeader">The current frame header.</param>
     private static void WriteFrameReferenceMode(ref Av1BitStreamWriter writer, ObuFrameHeader frameHeader)
     {
-        _ = writer;
-
         if (frameHeader.IsIntra)
         {
-            // Nothing to be written for INTRA frames.
             return;
         }
 
-        throw new InvalidImageContentException("AVIF files can only contain INTRA frames.");
+        writer.WriteBoolean(frameHeader.ReferenceMode == ObuReferenceMode.ReferenceModeSelect);
     }
 
     /// <summary>
@@ -882,7 +1374,7 @@ internal sealed class ObuWriter
     }
 
     /// <summary>
-    /// Writes film-grain synthesis parameters for a displayed still-image frame.
+    /// Writes film-grain synthesis parameters for a displayed frame.
     /// </summary>
     /// <param name="writer">The bit writer receiving the film-grain parameters.</param>
     /// <param name="sequenceHeader">The sequence header defining film-grain availability and color sampling.</param>

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1EncoderBlockWorkspace.cs

@@ -3,6 +3,7 @@
 
 using System.Buffers;
 using System.Runtime.InteropServices;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -39,40 +40,49 @@ internal sealed class Av1EncoderBlockWorkspace : IDisposable
     private const int TransformCoefficientOffset = ResidualStorageLength;
     private const int DequantizedCoefficientOffset = TransformCoefficientOffset + MaximumCoefficientCount;
     private const int TransformWorkspaceOffset = DequantizedCoefficientOffset + MaximumCoefficientCount;
-    private const int IntraBlockCopySampleStorageOffset = TransformWorkspaceOffset + Av1TransformWorkspace.MaximumLength;
-    private const int IntraBlockCopySampleStorageLength =
-        Av1EncoderIntraBlockCopyWorkspace<ushort>.SampleBufferCount *
-        Av1EncoderIntraBlockCopyWorkspace<ushort>.MaximumSampleCount *
+    private const int InterPredictionSampleStorageOffset = TransformWorkspaceOffset + Av1TransformWorkspace.MaximumLength;
+    private const int InterPredictionSampleStorageLength =
+        Av1EncoderInterPredictionWorkspace<ushort>.SampleBufferCount *
+        Av1EncoderInterPredictionWorkspace<ushort>.MaximumSampleCount *
         sizeof(ushort) /
         sizeof(int);
 
-    private const int IntraBlockCopyResidualStorageOffset =
-        IntraBlockCopySampleStorageOffset + IntraBlockCopySampleStorageLength;
+    private const int InterPredictionResidualStorageOffset =
+        InterPredictionSampleStorageOffset + InterPredictionSampleStorageLength;
 
-    private const int IntraBlockCopyResidualStorageLength =
-        Av1EncoderIntraBlockCopyWorkspace<ushort>.MaximumSampleCount *
+    private const int InterPredictionResidualStorageLength =
+        Av1EncoderInterPredictionWorkspace<ushort>.MaximumSampleCount *
         sizeof(short) /
         sizeof(int);
 
-    private const int IntraBlockCopyCoefficientStorageOffset =
-        IntraBlockCopyResidualStorageOffset + IntraBlockCopyResidualStorageLength;
+    private const int InterPredictionScratchStorageOffset =
+        InterPredictionResidualStorageOffset + InterPredictionResidualStorageLength;
 
-    private const int IntraBlockCopyCoefficientStorageLength =
-        Av1EncoderIntraBlockCopyWorkspace<ushort>.CoefficientBufferCount *
-        Av1EncoderIntraBlockCopyWorkspace<ushort>.MaximumSampleCount;
+    private const int InterPredictionScratchStorageLength =
+        Av1EncoderInterPredictionWorkspace<ushort>.PredictionScratchCount *
+        sizeof(short) /
+        sizeof(int);
+
+    private const int InterPredictionCoefficientStorageOffset =
+        InterPredictionScratchStorageOffset + InterPredictionScratchStorageLength;
 
-    private const int IntraBlockCopyStorageLength =
-        IntraBlockCopySampleStorageLength +
-        IntraBlockCopyResidualStorageLength +
-        IntraBlockCopyCoefficientStorageLength;
+    private const int InterPredictionCoefficientStorageLength =
+        Av1EncoderInterPredictionWorkspace<ushort>.CoefficientBufferCount *
+        Av1EncoderInterPredictionWorkspace<ushort>.MaximumSampleCount;
+
+    private const int InterPredictionStorageLength =
+        InterPredictionSampleStorageLength +
+        InterPredictionResidualStorageLength +
+        InterPredictionScratchStorageLength +
+        InterPredictionCoefficientStorageLength;
 
     private const int ModeDecisionStorageLength = Av1EncoderModeDecisionWorkspace<ushort>.StorageLength;
-    private const int SharedModeDecisionStorageLength = ModeDecisionStorageLength > IntraBlockCopyStorageLength
+    private const int SharedModeDecisionStorageLength = ModeDecisionStorageLength > InterPredictionStorageLength
         ? ModeDecisionStorageLength
-        : IntraBlockCopyStorageLength;
+        : InterPredictionStorageLength;
 
     private const int PartitionContextStorageOffset =
-        IntraBlockCopySampleStorageOffset + SharedModeDecisionStorageLength;
+        InterPredictionSampleStorageOffset + SharedModeDecisionStorageLength;
 
     private const int MaximumPartitionEdgeUnitCount =
         2 * (1 << (Av1Constants.MaxSuperBlockSizeLog2 - Av1Constants.ModeInfoSizeLog2));
@@ -97,6 +107,11 @@ internal sealed class Av1EncoderBlockWorkspace : IDisposable
     /// </summary>
     private readonly IMemoryOwner<int> owner;
 
+    /// <summary>
+    /// Reuses the fixed-capacity reference-vector stack for every inter block in the frame.
+    /// </summary>
+    private Av1ReferenceMotionVectors referenceMotionVectors;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="Av1EncoderBlockWorkspace"/> class.
     /// </summary>
@@ -128,6 +143,11 @@ internal sealed class Av1EncoderBlockWorkspace : IDisposable
     public Span<int> TransformWorkspace
         => this.owner.Memory.Span.Slice(TransformWorkspaceOffset, Av1TransformWorkspace.MaximumLength);
 
+    /// <summary>
+    /// Gets the reusable reference-vector stack used by inter mode decision and syntax writing.
+    /// </summary>
+    public ref Av1ReferenceMotionVectors ReferenceMotionVectors => ref this.referenceMotionVectors;
+
     /// <summary>
     /// Gets the disjoint edge snapshot used to restore one square partition-search level.
     /// </summary>
@@ -152,43 +172,49 @@ internal sealed class Av1EncoderBlockWorkspace : IDisposable
     public Av1EncoderModeDecisionWorkspace<TSample> GetModeDecisionWorkspace<TSample>()
         where TSample : unmanaged
     {
-        // Conventional intra search finishes before intra-block-copy search begins for the same block.
+        // Conventional intra search finishes before reference prediction begins for the same block.
         // Both phases can therefore reuse this aligned region without extending the owner or preserving stale scratch.
         Span<int> storage = this.owner.Memory.Span.Slice(
-            IntraBlockCopySampleStorageOffset,
+            InterPredictionSampleStorageOffset,
             SharedModeDecisionStorageLength);
 
         return new(storage[..Av1EncoderModeDecisionWorkspace<TSample>.StorageLength]);
     }
 
     /// <summary>
-    /// Gets the reusable storage used while comparing intra-block-copy candidates.
+    /// Gets the reusable storage used while comparing single-reference or intra-block-copy candidates.
     /// </summary>
     /// <typeparam name="TSample">The native sample type selected by the encoder pipeline.</typeparam>
-    /// <returns>The typed intra-block-copy workspace.</returns>
-    public Av1EncoderIntraBlockCopyWorkspace<TSample> GetIntraBlockCopyWorkspace<TSample>()
+    /// <returns>The typed inter-prediction workspace.</returns>
+    public Av1EncoderInterPredictionWorkspace<TSample> GetInterPredictionWorkspace<TSample>()
         where TSample : unmanaged
     {
         Span<int> storage = this.owner.Memory.Span;
         Span<TSample> sampleStorage = MemoryMarshal
-            .Cast<int, TSample>(storage.Slice(IntraBlockCopySampleStorageOffset, IntraBlockCopySampleStorageLength));
+            .Cast<int, TSample>(storage.Slice(InterPredictionSampleStorageOffset, InterPredictionSampleStorageLength));
 
         sampleStorage = sampleStorage[
-            ..(Av1EncoderIntraBlockCopyWorkspace<TSample>.SampleBufferCount *
-                Av1EncoderIntraBlockCopyWorkspace<TSample>.MaximumSampleCount)];
+            ..(Av1EncoderInterPredictionWorkspace<TSample>.SampleBufferCount *
+                Av1EncoderInterPredictionWorkspace<TSample>.MaximumSampleCount)];
 
         Span<short> residualStorage = MemoryMarshal
-            .Cast<int, short>(storage.Slice(IntraBlockCopyResidualStorageOffset, IntraBlockCopyResidualStorageLength));
+            .Cast<int, short>(storage.Slice(InterPredictionResidualStorageOffset, InterPredictionResidualStorageLength));
+
+        residualStorage = residualStorage[..Av1EncoderInterPredictionWorkspace<TSample>.MaximumSampleCount];
+
+        Span<short> predictionScratch = MemoryMarshal
+            .Cast<int, short>(storage.Slice(InterPredictionScratchStorageOffset, InterPredictionScratchStorageLength));
 
-        residualStorage = residualStorage[..Av1EncoderIntraBlockCopyWorkspace<TSample>.MaximumSampleCount];
+        predictionScratch = predictionScratch[..Av1EncoderInterPredictionWorkspace<TSample>.PredictionScratchCount];
 
         Span<int> coefficientStorage = storage.Slice(
-            IntraBlockCopyCoefficientStorageOffset,
-            IntraBlockCopyCoefficientStorageLength);
+            InterPredictionCoefficientStorageOffset,
+            InterPredictionCoefficientStorageLength);
 
-        return new Av1EncoderIntraBlockCopyWorkspace<TSample>(
+        return new Av1EncoderInterPredictionWorkspace<TSample>(
             sampleStorage,
             residualStorage,
+            predictionScratch,
             coefficientStorage);
     }
 

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1EncoderFrame.cs

@@ -345,6 +345,44 @@ internal readonly struct Av1EncoderFrame<TSample>
                 _ => this.chromaRed
             };
 
+        /// <summary>
+        /// Gets a top-left view whose visible dimensions can be smaller than the backing coded planes.
+        /// </summary>
+        /// <param name="width">The visible luma width.</param>
+        /// <param name="height">The visible luma height.</param>
+        /// <returns>The requested non-owning planar view.</returns>
+        public PlanarView GetSubView(int width, int height)
+        {
+            Av1ColorFormat colorFormat = this.IsMonochrome
+                ? Av1ColorFormat.Yuv400
+                : this.ChromaSubsamplingX == 0
+                    ? Av1ColorFormat.Yuv444
+                    : this.ChromaSubsamplingY == 0
+                        ? Av1ColorFormat.Yuv422
+                        : Av1ColorFormat.Yuv420;
+
+            int chromaWidth = (width + this.ChromaSubsamplingX) >> this.ChromaSubsamplingX;
+            int chromaHeight = (height + this.ChromaSubsamplingY) >> this.ChromaSubsamplingY;
+            Buffer2DRegion<TSample> blue = this.IsMonochrome
+                ? default
+                : this.chromaBlue.GetSubRegion(0, 0, chromaWidth, chromaHeight);
+
+            Buffer2DRegion<TSample> red = this.IsMonochrome
+                ? default
+                : this.chromaRed.GetSubRegion(0, 0, chromaWidth, chromaHeight);
+
+            return new PlanarView(
+                this.luma.GetSubRegion(0, 0, width, height),
+                blue,
+                red,
+                width,
+                height,
+                this.LumaBitDepth,
+                colorFormat,
+                this.ChromaPositionX,
+                this.ChromaPositionY);
+        }
+
         /// <inheritdoc/>
         public Span<TSample> GetLumaRowSpan(int row) => this.luma.DangerousGetRowSpan(row);
 

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1EncoderFrameBuffer.cs

@@ -14,6 +14,11 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 internal sealed class Av1EncoderFrameBuffer<TSample> : IDisposable
     where TSample : unmanaged
 {
+    /// <summary>
+    /// The byte boundary used by libaom for SIMD-accessible component planes.
+    /// </summary>
+    private const int PlaneAlignmentBytes = 32;
+
     /// <summary>
     /// The complete frame owner, or <see langword="null"/> after disposal.
     /// </summary>
@@ -48,7 +53,7 @@ internal sealed class Av1EncoderFrameBuffer<TSample> : IDisposable
             : Av1EncoderFrame<TSample>.GetPlaneBufferSize(width, height, subsamplingX, subsamplingY);
 
         int chromaElementCount = checked(chromaSize.Width * chromaSize.Height);
-        int planeAlignment = Math.Max(32 / Unsafe.SizeOf<TSample>(), 1);
+        int planeAlignment = Math.Max(PlaneAlignmentBytes / Unsafe.SizeOf<TSample>(), 1);
         int chromaBlueOffset = Align(lumaElementCount, planeAlignment);
         int chromaRedOffset = Align(checked(chromaBlueOffset + chromaElementCount), planeAlignment);
         int storageLength = colorFormat == Av1ColorFormat.Yuv400

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1EncoderIntraBlockCopyWorkspace.cs → src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1EncoderInterPredictionWorkspace.cs

@@ -1,19 +1,33 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
+
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 
 /// <summary>
-/// Provides disjoint reusable buffers for intra-block-copy mode decisions.
+/// Provides disjoint reusable buffers for single-reference and intra-block-copy mode decisions.
 /// </summary>
 /// <typeparam name="TSample">The native sample type selected by the encoder pipeline.</typeparam>
-internal readonly ref struct Av1EncoderIntraBlockCopyWorkspace<TSample>
+internal readonly ref struct Av1EncoderInterPredictionWorkspace<TSample>
     where TSample : unmanaged
 {
     /// <summary>
-    /// The number of samples in the fixed 8x8 intra-block-copy transform.
+    /// The width and height of the fixed prediction block handled by the current inter search.
+    /// </summary>
+    private const int MaximumBlockDimension = 8;
+
+    /// <summary>
+    /// The number of samples in the fixed prediction block.
+    /// </summary>
+    public const int MaximumSampleCount = MaximumBlockDimension * MaximumBlockDimension;
+
+    /// <summary>
+    /// The signed intermediate capacity needed when both translational interpolation axes are filtered.
     /// </summary>
-    public const int MaximumSampleCount = 8 * 8;
+    public const int PredictionScratchCount =
+        Av1TranslationalInterPredictor.MinimumScratchStride *
+        (MaximumBlockDimension + Av1TranslationalInterPredictor.MaximumExtraRows);
 
     /// <summary>
     /// The number of sample buffers retained by one mode decision.
@@ -27,21 +41,25 @@ internal readonly ref struct Av1EncoderIntraBlockCopyWorkspace<TSample>
 
     private readonly Span<TSample> samples;
     private readonly Span<short> residual;
+    private readonly Span<short> predictionScratch;
     private readonly Span<int> coefficients;
 
     /// <summary>
-    /// Initializes a new instance of the <see cref="Av1EncoderIntraBlockCopyWorkspace{TSample}"/> struct.
+    /// Initializes a new instance of the <see cref="Av1EncoderInterPredictionWorkspace{TSample}"/> struct.
     /// </summary>
     /// <param name="samples">The sample storage.</param>
     /// <param name="residual">The residual storage shared by sequential plane evaluations.</param>
+    /// <param name="predictionScratch">The intermediate storage used by two-dimensional interpolation.</param>
     /// <param name="coefficients">The coefficient storage.</param>
-    public Av1EncoderIntraBlockCopyWorkspace(
+    public Av1EncoderInterPredictionWorkspace(
         Span<TSample> samples,
         Span<short> residual,
+        Span<short> predictionScratch,
         Span<int> coefficients)
     {
         this.samples = samples;
         this.residual = residual;
+        this.predictionScratch = predictionScratch;
         this.coefficients = coefficients;
     }
 
@@ -100,6 +118,11 @@ internal readonly ref struct Av1EncoderIntraBlockCopyWorkspace<TSample>
     /// </summary>
     public Span<short> Residual => this.residual;
 
+    /// <summary>
+    /// Gets the intermediate scratch used when both translational interpolation axes are filtered.
+    /// </summary>
+    public Span<short> PredictionScratch => this.predictionScratch;
+
     /// <summary>
     /// Gets the selected luma coefficients.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1IntraSuperblockEncoder.IntraBlockCopyModeDecision.cs

@@ -1,586 +0,0 @@
-// Copyright (c) Six Labors.
-// Licensed under the Six Labors Split License.
-
-using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
-using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
-using SixLabors.ImageSharp.Memory;
-
-namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
-
-/// <content>
-/// Provides full rate-distortion selection for intra-block-copy candidates.
-/// </content>
-internal static partial class Av1IntraSuperblockEncoder
-{
-    internal partial struct ModeDecision<TSample, TOperator>
-        where TSample : unmanaged
-        where TOperator : struct, IBlockEncodingOperator<TSample>
-    {
-        private long SelectIntraBlockCopy(
-            Av1SymbolEncoder writer,
-            Av1MacroBlockD macroBlock,
-            Point blockOrigin,
-            ushort tileIndex,
-            long regularCost,
-            ref Av1MacroBlockModeInfo modeInfo,
-            ref Av1EncoderBlockStruct block,
-            ref Av1EncoderPaletteInfo paletteInfo)
-        {
-            const Av1BlockSize BlockSize = Av1BlockSize.Block8x8;
-            const Av1TransformSize LumaTransformSize = Av1TransformSize.Size8x8;
-            Buffer2DRegion<TSample> lumaSource = this.source.GetPlane(Av1Plane.Y);
-            Buffer2DRegion<TSample> lumaReconstruction = this.reconstruction.GetPlane(Av1Plane.Y);
-            Point modeInfoPosition = new(
-                blockOrigin.X >> Av1Constants.ModeInfoSizeLog2,
-                blockOrigin.Y >> Av1Constants.ModeInfoSizeLog2);
-
-            Span<Av1MotionVector> referenceCandidates = stackalloc Av1MotionVector[8];
-            Span<int> referenceWeights = stackalloc int[8];
-            Av1MotionVector reference = Av1IntraBlockCopy.FindReference(
-                this.picture,
-                macroBlock,
-                modeInfoPosition,
-                BlockSize,
-                Av1PartitionType.None,
-                referenceCandidates,
-                referenceWeights);
-
-            Span<Av1MotionVector> candidates = stackalloc Av1MotionVector[4];
-            Av1IntraBlockCopySearchIndex search = this.picture.IntraBlockCopySearch;
-            int candidateCount = search.FindCandidates<TSample, TOperator>(
-                lumaSource,
-                lumaReconstruction,
-                blockOrigin,
-                macroBlock.Tile,
-                this.picture.Sequence.SequenceHeader,
-                writer,
-                reference,
-                this.rateMultiplier,
-                candidates);
-
-            candidateCount += search.FindPixelCandidates<TSample, TOperator>(
-                lumaSource,
-                lumaReconstruction,
-                blockOrigin,
-                macroBlock.Tile,
-                this.picture.Sequence.SequenceHeader,
-                writer,
-                reference,
-                this.quantization.QIndex[0],
-                this.rateMultiplier,
-                candidates[candidateCount..]);
-
-            // Hash and full-pixel searches can converge on the same vector. Preserve the first search-order
-            // occurrence so repeated vectors do not pay for duplicate transform searches or alter ties.
-            int uniqueCandidateCount = 0;
-            for (int candidateIndex = 0; candidateIndex < candidateCount; candidateIndex++)
-            {
-                Av1MotionVector candidate = candidates[candidateIndex];
-                bool duplicate = false;
-                for (int uniqueIndex = 0; uniqueIndex < uniqueCandidateCount; uniqueIndex++)
-                {
-                    if (candidate == candidates[uniqueIndex])
-                    {
-                        duplicate = true;
-                        break;
-                    }
-                }
-
-                if (!duplicate)
-                {
-                    candidates[uniqueCandidateCount++] = candidate;
-                }
-            }
-
-            if (uniqueCandidateCount == 0)
-            {
-                return regularCost;
-            }
-
-            int skipContext = Av1TileWriter.GetSkipContext(macroBlock);
-            long bestCost = regularCost;
-            bool hasSelectedCandidate = false;
-            bool selectedSkip = false;
-            Av1MotionVector selectedVector = default;
-            Av1EncoderTransformBlockState selectedLumaState = default;
-            Av1EncoderTransformBlockState selectedBlueState = default;
-            Av1EncoderTransformBlockState selectedRedState = default;
-            Av1EncoderIntraBlockCopyWorkspace<TSample> workspace =
-                this.blockWorkspace.GetIntraBlockCopyWorkspace<TSample>();
-
-            Av1TransformBlockContext lumaContext = Av1TileWriter.GetTransformBlockContexts(
-                Av1ComponentType.Luminance,
-                this.picture.LuminanceDcSignLevelCoefficientNeighbors[tileIndex],
-                blockOrigin,
-                BlockSize,
-                LumaTransformSize);
-
-            // A coded IBC residual uses the unsplit transform root at this fixed block size. A skipped block
-            // omits both the transform-partition bit and coefficient syntax, so this rate is added only below.
-            int transformPartitionRate = 0;
-            if (this.picture.Parent.FrameHeader.TransformMode == Av1TransformMode.Select)
-            {
-                Av1NeighborArrayUnit<byte> transformContexts = this.picture.TransformFunctionContexts[tileIndex];
-                int topIndex = transformContexts.GetTopIndex(blockOrigin);
-                int leftIndex = transformContexts.GetLeftIndex(blockOrigin);
-                int transformPartitionContext = Av1SymbolContextHelper.GetTransformPartitionContext(
-                    transformContexts.Top[topIndex],
-                    transformContexts.Left[leftIndex],
-                    BlockSize,
-                    LumaTransformSize);
-
-                transformPartitionRate = writer.GetTransformPartitionCost(
-                    false,
-                    transformPartitionContext);
-            }
-
-            ObuColorConfig colorConfig = this.picture.Sequence.SequenceHeader.ColorConfig;
-            int subsamplingX = colorConfig.SubSamplingX ? 1 : 0;
-            int subsamplingY = colorConfig.SubSamplingY ? 1 : 0;
-            Point chromaOrigin = new(blockOrigin.X >> subsamplingX, blockOrigin.Y >> subsamplingY);
-            Av1TransformSize chromaTransformSize = BlockSize.GetMaxUvTransformSize(
-                colorConfig.SubSamplingX,
-                colorConfig.SubSamplingY);
-
-            Av1TransformBlockContext blueContext = default;
-            Av1TransformBlockContext redContext = default;
-            if (!this.source.IsMonochrome)
-            {
-                Av1BlockSize chromaBlockSize = BlockSize.GetSubsampled(
-                    colorConfig.SubSamplingX,
-                    colorConfig.SubSamplingY);
-
-                blueContext = Av1TileWriter.GetTransformBlockContexts(
-                    Av1ComponentType.Chroma,
-                    this.picture.CbDcSignLevelCoefficientNeighbors[tileIndex],
-                    chromaOrigin,
-                    chromaBlockSize,
-                    chromaTransformSize);
-
-                redContext = Av1TileWriter.GetTransformBlockContexts(
-                    Av1ComponentType.Chroma,
-                    this.picture.CrDcSignLevelCoefficientNeighbors[tileIndex],
-                    chromaOrigin,
-                    chromaBlockSize,
-                    chromaTransformSize);
-            }
-
-            // Per-vector plane results reuse candidate scratch. Separate selected spans retain only a new
-            // global winner, allowing the complete search to finish before committed reconstruction changes.
-            for (int candidateIndex = 0; candidateIndex < uniqueCandidateCount; candidateIndex++)
-            {
-                Av1MotionVector candidate = candidates[candidateIndex];
-                this.EvaluateIntraBlockCopyPlane(
-                    writer,
-                    candidate,
-                    Av1Plane.Y,
-                    Av1ComponentType.Luminance,
-                    blockOrigin,
-                    0,
-                    0,
-                    LumaTransformSize,
-                    Av1TransformType.AllTransformTypes,
-                    lumaContext,
-                    workspace.LumaPrediction,
-                    workspace.Residual,
-                    workspace.TransformReconstruction,
-                    workspace.TransformCoefficients,
-                    workspace.LumaCandidateReconstruction,
-                    workspace.LumaCandidateCoefficients,
-                    out Av1EncoderTransformBlockState lumaCandidateState,
-                    out int lumaRate,
-                    out long lumaDistortion,
-                    out bool hasEmptyLuma,
-                    out Av1EncoderTransformBlockState emptyLumaState,
-                    out long emptyLumaDistortion);
-
-                int blueRate = 0;
-                int redRate = 0;
-                long blueDistortion = 0;
-                long redDistortion = 0;
-                long emptyBlueDistortion = 0;
-                long emptyRedDistortion = 0;
-                bool hasEmptyBlue = true;
-                bool hasEmptyRed = true;
-                Av1EncoderTransformBlockState blueCandidateState = default;
-                Av1EncoderTransformBlockState redCandidateState = default;
-                Av1EncoderTransformBlockState emptyBlueState = default;
-                Av1EncoderTransformBlockState emptyRedState = default;
-                if (!this.source.IsMonochrome)
-                {
-                    Av1TransformType chromaTransformType = lumaCandidateState.TransformType;
-                    Av1TransformSetType chromaTransformSet = Av1SymbolContextHelper.GetExtendedTransformSetType(
-                        chromaTransformSize,
-                        isInter: true,
-                        this.picture.Parent.FrameHeader.UseReducedTransformSet);
-
-                    // Inter prediction does not signal an independent chroma transform type. Chroma reuses the
-                    // selected luma type when that type belongs to its transform set and otherwise falls back to DCT.
-                    if (!chromaTransformType.IsExtendedSetUsed(chromaTransformSet))
-                    {
-                        chromaTransformType = Av1TransformType.DctDct;
-                    }
-
-                    this.EvaluateIntraBlockCopyPlane(
-                        writer,
-                        candidate,
-                        Av1Plane.U,
-                        Av1ComponentType.Chroma,
-                        blockOrigin,
-                        subsamplingX,
-                        subsamplingY,
-                        chromaTransformSize,
-                        chromaTransformType,
-                        blueContext,
-                        workspace.BluePrediction,
-                        workspace.Residual,
-                        workspace.TransformReconstruction,
-                        workspace.TransformCoefficients,
-                        workspace.BlueCandidateReconstruction,
-                        workspace.BlueCandidateCoefficients,
-                        out blueCandidateState,
-                        out blueRate,
-                        out blueDistortion,
-                        out hasEmptyBlue,
-                        out emptyBlueState,
-                        out emptyBlueDistortion);
-
-                    this.EvaluateIntraBlockCopyPlane(
-                        writer,
-                        candidate,
-                        Av1Plane.V,
-                        Av1ComponentType.Chroma,
-                        blockOrigin,
-                        subsamplingX,
-                        subsamplingY,
-                        chromaTransformSize,
-                        chromaTransformType,
-                        redContext,
-                        workspace.RedPrediction,
-                        workspace.Residual,
-                        workspace.TransformReconstruction,
-                        workspace.TransformCoefficients,
-                        workspace.RedCandidateReconstruction,
-                        workspace.RedCandidateCoefficients,
-                        out redCandidateState,
-                        out redRate,
-                        out redDistortion,
-                        out hasEmptyRed,
-                        out emptyRedState,
-                        out emptyRedDistortion);
-                }
-
-                int displacementRate = writer.GetDisplacementVectorCost(candidate, reference);
-                int candidateRate = writer.GetUseIntraBlockCopyCost(true) +
-                    displacementRate +
-                    writer.GetSkipCost(false, skipContext) +
-                    transformPartitionRate +
-                    lumaRate +
-                    blueRate +
-                    redRate;
-
-                long candidateDistortion = lumaDistortion + blueDistortion + redDistortion;
-                long candidateCost = Av1RateDistortion.GetCost(this.rateMultiplier, candidateRate, candidateDistortion);
-                bool candidateSkip = false;
-
-                // The skip alternative is available only when every coded plane has an empty transform. Its
-                // distortion comes from prediction alone and its rate excludes the transform tree and coefficients.
-                if (hasEmptyLuma && hasEmptyBlue && hasEmptyRed)
-                {
-                    int skipRate = writer.GetUseIntraBlockCopyCost(true) +
-                        displacementRate +
-                        writer.GetSkipCost(true, skipContext);
-
-                    long skipDistortion = emptyLumaDistortion + emptyBlueDistortion + emptyRedDistortion;
-                    long skipCost = Av1RateDistortion.GetCost(this.rateMultiplier, skipRate, skipDistortion);
-                    if (skipCost < candidateCost)
-                    {
-                        candidateCost = skipCost;
-                        candidateSkip = true;
-                    }
-                }
-
-                // Conventional intra and earlier IBC vectors retain strict search-order precedence on equal RD.
-                if (candidateCost >= bestCost)
-                {
-                    continue;
-                }
-
-                bestCost = candidateCost;
-                hasSelectedCandidate = true;
-                selectedSkip = candidateSkip;
-                selectedVector = candidate;
-                if (candidateSkip)
-                {
-                    workspace.LumaPrediction.CopyTo(workspace.SelectedLumaReconstruction);
-                    workspace.SelectedLumaCoefficients.Clear();
-                    selectedLumaState = emptyLumaState;
-                    if (!this.source.IsMonochrome)
-                    {
-                        int chromaSampleCount = chromaTransformSize.GetSize2d();
-                        workspace.BluePrediction[..chromaSampleCount].CopyTo(workspace.SelectedBlueReconstruction);
-                        workspace.RedPrediction[..chromaSampleCount].CopyTo(workspace.SelectedRedReconstruction);
-                        workspace.SelectedBlueCoefficients[..chromaSampleCount].Clear();
-                        workspace.SelectedRedCoefficients[..chromaSampleCount].Clear();
-                        selectedBlueState = emptyBlueState;
-                        selectedRedState = emptyRedState;
-                    }
-                }
-                else
-                {
-                    workspace.LumaCandidateReconstruction.CopyTo(workspace.SelectedLumaReconstruction);
-                    workspace.LumaCandidateCoefficients.CopyTo(workspace.SelectedLumaCoefficients);
-                    selectedLumaState = lumaCandidateState;
-                    if (!this.source.IsMonochrome)
-                    {
-                        int chromaSampleCount = chromaTransformSize.GetSize2d();
-                        workspace.BlueCandidateReconstruction[..chromaSampleCount]
-                            .CopyTo(workspace.SelectedBlueReconstruction);
-
-                        workspace.RedCandidateReconstruction[..chromaSampleCount]
-                            .CopyTo(workspace.SelectedRedReconstruction);
-
-                        workspace.BlueCandidateCoefficients[..chromaSampleCount]
-                            .CopyTo(workspace.SelectedBlueCoefficients);
-
-                        workspace.RedCandidateCoefficients[..chromaSampleCount]
-                            .CopyTo(workspace.SelectedRedCoefficients);
-
-                        selectedBlueState = blueCandidateState;
-                        selectedRedState = redCandidateState;
-                    }
-                }
-            }
-
-            if (!hasSelectedCandidate)
-            {
-                return bestCost;
-            }
-
-            // Only the winning vector is now visible to later coding blocks. This single publication keeps
-            // rejected motion vectors from contaminating intra references or entropy contexts.
-            Span<int> retainedLumaCoefficients = this.coefficientBuffer.GetPlaneSpan(this.superblock.Index, Av1Plane.Y);
-            Span<Av1EncoderTransformBlockState> retainedLumaTransformBlocks =
-                this.coefficientBuffer.GetTransformBlockSpan(this.superblock.Index, Av1Plane.Y);
-
-            int lumaTransformIndex = this.codedAreaLuma /
-                Av1EncoderCoefficientBuffer.TransformBlockUnitCoefficientCount;
-
-            ref Av1EncoderTransformBlockState retainedLumaState = ref retainedLumaTransformBlocks[lumaTransformIndex];
-            CopyCandidate(
-                workspace.SelectedLumaReconstruction,
-                workspace.SelectedLumaCoefficients,
-                lumaReconstruction,
-                blockOrigin,
-                retainedLumaCoefficients[this.codedAreaLuma..],
-                LumaTransformSize,
-                selectedLumaState,
-                ref retainedLumaState);
-
-            if (!this.source.IsMonochrome)
-            {
-                Span<int> retainedBlueCoefficients = this.coefficientBuffer.GetPlaneSpan(this.superblock.Index, Av1Plane.U);
-                Span<int> retainedRedCoefficients = this.coefficientBuffer.GetPlaneSpan(this.superblock.Index, Av1Plane.V);
-                Span<Av1EncoderTransformBlockState> retainedBlueTransformBlocks =
-                    this.coefficientBuffer.GetTransformBlockSpan(this.superblock.Index, Av1Plane.U);
-
-                Span<Av1EncoderTransformBlockState> retainedRedTransformBlocks =
-                    this.coefficientBuffer.GetTransformBlockSpan(this.superblock.Index, Av1Plane.V);
-
-                int chromaTransformIndex = this.codedAreaChroma /
-                    Av1EncoderCoefficientBuffer.TransformBlockUnitCoefficientCount;
-
-                ref Av1EncoderTransformBlockState retainedBlueState = ref retainedBlueTransformBlocks[chromaTransformIndex];
-                ref Av1EncoderTransformBlockState retainedRedState = ref retainedRedTransformBlocks[chromaTransformIndex];
-                CopyCandidate(
-                    workspace.SelectedBlueReconstruction,
-                    workspace.SelectedBlueCoefficients,
-                    this.reconstruction.GetPlane(Av1Plane.U),
-                    chromaOrigin,
-                    retainedBlueCoefficients[this.codedAreaChroma..],
-                    chromaTransformSize,
-                    selectedBlueState,
-                    ref retainedBlueState);
-
-                CopyCandidate(
-                    workspace.SelectedRedReconstruction,
-                    workspace.SelectedRedCoefficients,
-                    this.reconstruction.GetPlane(Av1Plane.V),
-                    chromaOrigin,
-                    retainedRedCoefficients[this.codedAreaChroma..],
-                    chromaTransformSize,
-                    selectedRedState,
-                    ref retainedRedState);
-            }
-
-            modeInfo.Block.Mode = Av1PredictionMode.DC;
-            modeInfo.Block.UvMode = Av1ChromaPredictionMode.DC;
-            modeInfo.Block.TransformSize = LumaTransformSize;
-            modeInfo.Block.Skip = selectedSkip;
-            modeInfo.Block.UseIntraBlockCopy = true;
-            block.FilterIntraMode = Av1FilterIntraMode.AllFilterIntraModes;
-            block.PredictionUnit.AngleDelta[(int)Av1PlaneType.Y] = 0;
-            block.PredictionUnit.AngleDelta[(int)Av1PlaneType.Uv] = 0;
-            block.PredictionUnit.ChromaFromLumaIndex = 0;
-            block.PredictionUnit.ChromaFromLumaSigns = 0;
-            paletteInfo = default;
-            this.picture.SetDisplacementVector(modeInfoPosition, selectedVector);
-            return bestCost;
-        }
-
-        private void EvaluateIntraBlockCopyPlane(
-            Av1SymbolEncoder writer,
-            Av1MotionVector vector,
-            Av1Plane plane,
-            Av1ComponentType componentType,
-            Point lumaOrigin,
-            int subsamplingX,
-            int subsamplingY,
-            Av1TransformSize transformSize,
-            Av1TransformType transformTypeSelection,
-            Av1TransformBlockContext blockContext,
-            Span<TSample> prediction,
-            Span<short> residual,
-            Span<TSample> transformReconstruction,
-            Span<int> transformCoefficients,
-            Span<TSample> selectedReconstruction,
-            Span<int> selectedCoefficients,
-            out Av1EncoderTransformBlockState selectedState,
-            out int selectedRate,
-            out long selectedDistortion,
-            out bool hasEmptyTransform,
-            out Av1EncoderTransformBlockState emptyState,
-            out long emptyDistortion)
-        {
-            Point planeOrigin = new(lumaOrigin.X >> subsamplingX, lumaOrigin.Y >> subsamplingY);
-            int sourceColumnQ4 = (planeOrigin.X << 4) + (vector.Column << (1 - subsamplingX));
-            int sourceRowQ4 = (planeOrigin.Y << 4) + (vector.Row << (1 - subsamplingY));
-            Point predictionOrigin = new(sourceColumnQ4 >> 4, sourceRowQ4 >> 4);
-            int sampleCount = transformSize.GetSize2d();
-            Buffer2DRegion<TSample> sourcePlane = this.source.GetPlane(plane);
-            Buffer2DRegion<TSample> reconstructionPlane = this.reconstruction.GetPlane(plane);
-            TOperator.PrepareIntraBlockCopy(
-                sourcePlane,
-                planeOrigin,
-                reconstructionPlane,
-                predictionOrigin,
-                (sourceColumnQ4 & 15) != 0,
-                (sourceRowQ4 & 15) != 0,
-                prediction[..sampleCount],
-                residual[..sampleCount],
-                transformSize);
-
-            // Motion compensation and subtraction do not depend on transform type. Keep them outside the
-            // transform loop so exhaustive luma search traverses the source and reference blocks only once.
-            Av1TransformSetType transformSetType = Av1SymbolContextHelper.GetExtendedTransformSetType(
-                transformSize,
-                isInter: true,
-                this.picture.Parent.FrameHeader.UseReducedTransformSet);
-
-            Av1TransformType firstTransformType = transformTypeSelection == Av1TransformType.AllTransformTypes
-                ? Av1TransformType.DctDct
-                : transformTypeSelection;
-            Av1TransformType transformTypeLimit = transformTypeSelection == Av1TransformType.AllTransformTypes
-                ? Av1TransformType.AllTransformTypes
-                : (Av1TransformType)((int)transformTypeSelection + 1);
-
-            long bestCost = long.MaxValue;
-            selectedState = default;
-            selectedRate = 0;
-            selectedDistortion = 0;
-            hasEmptyTransform = false;
-            emptyState = default;
-            emptyDistortion = 0;
-
-            // The candidate and best spans alternate ownership whenever a transform improves the result.
-            // This mirrors the reference's buffer-pointer swap and replaces a copy on every improvement
-            // with at most one normalization copy after the transform search.
-            Span<TSample> candidateReconstruction = transformReconstruction[..sampleCount];
-            Span<int> candidateCoefficients = transformCoefficients[..sampleCount];
-            Span<TSample> bestReconstruction = selectedReconstruction[..sampleCount];
-            Span<int> bestCoefficients = selectedCoefficients[..sampleCount];
-            bool bestUsesSelectedStorage = true;
-            for (Av1TransformType transformType = firstTransformType;
-                transformType < transformTypeLimit;
-                transformType++)
-            {
-                if (!transformType.IsExtendedSetUsed(transformSetType))
-                {
-                    continue;
-                }
-
-                Av1EncoderTransformBlockState candidateState = default;
-                long candidateDistortion = TOperator.EncodePredictionCandidate(
-                    this.blockWorkspace,
-                    sourcePlane,
-                    planeOrigin,
-                    prediction[..sampleCount],
-                    residual[..sampleCount],
-                    candidateReconstruction,
-                    transformSize.GetWidth(),
-                    candidateCoefficients,
-                    transformSize,
-                    transformType,
-                    plane,
-                    this.quantization.QIndex[0],
-                    this.quantization.DeltaQDc[(int)plane],
-                    this.quantization.DeltaQAc[(int)plane],
-                    this.bitDepth,
-                    ref candidateState);
-
-                int candidateRate = writer.GetCoefficientCost(
-                    transformSize,
-                    transformType,
-                    Av1PredictionMode.DC,
-                    candidateCoefficients,
-                    componentType,
-                    blockContext,
-                    candidateState.EndOfBlock,
-                    this.picture.Parent.FrameHeader.UseReducedTransformSet,
-                    Av1FilterIntraMode.AllFilterIntraModes,
-                    usesInterTransformSet: true);
-
-                long candidateCost = Av1RateDistortion.GetCost(
-                    this.rateMultiplier,
-                    candidateRate,
-                    candidateDistortion);
-
-                if (candidateCost < bestCost)
-                {
-                    Span<TSample> previousBestReconstruction = bestReconstruction;
-                    bestReconstruction = candidateReconstruction;
-                    candidateReconstruction = previousBestReconstruction;
-
-                    Span<int> previousBestCoefficients = bestCoefficients;
-                    bestCoefficients = candidateCoefficients;
-                    candidateCoefficients = previousBestCoefficients;
-                    bestUsesSelectedStorage = !bestUsesSelectedStorage;
-                    bestCost = candidateCost;
-                    selectedState = candidateState;
-                    selectedRate = candidateRate;
-                    selectedDistortion = candidateDistortion;
-                }
-
-                if (candidateState.EndOfBlock == 0 &&
-                    (!hasEmptyTransform || candidateDistortion < emptyDistortion))
-                {
-                    hasEmptyTransform = true;
-                    emptyState = candidateState;
-                    emptyDistortion = candidateDistortion;
-                }
-            }
-
-            // Callers retain the designated selected spans after this scratch workspace is reused by the
-            // next plane or motion vector, so normalize only when the final best result occupies scratch.
-            if (!bestUsesSelectedStorage)
-            {
-                bestReconstruction.CopyTo(selectedReconstruction);
-                bestCoefficients.CopyTo(selectedCoefficients);
-            }
-        }
-    }
-}

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1IntraSuperblockEncoder.ModeDecision.cs

@@ -127,6 +127,7 @@ internal static partial class Av1IntraSuperblockEncoder
         where TOperator : struct, IBlockEncodingOperator<TSample>
     {
         private readonly Av1EncoderFrame<TSample>.PlanarView source;
+        private readonly Av1EncoderFrame<TSample>.PlanarView reference;
         private readonly Av1EncoderFrame<TSample>.PlanarView reconstruction;
         private readonly Av1PictureControlSet picture;
         private readonly Av1Superblock superblock;
@@ -144,6 +145,7 @@ internal static partial class Av1IntraSuperblockEncoder
         /// Initializes a new instance of the <see cref="ModeDecision{TSample, TOperator}"/> struct.
         /// </summary>
         /// <param name="source">The coded source frame.</param>
+        /// <param name="reference">The reconstructed inter reference, or the current reconstruction for an intra frame.</param>
         /// <param name="reconstruction">The reconstructed frame updated by winning candidates.</param>
         /// <param name="picture">The frame coding and mode-information state.</param>
         /// <param name="superblock">The current superblock.</param>
@@ -152,6 +154,7 @@ internal static partial class Av1IntraSuperblockEncoder
         /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
         public ModeDecision(
             Av1EncoderFrame<TSample> source,
+            Av1EncoderFrame<TSample> reference,
             Av1EncoderFrame<TSample> reconstruction,
             Av1PictureControlSet picture,
             Av1Superblock superblock,
@@ -160,6 +163,7 @@ internal static partial class Av1IntraSuperblockEncoder
             int effort)
         {
             this.source = source.CodedView;
+            this.reference = reference.CodedView;
             this.reconstruction = reconstruction.CodedView;
             this.picture = picture;
             this.superblock = superblock;
@@ -167,7 +171,10 @@ internal static partial class Av1IntraSuperblockEncoder
             this.blockWorkspace = blockWorkspace;
             this.quantization = picture.Parent.FrameHeader.QuantizationParameters;
             this.bitDepth = picture.Sequence.SequenceHeader.ColorConfig.BitDepth;
-            this.rateMultiplier = Av1RateDistortion.GetKeyFrameRateMultiplier(this.quantization.QIndex[0], this.bitDepth);
+            this.rateMultiplier = picture.Parent.FrameHeader.IsIntra
+                ? Av1RateDistortion.GetKeyFrameRateMultiplier(this.quantization.QIndex[0], this.bitDepth)
+                : Av1RateDistortion.GetInterFrameRateMultiplier(this.quantization.QIndex[0], this.bitDepth);
+
             this.effort = effort;
             this.codedAreaLuma = 0;
             this.codedAreaChroma = 0;
@@ -183,6 +190,13 @@ internal static partial class Av1IntraSuperblockEncoder
             Av1BlockSize blockSize,
             Av1PartitionType preparedPartition)
         {
+            if (!this.picture.Parent.FrameHeader.IsIntra)
+            {
+                // The first inter implementation retains the prepared 8x8 tree so every prediction and residual
+                // transform fits the single reusable block workspace while larger inter partitions remain unsearched.
+                return preparedPartition;
+            }
+
             bool searchPartition = blockSize is Av1BlockSize.Block8x8 or Av1BlockSize.Block16x16 ||
                 (this.effort == 10 &&
                     blockSize is Av1BlockSize.Block32x32 or Av1BlockSize.Block64x64 or Av1BlockSize.Block128x128);
@@ -640,8 +654,9 @@ internal static partial class Av1IntraSuperblockEncoder
                     modeInfo.Block.Skip,
                     allowIntraBlockCopy);
 
-                this.selectedBlockCost = allowIntraBlockCopy
-                    ? this.SelectIntraBlockCopy(
+                if (!this.picture.Parent.FrameHeader.IsIntra)
+                {
+                    this.selectedBlockCost = this.SelectInterPrediction(
                         writer,
                         macroBlock,
                         blockOrigin,
@@ -649,8 +664,22 @@ internal static partial class Av1IntraSuperblockEncoder
                         regularCost,
                         ref modeInfo,
                         ref block,
-                        ref paletteInfo)
-                    : regularCost;
+                        ref paletteInfo);
+                }
+                else
+                {
+                    this.selectedBlockCost = allowIntraBlockCopy
+                        ? this.SelectIntraBlockCopy(
+                            writer,
+                            macroBlock,
+                            blockOrigin,
+                            tileIndex,
+                            regularCost,
+                            ref modeInfo,
+                            ref block,
+                            ref paletteInfo)
+                        : regularCost;
+                }
 
                 this.codedAreaLuma += blockSize.GetWidth() * blockSize.GetHeight();
                 return;
@@ -787,8 +816,9 @@ internal static partial class Av1IntraSuperblockEncoder
                 modeInfo.Block.Skip,
                 allowColorIntraBlockCopy);
 
-            this.selectedBlockCost = allowColorIntraBlockCopy
-                ? this.SelectIntraBlockCopy(
+            if (!this.picture.Parent.FrameHeader.IsIntra)
+            {
+                this.selectedBlockCost = this.SelectInterPrediction(
                     writer,
                     macroBlock,
                     blockOrigin,
@@ -796,8 +826,22 @@ internal static partial class Av1IntraSuperblockEncoder
                     regularColorCost,
                     ref modeInfo,
                     ref block,
-                    ref paletteInfo)
-                : regularColorCost;
+                    ref paletteInfo);
+            }
+            else
+            {
+                this.selectedBlockCost = allowColorIntraBlockCopy
+                    ? this.SelectIntraBlockCopy(
+                        writer,
+                        macroBlock,
+                        blockOrigin,
+                        tileIndex,
+                        regularColorCost,
+                        ref modeInfo,
+                        ref block,
+                        ref paletteInfo)
+                    : regularColorCost;
+            }
 
             this.codedAreaLuma += blockSize.GetWidth() * blockSize.GetHeight();
             if (block.HasChroma)
@@ -1237,6 +1281,12 @@ internal static partial class Av1IntraSuperblockEncoder
             bool allowIntraBlockCopy)
         {
             int rateAdjustment = writer.GetSkipCost(skip, Av1TileWriter.GetSkipContext(macroBlock));
+            if (!this.picture.Parent.FrameHeader.IsIntra)
+            {
+                int intraInterContext = Av1TileWriter.GetIntraInterContext(macroBlock);
+                rateAdjustment += writer.GetIsInterCost(false, intraInterContext);
+            }
+
             if (allowIntraBlockCopy)
             {
                 rateAdjustment += writer.GetUseIntraBlockCopyCost(false);
@@ -2082,7 +2132,14 @@ internal static partial class Av1IntraSuperblockEncoder
                     leftContexts,
                     out int coefficientRate);
 
-                int rate = Av1TileWriter.GetLumaModeCost(writer, macroBlock, blockSize, mode, angleDelta);
+                int rate = Av1TileWriter.GetLumaModeCost(
+                    writer,
+                    macroBlock,
+                    blockSize,
+                    mode,
+                    angleDelta,
+                    this.picture.Parent.FrameHeader.IsIntra);
+
                 rate += transformSizeRate + coefficientRate;
                 if (mode == Av1PredictionMode.DC)
                 {
@@ -2193,7 +2250,14 @@ internal static partial class Av1IntraSuperblockEncoder
             }
             else
             {
-                rate += Av1TileWriter.GetLumaModeCost(writer, macroBlock, BlockSize, mode, angleDelta);
+                rate += Av1TileWriter.GetLumaModeCost(
+                    writer,
+                    macroBlock,
+                    BlockSize,
+                    mode,
+                    angleDelta,
+                    this.picture.Parent.FrameHeader.IsIntra);
+
                 if (mode == Av1PredictionMode.DC)
                 {
                     rate += paletteDisabledCost;
@@ -2643,7 +2707,14 @@ internal static partial class Av1IntraSuperblockEncoder
 
             // Charge every block-level choice that distinguishes this spatial candidate before adding
             // coefficient syntax derived from the live neighboring-transform context.
-            int rate = Av1TileWriter.GetLumaModeCost(writer, macroBlock, blockSize, mode, angleDelta);
+            int rate = Av1TileWriter.GetLumaModeCost(
+                writer,
+                macroBlock,
+                blockSize,
+                mode,
+                angleDelta,
+                this.picture.Parent.FrameHeader.IsIntra);
+
             rate += transformSizeRate;
             if (mode == Av1PredictionMode.DC)
             {
@@ -2714,7 +2785,8 @@ internal static partial class Av1IntraSuperblockEncoder
                 macroBlock,
                 blockSize,
                 Av1PredictionMode.DC,
-                0);
+                0,
+                this.picture.Parent.FrameHeader.IsIntra);
 
             rate += transformSizeRate;
             rate += paletteDisabledCost;

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1IntraSuperblockEncoder.Operator.cs

@@ -6,6 +6,7 @@ using System.Runtime.Intrinsics;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.IntraBlockCopy;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
@@ -18,6 +19,11 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 /// </content>
 internal static partial class Av1IntraSuperblockEncoder
 {
+    /// <summary>
+    /// The width and height of the fixed block currently used by inter motion search.
+    /// </summary>
+    private const int InterSearchBlockDimension = 8;
+
     /// <summary>
     /// Defines type-specific block encoding without coupling traversal to sample storage width.
     /// </summary>
@@ -259,7 +265,7 @@ internal static partial class Av1IntraSuperblockEncoder
         /// <param name="prediction">The contiguous prediction destination.</param>
         /// <param name="residual">The contiguous source-minus-prediction destination.</param>
         /// <param name="transformSize">The prediction dimensions.</param>
-        public static abstract void PrepareIntraBlockCopy(
+        public static abstract void PrepareIntraBlockCopyPrediction(
             Buffer2DRegion<TSample> source,
             Point blockOrigin,
             Buffer2DRegion<TSample> reconstruction,
@@ -270,6 +276,68 @@ internal static partial class Av1IntraSuperblockEncoder
             Span<short> residual,
             Av1TransformSize transformSize);
 
+        /// <summary>
+        /// Subtracts a retained prediction from its source without rebuilding the inter predictor.
+        /// </summary>
+        /// <param name="source">The source plane.</param>
+        /// <param name="blockOrigin">The block origin in plane samples.</param>
+        /// <param name="prediction">The tightly packed prediction samples.</param>
+        /// <param name="residual">The destination signed residual samples.</param>
+        /// <param name="transformSize">The plane block geometry.</param>
+        public static abstract void SubtractPrediction(
+            Buffer2DRegion<TSample> source,
+            Point blockOrigin,
+            ReadOnlySpan<TSample> prediction,
+            Span<short> residual,
+            Av1TransformSize transformSize);
+
+        /// <summary>
+        /// Builds a translational prediction from a retained reference frame and the matching source residual.
+        /// </summary>
+        /// <param name="source">The coded source plane.</param>
+        /// <param name="blockOrigin">The destination block origin in plane samples.</param>
+        /// <param name="reference">The padded retained reference plane.</param>
+        /// <param name="predictionOrigin">The integer reference origin preceding the subpixel phase.</param>
+        /// <param name="horizontalFilter">The horizontal interpolation filter.</param>
+        /// <param name="verticalFilter">The vertical interpolation filter.</param>
+        /// <param name="horizontalPhase">The horizontal phase in one-sixteenth-sample units.</param>
+        /// <param name="verticalPhase">The vertical phase in one-sixteenth-sample units.</param>
+        /// <param name="prediction">The contiguous prediction destination.</param>
+        /// <param name="residual">The contiguous source-minus-prediction destination.</param>
+        /// <param name="predictionScratch">The intermediate storage used by two-dimensional filtering.</param>
+        /// <param name="transformSize">The prediction dimensions.</param>
+        /// <param name="bitDepth">The coded sample bit depth.</param>
+        public static abstract void PrepareTranslationalInterPrediction(
+            Buffer2DRegion<TSample> source,
+            Point blockOrigin,
+            Buffer2DRegion<TSample> reference,
+            Point predictionOrigin,
+            Av1InterpolationFilter horizontalFilter,
+            Av1InterpolationFilter verticalFilter,
+            int horizontalPhase,
+            int verticalPhase,
+            Span<TSample> prediction,
+            Span<short> residual,
+            Span<short> predictionScratch,
+            Av1TransformSize transformSize,
+            Av1BitDepth bitDepth);
+
+        /// <summary>
+        /// Measures an 8x8 full-pixel reference candidate through the bordered plane storage.
+        /// </summary>
+        /// <param name="source">The coded source plane.</param>
+        /// <param name="sourceOrigin">The source block origin in visible-plane coordinates.</param>
+        /// <param name="reference">The padded retained reference plane.</param>
+        /// <param name="predictionOrigin">The candidate origin, which may lie inside the physical border.</param>
+        /// <param name="bitDepth">The coded sample precision.</param>
+        /// <returns>The squared error normalized to the eight-bit distortion domain.</returns>
+        public static abstract long GetInterPredictionError(
+            Buffer2DRegion<TSample> source,
+            Point sourceOrigin,
+            Buffer2DRegion<TSample> reference,
+            Point predictionOrigin,
+            Av1BitDepth bitDepth);
+
         /// <summary>
         /// Encodes one prepared prediction with the selected transform into decision scratch.
         /// </summary>
@@ -384,9 +452,8 @@ internal static partial class Av1IntraSuperblockEncoder
                 ReadOnlySpan<byte> firstRow = plane.DangerousGetRowSpan(first.Y + row)[first.X..];
                 ReadOnlySpan<byte> secondRow = plane.DangerousGetRowSpan(second.Y + row)[second.X..];
 
-                // An 8x8 search row occupies one machine word, so one unaligned load and comparison replaces
-                // eight dependent scalar branches while retaining exact collision rejection.
-                if (MemoryMarshal.Read<ulong>(firstRow) != MemoryMarshal.Read<ulong>(secondRow))
+                // Compare the complete row as byte lanes so collision rejection remains independent of native endianness.
+                if (Vector64.Create(firstRow) != Vector64.Create(secondRow))
                 {
                     return false;
                 }
@@ -395,49 +462,47 @@ internal static partial class Av1IntraSuperblockEncoder
             return true;
         }
 
+        /// <inheritdoc/>
+        public static long GetInterPredictionError(
+            Buffer2DRegion<byte> source,
+            Point sourceOrigin,
+            Buffer2DRegion<byte> reference,
+            Point predictionOrigin,
+            Av1BitDepth bitDepth)
+        {
+            Rectangle sourceBounds = source.Bounds;
+            Rectangle referenceBounds = reference.Bounds;
+            int sourceIndex =
+                ((sourceBounds.Y + sourceOrigin.Y) * source.Stride) +
+                sourceBounds.X +
+                sourceOrigin.X;
+
+            int referenceIndex =
+                ((referenceBounds.Y + predictionOrigin.Y) * reference.Stride) +
+                referenceBounds.X +
+                predictionOrigin.X;
+
+            // The shared residual kernel selects the widest available vector width and handles the scalar tail.
+            return Av1ResidualBuilder.SumSquaredError(
+                source.Buffer.DangerousGetSingleSpan()[sourceIndex..],
+                source.Stride,
+                reference.Buffer.DangerousGetSingleSpan()[referenceIndex..],
+                reference.Stride,
+                InterSearchBlockDimension,
+                InterSearchBlockDimension);
+        }
+
         /// <inheritdoc/>
         public static int GetSumOfAbsoluteDifferences(
             Buffer2DRegion<byte> source,
             Point sourceOrigin,
             Buffer2DRegion<byte> reconstruction,
             Point predictionOrigin)
-        {
-            int sum = 0;
-            if (Vector128.IsHardwareAccelerated)
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<byte> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<byte> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    Vector128<short> difference =
-                        (Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(sourceRow)).AsByte()) -
-                         Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(predictionRow)).AsByte()))
-                        .AsInt16();
-
-                    // Widened signed differences retain both subtraction directions; absolute values then reduce
-                    // the complete eight-sample row without scalar extraction or per-sample branches.
-                    sum += Vector128.Sum(Vector128.Abs(difference));
-                }
-            }
-            else
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<byte> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<byte> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    for (int column = 0; column < 8; column++)
-                    {
-                        sum += Math.Abs(sourceRow[column] - predictionRow[column]);
-                    }
-                }
-            }
-
-            return sum;
-        }
+            => Av1ResidualBuilder.SumAbsoluteDifferences8x8(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, sourceOrigin),
+                source.Stride,
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, predictionOrigin),
+                reconstruction.Stride);
 
         /// <inheritdoc/>
         public static void GetFourSumsOfAbsoluteDifferences(
@@ -446,66 +511,12 @@ internal static partial class Av1IntraSuperblockEncoder
             Buffer2DRegion<byte> reconstruction,
             Point firstPredictionOrigin,
             Span<int> sums)
-        {
-            int sum0 = 0;
-            int sum1 = 0;
-            int sum2 = 0;
-            int sum3 = 0;
-            if (Vector128.IsHardwareAccelerated)
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<byte> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<byte> predictionRow =
-                        reconstruction.DangerousGetRowSpan(firstPredictionOrigin.Y + row)[firstPredictionOrigin.X..];
-
-                    // The four candidates reuse one widened source vector; only their overlapping predictor
-                    // windows are loaded separately before the packed absolute-difference reductions.
-                    Vector128<short> sourceSamples =
-                        Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(sourceRow)).AsByte()).AsInt16();
-
-                    Vector128<short> prediction0 =
-                        Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(predictionRow)).AsByte()).AsInt16();
-
-                    Vector128<short> prediction1 =
-                        Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(predictionRow[1..])).AsByte()).AsInt16();
-
-                    Vector128<short> prediction2 =
-                        Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(predictionRow[2..])).AsByte()).AsInt16();
-
-                    Vector128<short> prediction3 =
-                        Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(predictionRow[3..])).AsByte()).AsInt16();
-
-                    sum0 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction0));
-                    sum1 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction1));
-                    sum2 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction2));
-                    sum3 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction3));
-                }
-            }
-            else
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<byte> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<byte> predictionRow =
-                        reconstruction.DangerousGetRowSpan(firstPredictionOrigin.Y + row)[firstPredictionOrigin.X..];
-
-                    for (int column = 0; column < 8; column++)
-                    {
-                        int sourceSample = sourceRow[column];
-                        sum0 += Math.Abs(sourceSample - predictionRow[column]);
-                        sum1 += Math.Abs(sourceSample - predictionRow[column + 1]);
-                        sum2 += Math.Abs(sourceSample - predictionRow[column + 2]);
-                        sum3 += Math.Abs(sourceSample - predictionRow[column + 3]);
-                    }
-                }
-            }
-
-            sums[0] = sum0;
-            sums[1] = sum1;
-            sums[2] = sum2;
-            sums[3] = sum3;
-        }
+            => Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, sourceOrigin),
+                source.Stride,
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, firstPredictionOrigin),
+                reconstruction.Stride,
+                sums);
 
         /// <inheritdoc/>
         public static int GetVariance(
@@ -515,44 +526,13 @@ internal static partial class Av1IntraSuperblockEncoder
             Point predictionOrigin,
             Av1BitDepth bitDepth)
         {
-            int sum = 0;
-            int sumOfSquares = 0;
-            if (Vector128.IsHardwareAccelerated)
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<byte> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<byte> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    Vector128<short> difference =
-                        (Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(sourceRow)).AsByte()) -
-                         Vector128.WidenLower(Vector128.CreateScalarUnsafe(MemoryMarshal.Read<ulong>(predictionRow)).AsByte()))
-                        .AsInt16();
-
-                    // Widen before squaring so signed residuals cannot wrap in 16-bit lanes.
-                    Vector128<int> lower = Vector128.WidenLower(difference);
-                    Vector128<int> upper = Vector128.WidenUpper(difference);
-                    sum += Vector128.Sum(difference);
-                    sumOfSquares += Vector128.Sum(lower * lower) + Vector128.Sum(upper * upper);
-                }
-            }
-            else
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<byte> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<byte> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    for (int column = 0; column < 8; column++)
-                    {
-                        int difference = sourceRow[column] - predictionRow[column];
-                        sum += difference;
-                        sumOfSquares += difference * difference;
-                    }
-                }
-            }
+            Av1ResidualBuilder.GetMoments8x8(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, sourceOrigin),
+                source.Stride,
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, predictionOrigin),
+                reconstruction.Stride,
+                out int sum,
+                out int sumOfSquares);
 
             return GetNormalizedVariance(sum, sumOfSquares, bitDepth);
         }
@@ -575,10 +555,7 @@ internal static partial class Av1IntraSuperblockEncoder
                 // A complete row widens in one vector; clipped edge rows retain scalar bounds.
                 if (columns == 8 && Vector128.IsHardwareAccelerated)
                 {
-                    ulong packed = MemoryMarshal.Read<ulong>(sourceRow);
-                    Vector128.WidenLower(Vector128.CreateScalarUnsafe(packed).AsByte())
-                        .AsInt16()
-                        .CopyTo(samples[sampleOffset..]);
+                    Vector128.WidenLower(Vector128.Create(Vector64.Create(sourceRow), Vector64<byte>.Zero)).AsInt16().CopyTo(samples[sampleOffset..]);
 
                     sampleOffset += columns;
                     continue;
@@ -803,7 +780,7 @@ internal static partial class Av1IntraSuperblockEncoder
         }
 
         /// <inheritdoc/>
-        public static void PrepareIntraBlockCopy(
+        public static void PrepareIntraBlockCopyPrediction(
             Buffer2DRegion<byte> source,
             Point blockOrigin,
             Buffer2DRegion<byte> reconstruction,
@@ -837,6 +814,64 @@ internal static partial class Av1IntraSuperblockEncoder
                 height);
         }
 
+        /// <inheritdoc/>
+        public static void SubtractPrediction(
+            Buffer2DRegion<byte> source,
+            Point blockOrigin,
+            ReadOnlySpan<byte> prediction,
+            Span<short> residual,
+            Av1TransformSize transformSize)
+            => Av1ResidualBuilder.Subtract(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, blockOrigin),
+                source.Stride,
+                prediction,
+                transformSize.GetWidth(),
+                residual,
+                transformSize.GetWidth(),
+                transformSize.GetWidth(),
+                transformSize.GetHeight());
+
+        /// <inheritdoc/>
+        public static void PrepareTranslationalInterPrediction(
+            Buffer2DRegion<byte> source,
+            Point blockOrigin,
+            Buffer2DRegion<byte> reference,
+            Point predictionOrigin,
+            Av1InterpolationFilter horizontalFilter,
+            Av1InterpolationFilter verticalFilter,
+            int horizontalPhase,
+            int verticalPhase,
+            Span<byte> prediction,
+            Span<short> residual,
+            Span<short> predictionScratch,
+            Av1TransformSize transformSize,
+            Av1BitDepth bitDepth)
+        {
+            int width = transformSize.GetWidth();
+            int height = transformSize.GetHeight();
+            Rectangle referenceBounds = reference.Bounds;
+            int referenceOrigin =
+                ((referenceBounds.Y + predictionOrigin.Y) * reference.Stride) +
+                referenceBounds.X +
+                predictionOrigin.X;
+
+            Av1TranslationalInterPredictor.Predict(
+                reference.Buffer.DangerousGetSingleSpan(),
+                reference.Stride,
+                referenceOrigin,
+                prediction,
+                width,
+                width,
+                height,
+                horizontalFilter,
+                verticalFilter,
+                horizontalPhase,
+                verticalPhase,
+                predictionScratch);
+
+            SubtractPrediction(source, blockOrigin, prediction, residual, transformSize);
+        }
+
         /// <inheritdoc/>
         public static long EncodePredictionCandidate(
             Av1EncoderBlockWorkspace workspace,
@@ -947,49 +982,49 @@ internal static partial class Av1IntraSuperblockEncoder
         }
 
         /// <inheritdoc/>
-        public static int GetSumOfAbsoluteDifferences(
+        public static long GetInterPredictionError(
             Buffer2DRegion<ushort> source,
             Point sourceOrigin,
-            Buffer2DRegion<ushort> reconstruction,
-            Point predictionOrigin)
+            Buffer2DRegion<ushort> reference,
+            Point predictionOrigin,
+            Av1BitDepth bitDepth)
         {
-            int sum = 0;
-            if (Vector128.IsHardwareAccelerated)
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<ushort> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<ushort> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    // Twelve-bit samples remain within signed 16-bit subtraction and absolute-value ranges,
-                    // allowing all eight row differences to stay packed until their horizontal reduction.
-                    Vector128<short> difference =
-                        (Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(sourceRow)) -
-                         Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(predictionRow)))
-                        .AsInt16();
-
-                    sum += Vector128.Sum(Vector128.Abs(difference));
-                }
-            }
-            else
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<ushort> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<ushort> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    for (int column = 0; column < 8; column++)
-                    {
-                        sum += Math.Abs(sourceRow[column] - predictionRow[column]);
-                    }
-                }
-            }
+            Rectangle sourceBounds = source.Bounds;
+            Rectangle referenceBounds = reference.Bounds;
+            int sourceIndex =
+                ((sourceBounds.Y + sourceOrigin.Y) * source.Stride) +
+                sourceBounds.X +
+                sourceOrigin.X;
+
+            int referenceIndex =
+                ((referenceBounds.Y + predictionOrigin.Y) * reference.Stride) +
+                referenceBounds.X +
+                predictionOrigin.X;
+
+            long error = Av1ResidualBuilder.SumSquaredError(
+                source.Buffer.DangerousGetSingleSpan()[sourceIndex..],
+                source.Stride,
+                reference.Buffer.DangerousGetSingleSpan()[referenceIndex..],
+                reference.Stride,
+                InterSearchBlockDimension,
+                InterSearchBlockDimension);
 
-            return sum;
+            int shift = (bitDepth.GetBitCount() - 8) * 2;
+            return shift == 0 ? error : (error + (1L << (shift - 1))) >> shift;
         }
 
+        /// <inheritdoc/>
+        public static int GetSumOfAbsoluteDifferences(
+            Buffer2DRegion<ushort> source,
+            Point sourceOrigin,
+            Buffer2DRegion<ushort> reconstruction,
+            Point predictionOrigin)
+            => Av1ResidualBuilder.SumAbsoluteDifferences8x8(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, sourceOrigin),
+                source.Stride,
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, predictionOrigin),
+                reconstruction.Stride);
+
         /// <inheritdoc/>
         public static void GetFourSumsOfAbsoluteDifferences(
             Buffer2DRegion<ushort> source,
@@ -997,66 +1032,12 @@ internal static partial class Av1IntraSuperblockEncoder
             Buffer2DRegion<ushort> reconstruction,
             Point firstPredictionOrigin,
             Span<int> sums)
-        {
-            int sum0 = 0;
-            int sum1 = 0;
-            int sum2 = 0;
-            int sum3 = 0;
-            if (Vector128.IsHardwareAccelerated)
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<ushort> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<ushort> predictionRow =
-                        reconstruction.DangerousGetRowSpan(firstPredictionOrigin.Y + row)[firstPredictionOrigin.X..];
-
-                    // Signed 16-bit lanes preserve every AV1 sample difference while four horizontally adjacent
-                    // candidates reuse the same source load and stay packed through horizontal reduction.
-                    Vector128<short> sourceSamples =
-                        Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(sourceRow)).AsInt16();
-
-                    Vector128<short> prediction0 =
-                        Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(predictionRow)).AsInt16();
-
-                    Vector128<short> prediction1 =
-                        Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(predictionRow[1..])).AsInt16();
-
-                    Vector128<short> prediction2 =
-                        Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(predictionRow[2..])).AsInt16();
-
-                    Vector128<short> prediction3 =
-                        Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(predictionRow[3..])).AsInt16();
-
-                    sum0 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction0));
-                    sum1 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction1));
-                    sum2 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction2));
-                    sum3 += Vector128.Sum(Vector128.Abs(sourceSamples - prediction3));
-                }
-            }
-            else
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<ushort> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<ushort> predictionRow =
-                        reconstruction.DangerousGetRowSpan(firstPredictionOrigin.Y + row)[firstPredictionOrigin.X..];
-
-                    for (int column = 0; column < 8; column++)
-                    {
-                        int sourceSample = sourceRow[column];
-                        sum0 += Math.Abs(sourceSample - predictionRow[column]);
-                        sum1 += Math.Abs(sourceSample - predictionRow[column + 1]);
-                        sum2 += Math.Abs(sourceSample - predictionRow[column + 2]);
-                        sum3 += Math.Abs(sourceSample - predictionRow[column + 3]);
-                    }
-                }
-            }
-
-            sums[0] = sum0;
-            sums[1] = sum1;
-            sums[2] = sum2;
-            sums[3] = sum3;
-        }
+            => Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, sourceOrigin),
+                source.Stride,
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, firstPredictionOrigin),
+                reconstruction.Stride,
+                sums);
 
         /// <inheritdoc/>
         public static int GetVariance(
@@ -1066,45 +1047,13 @@ internal static partial class Av1IntraSuperblockEncoder
             Point predictionOrigin,
             Av1BitDepth bitDepth)
         {
-            int sum = 0;
-            int sumOfSquares = 0;
-            if (Vector128.IsHardwareAccelerated)
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<ushort> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<ushort> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    // AV1's high-bit-depth domain tops out at 4095, so signed 16-bit subtraction preserves
-                    // every possible sample difference before the square is widened to 32-bit lanes.
-                    Vector128<short> difference =
-                        (Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(sourceRow)) -
-                         Vector128.LoadUnsafe(ref MemoryMarshal.GetReference(predictionRow)))
-                        .AsInt16();
-
-                    Vector128<int> lower = Vector128.WidenLower(difference);
-                    Vector128<int> upper = Vector128.WidenUpper(difference);
-                    sum += Vector128.Sum(difference);
-                    sumOfSquares += Vector128.Sum(lower * lower) + Vector128.Sum(upper * upper);
-                }
-            }
-            else
-            {
-                for (int row = 0; row < 8; row++)
-                {
-                    ReadOnlySpan<ushort> sourceRow = source.DangerousGetRowSpan(sourceOrigin.Y + row)[sourceOrigin.X..];
-                    ReadOnlySpan<ushort> predictionRow =
-                        reconstruction.DangerousGetRowSpan(predictionOrigin.Y + row)[predictionOrigin.X..];
-
-                    for (int column = 0; column < 8; column++)
-                    {
-                        int difference = sourceRow[column] - predictionRow[column];
-                        sum += difference;
-                        sumOfSquares += difference * difference;
-                    }
-                }
-            }
+            Av1ResidualBuilder.GetMoments8x8(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, sourceOrigin),
+                source.Stride,
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, predictionOrigin),
+                reconstruction.Stride,
+                out int sum,
+                out int sumOfSquares);
 
             return GetNormalizedVariance(sum, sumOfSquares, bitDepth);
         }
@@ -1353,7 +1302,7 @@ internal static partial class Av1IntraSuperblockEncoder
         }
 
         /// <inheritdoc/>
-        public static void PrepareIntraBlockCopy(
+        public static void PrepareIntraBlockCopyPrediction(
             Buffer2DRegion<ushort> source,
             Point blockOrigin,
             Buffer2DRegion<ushort> reconstruction,
@@ -1387,6 +1336,65 @@ internal static partial class Av1IntraSuperblockEncoder
                 height);
         }
 
+        /// <inheritdoc/>
+        public static void SubtractPrediction(
+            Buffer2DRegion<ushort> source,
+            Point blockOrigin,
+            ReadOnlySpan<ushort> prediction,
+            Span<short> residual,
+            Av1TransformSize transformSize)
+            => Av1ResidualBuilder.Subtract(
+                Av1TransformBlockEncoder.GetPlaneSpan(source, blockOrigin),
+                source.Stride,
+                prediction,
+                transformSize.GetWidth(),
+                residual,
+                transformSize.GetWidth(),
+                transformSize.GetWidth(),
+                transformSize.GetHeight());
+
+        /// <inheritdoc/>
+        public static void PrepareTranslationalInterPrediction(
+            Buffer2DRegion<ushort> source,
+            Point blockOrigin,
+            Buffer2DRegion<ushort> reference,
+            Point predictionOrigin,
+            Av1InterpolationFilter horizontalFilter,
+            Av1InterpolationFilter verticalFilter,
+            int horizontalPhase,
+            int verticalPhase,
+            Span<ushort> prediction,
+            Span<short> residual,
+            Span<short> predictionScratch,
+            Av1TransformSize transformSize,
+            Av1BitDepth bitDepth)
+        {
+            int width = transformSize.GetWidth();
+            int height = transformSize.GetHeight();
+            Rectangle referenceBounds = reference.Bounds;
+            int referenceOrigin =
+                ((referenceBounds.Y + predictionOrigin.Y) * reference.Stride) +
+                referenceBounds.X +
+                predictionOrigin.X;
+
+            Av1TranslationalInterPredictor.Predict(
+                reference.Buffer.DangerousGetSingleSpan(),
+                reference.Stride,
+                referenceOrigin,
+                prediction,
+                width,
+                width,
+                height,
+                horizontalFilter,
+                verticalFilter,
+                horizontalPhase,
+                verticalPhase,
+                bitDepth.GetBitCount(),
+                predictionScratch);
+
+            SubtractPrediction(source, blockOrigin, prediction, residual, transformSize);
+        }
+
         /// <inheritdoc/>
         public static long EncodePredictionCandidate(
             Av1EncoderBlockWorkspace workspace,

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1IntraSuperblockEncoder.PaletteModeDecision.cs

@@ -372,7 +372,8 @@ internal static partial class Av1IntraSuperblockEncoder
                 macroBlock,
                 BlockSize,
                 Av1PredictionMode.DC,
-                0);
+                0,
+                this.picture.Parent.FrameHeader.IsIntra);
 
             rate += writer.GetPaletteYModeCost(true, blockSizeContext, neighborContext);
             rate += writer.GetPaletteSizeCost(paletteSize, blockSizeContext, Av1PlaneType.Y);

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1IntraTileWriter.cs

@@ -1,170 +0,0 @@
-// Copyright (c) Six Labors.
-// Licensed under the Six Labors Split License.
-
-using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
-using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
-
-namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
-
-/// <summary>
-/// Encodes one range-coded all-intra tile payload.
-/// </summary>
-internal sealed partial class Av1IntraTileWriter : IAv1TileWriter
-{
-    private readonly ReadOnlyMemory<byte> tileData;
-    private readonly int tileDataLength;
-
-    /// <summary>
-    /// Initializes a new instance of the <see cref="Av1IntraTileWriter"/> class for eight-bit samples.
-    /// </summary>
-    /// <param name="writer">The operation-owned symbol encoder that retains the tile output memory.</param>
-    /// <param name="source">The coded source frame.</param>
-    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
-    /// <param name="picture">The frame coding and mode-information state.</param>
-    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
-    /// <param name="superblockWorkspace">The reusable partition and final-block decision workspace.</param>
-    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
-    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
-    public Av1IntraTileWriter(
-        Av1SymbolEncoder writer,
-        Av1EncoderFrame<byte> source,
-        Av1EncoderFrame<byte> reconstruction,
-        Av1PictureControlSet picture,
-        Av1EncoderCoefficientBuffer coefficientBuffer,
-        Av1EncoderSuperblockWorkspace superblockWorkspace,
-        Av1EncoderBlockWorkspace blockWorkspace,
-        int effort)
-    {
-        this.tileData = Encode<byte, Av1IntraSuperblockEncoder.ByteOperator>(
-            writer,
-            source,
-            reconstruction,
-            picture,
-            coefficientBuffer,
-            superblockWorkspace,
-            blockWorkspace,
-            effort,
-            out this.tileDataLength);
-    }
-
-    /// <summary>
-    /// Initializes a new instance of the <see cref="Av1IntraTileWriter"/> class for high-bit-depth samples.
-    /// </summary>
-    /// <param name="writer">The operation-owned symbol encoder that retains the tile output memory.</param>
-    /// <param name="source">The coded source frame.</param>
-    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
-    /// <param name="picture">The frame coding and mode-information state.</param>
-    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
-    /// <param name="superblockWorkspace">The reusable partition and final-block decision workspace.</param>
-    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
-    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
-    public Av1IntraTileWriter(
-        Av1SymbolEncoder writer,
-        Av1EncoderFrame<ushort> source,
-        Av1EncoderFrame<ushort> reconstruction,
-        Av1PictureControlSet picture,
-        Av1EncoderCoefficientBuffer coefficientBuffer,
-        Av1EncoderSuperblockWorkspace superblockWorkspace,
-        Av1EncoderBlockWorkspace blockWorkspace,
-        int effort)
-    {
-        this.tileData = Encode<ushort, Av1IntraSuperblockEncoder.UInt16Operator>(
-            writer,
-            source,
-            reconstruction,
-            picture,
-            coefficientBuffer,
-            superblockWorkspace,
-            blockWorkspace,
-            effort,
-            out this.tileDataLength);
-    }
-
-    /// <inheritdoc/>
-    public ReadOnlySpan<byte> GetTileData(int tileNum) => this.tileData.Span[..this.tileDataLength];
-
-    private static ReadOnlyMemory<byte> Encode<TSample, TOperator>(
-        Av1SymbolEncoder writer,
-        Av1EncoderFrame<TSample> source,
-        Av1EncoderFrame<TSample> reconstruction,
-        Av1PictureControlSet picture,
-        Av1EncoderCoefficientBuffer coefficientBuffer,
-        Av1EncoderSuperblockWorkspace superblockWorkspace,
-        Av1EncoderBlockWorkspace blockWorkspace,
-        int effort,
-        out int tileDataLength)
-        where TSample : unmanaged
-        where TOperator : struct, Av1IntraSuperblockEncoder.IBlockEncodingOperator<TSample>
-    {
-        ObuFrameHeader frameHeader = picture.Parent.FrameHeader;
-        ObuSequenceHeader sequenceHeader = picture.Sequence.SequenceHeader;
-        const ushort TileIndex = 0;
-        Av1TileInfo tile = new(0, 0, frameHeader);
-        Av1Superblock superblock = new()
-        {
-            Workspace = superblockWorkspace,
-            TileInfo = tile
-        };
-
-        Point firstModeInfoPosition = new(tile.ModeInfoColumnStart, tile.ModeInfoRowStart);
-        Av1TileWriter.Av1EntropyCodingContext entropyContext = new()
-        {
-            MacroBlock = new Av1MacroBlockD { Tile = tile },
-            MacroBlockModeInfo = picture.GetMacroBlockModeInfo(firstModeInfoPosition)
-        };
-
-        int superblockModeInfoSize = sequenceHeader.SuperblockModeInfoSize;
-        int superblockShift = sequenceHeader.SuperblockSizeLog2 - Av1Constants.ModeInfoSizeLog2;
-        if (frameHeader.AllowIntraBlockCopy)
-        {
-            // Hash the visible source once before reconstruction begins so candidate discovery never depends
-            // on coding order and the workspace can be reused as compact bucket links afterward.
-            picture.IntraBlockCopySearch.Initialize<TSample, TOperator>(
-                source.View.GetPlane(Av1Plane.Y));
-        }
-
-        for (int modeInfoRow = tile.ModeInfoRowStart;
-            modeInfoRow < tile.ModeInfoRowEnd;
-            modeInfoRow += superblockModeInfoSize)
-        {
-            for (int modeInfoColumn = tile.ModeInfoColumnStart;
-                modeInfoColumn < tile.ModeInfoColumnEnd;
-                modeInfoColumn += superblockModeInfoSize)
-            {
-                int superblockRow = modeInfoRow >> superblockShift;
-                int superblockColumn = modeInfoColumn >> superblockShift;
-                superblock.Index = (superblockRow * coefficientBuffer.SuperblockColumnCount) + superblockColumn;
-                entropyContext.SuperblockOrigin = new Point(
-                    modeInfoColumn << Av1Constants.ModeInfoSizeLog2,
-                    modeInfoRow << Av1Constants.ModeInfoSizeLog2);
-
-                Av1IntraSuperblockEncoder.Prepare(
-                    picture,
-                    superblock,
-                    entropyContext.SuperblockOrigin);
-
-                Av1IntraSuperblockEncoder.ModeDecision<TSample, TOperator> blockEncoder = new(
-                    source,
-                    reconstruction,
-                    picture,
-                    superblock,
-                    coefficientBuffer,
-                    blockWorkspace,
-                    effort);
-
-                Av1TileWriter.WriteSuperblock(
-                    picture,
-                    entropyContext,
-                    writer,
-                    superblock,
-                    coefficientBuffer,
-                    TileIndex,
-                    ref blockEncoder);
-            }
-        }
-
-        return writer.Exit(out tileDataLength);
-    }
-}

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1ResidualBuilder.Operator.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using System.Runtime.CompilerServices;
 using System.Runtime.Intrinsics;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
@@ -51,6 +52,57 @@ internal static partial class Av1ResidualBuilder
         /// <param name="prediction">The prediction sample.</param>
         /// <returns>The signed residual.</returns>
         public static abstract short Subtract(TSample source, TSample prediction);
+
+        /// <summary>
+        /// Measures one scalar absolute sample difference.
+        /// </summary>
+        /// <param name="source">The source sample.</param>
+        /// <param name="prediction">The prediction sample.</param>
+        /// <returns>The sum of absolute differences.</returns>
+        public static abstract int SumAbsoluteDifferences(TSample source, TSample prediction);
+
+        /// <summary>
+        /// Measures eight absolute sample differences; byte inputs occupy only the lower eight lanes.
+        /// </summary>
+        /// <param name="source">The eight source samples.</param>
+        /// <param name="prediction">The eight prediction samples.</param>
+        /// <returns>The sum of absolute differences.</returns>
+        public static abstract int SumAbsoluteDifferences(Vector128<TSample> source, Vector128<TSample> prediction);
+
+        /// <summary>
+        /// Measures four eight-sample predictions, returning their costs in candidate order.
+        /// Byte inputs occupy only the lower eight lanes.
+        /// </summary>
+        /// <param name="source">The eight source samples.</param>
+        /// <param name="prediction0">The eight samples for candidate 0.</param>
+        /// <param name="prediction1">The eight samples for candidate 1.</param>
+        /// <param name="prediction2">The eight samples for candidate 2.</param>
+        /// <param name="prediction3">The eight samples for candidate 3.</param>
+        /// <returns>Four absolute-difference sums in increasing candidate order.</returns>
+        public static abstract Vector128<int> SumFourAbsoluteDifferences(
+            Vector128<TSample> source,
+            Vector128<TSample> prediction0,
+            Vector128<TSample> prediction1,
+            Vector128<TSample> prediction2,
+            Vector128<TSample> prediction3);
+
+        /// <summary>
+        /// Calculates one squared difference and returns its signed difference for the first moment.
+        /// </summary>
+        /// <param name="source">The source sample.</param>
+        /// <param name="prediction">The prediction sample.</param>
+        /// <param name="sum">The signed sum of source-minus-prediction differences.</param>
+        /// <returns>The sum of squared differences.</returns>
+        public static abstract int SumSquaredDifferences(TSample source, TSample prediction, out int sum);
+
+        /// <summary>
+        /// Calculates eight squared differences and their signed sum; byte inputs occupy only the lower eight lanes.
+        /// </summary>
+        /// <param name="source">The eight source samples.</param>
+        /// <param name="prediction">The eight prediction samples.</param>
+        /// <param name="sum">The signed sum of source-minus-prediction differences.</param>
+        /// <returns>The sum of squared differences.</returns>
+        public static abstract int SumSquaredDifferences(Vector128<TSample> source, Vector128<TSample> prediction, out int sum);
     }
 
     /// <summary>
@@ -58,6 +110,59 @@ internal static partial class Av1ResidualBuilder
     /// </summary>
     internal readonly struct ByteOperator : IResidualOperator<byte>
     {
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumAbsoluteDifferences(byte source, byte prediction) => Math.Abs(Subtract(source, prediction));
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumAbsoluteDifferences(Vector128<byte> source, Vector128<byte> prediction)
+        {
+            // Widen byte samples before subtraction; twelve-bit word samples already fit signed-short lanes.
+            // Eight absolute residuals sum to at most 32760, so the signed-short horizontal sum remains exact.
+            Vector128<short> difference = Vector128.WidenLower(source).AsInt16() - Vector128.WidenLower(prediction).AsInt16();
+            return Vector128.Sum(Vector128.Abs(difference));
+        }
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Vector128<int> SumFourAbsoluteDifferences(
+            Vector128<byte> source,
+            Vector128<byte> prediction0,
+            Vector128<byte> prediction1,
+            Vector128<byte> prediction2,
+            Vector128<byte> prediction3)
+        {
+            // Reuse the source conversion across all four candidates. Each reduction contributes one independent
+            // 32-bit lane, allowing the traversal to accumulate all eight rows without extracting candidate costs.
+            Vector128<short> sourceSamples = Vector128.WidenLower(source).AsInt16();
+            return Vector128.Create(
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - Vector128.WidenLower(prediction0).AsInt16())),
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - Vector128.WidenLower(prediction1).AsInt16())),
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - Vector128.WidenLower(prediction2).AsInt16())),
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - Vector128.WidenLower(prediction3).AsInt16())));
+        }
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumSquaredDifferences(byte source, byte prediction, out int sum)
+        {
+            sum = Subtract(source, prediction);
+            return sum * sum;
+        }
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumSquaredDifferences(Vector128<byte> source, Vector128<byte> prediction, out int sum)
+        {
+            Vector128<short> difference = Vector128.WidenLower(source).AsInt16() - Vector128.WidenLower(prediction).AsInt16();
+            sum = Vector128.Sum(difference);
+
+            // The shared square reduction widens to int before multiplying. All eight twelve-bit squares
+            // fit in the returned int; no short multiplication or premature bit-depth rounding is permitted.
+            return (int)SumSquares(difference);
+        }
+
         /// <inheritdoc/>
         public static Vector128<short> Subtract(Vector128<byte> source, Vector128<byte> prediction, out Vector128<short> upper)
         {
@@ -91,6 +196,59 @@ internal static partial class Av1ResidualBuilder
     /// </summary>
     internal readonly struct UInt16Operator : IResidualOperator<ushort>
     {
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumAbsoluteDifferences(ushort source, ushort prediction) => Math.Abs(Subtract(source, prediction));
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumAbsoluteDifferences(Vector128<ushort> source, Vector128<ushort> prediction)
+        {
+            // Widen byte samples before subtraction; twelve-bit word samples already fit signed-short lanes.
+            // Eight absolute residuals sum to at most 32760, so the signed-short horizontal sum remains exact.
+            Vector128<short> difference = source.AsInt16() - prediction.AsInt16();
+            return Vector128.Sum(Vector128.Abs(difference));
+        }
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Vector128<int> SumFourAbsoluteDifferences(
+            Vector128<ushort> source,
+            Vector128<ushort> prediction0,
+            Vector128<ushort> prediction1,
+            Vector128<ushort> prediction2,
+            Vector128<ushort> prediction3)
+        {
+            // Reuse the source conversion across all four candidates. Each reduction contributes one independent
+            // 32-bit lane, allowing the traversal to accumulate all eight rows without extracting candidate costs.
+            Vector128<short> sourceSamples = source.AsInt16();
+            return Vector128.Create(
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - prediction0.AsInt16())),
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - prediction1.AsInt16())),
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - prediction2.AsInt16())),
+                (int)Vector128.Sum(Vector128.Abs(sourceSamples - prediction3.AsInt16())));
+        }
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumSquaredDifferences(ushort source, ushort prediction, out int sum)
+        {
+            sum = Subtract(source, prediction);
+            return sum * sum;
+        }
+
+        /// <inheritdoc/>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int SumSquaredDifferences(Vector128<ushort> source, Vector128<ushort> prediction, out int sum)
+        {
+            Vector128<short> difference = source.AsInt16() - prediction.AsInt16();
+            sum = Vector128.Sum(difference);
+
+            // The shared square reduction widens to int before multiplying. All eight twelve-bit squares
+            // fit in the returned int; no short multiplication or premature bit-depth rounding is permitted.
+            return (int)SumSquares(difference);
+        }
+
         /// <inheritdoc/>
         public static Vector128<short> Subtract(Vector128<ushort> source, Vector128<ushort> prediction, out Vector128<short> upper)
         {

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1ResidualBuilder.cs

@@ -12,6 +12,226 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 /// </summary>
 internal static partial class Av1ResidualBuilder
 {
+    /// <summary>
+    /// The width and height of the encoder's fixed motion-search block, in samples.
+    /// </summary>
+    private const int SearchBlockDimension = 8;
+
+    /// <summary>
+    /// Calculates the sum of absolute differences for an 8x8 block.
+    /// </summary>
+    /// <param name="source">The source samples starting at the block origin.</param>
+    /// <param name="sourceStride">The source row stride, in samples.</param>
+    /// <param name="prediction">The prediction samples starting at the block origin.</param>
+    /// <param name="predictionStride">The prediction row stride, in samples.</param>
+    /// <returns>The sum of absolute sample differences.</returns>
+    public static int SumAbsoluteDifferences8x8(ReadOnlySpan<byte> source, int sourceStride, ReadOnlySpan<byte> prediction, int predictionStride)
+        => SumAbsoluteDifferences8x8<byte, ByteOperator>(source, sourceStride, prediction, predictionStride);
+
+    /// <summary>
+    /// Measures four horizontally adjacent 8x8 predictions against one source block.
+    /// </summary>
+    /// <param name="source">The source samples starting at the block origin.</param>
+    /// <param name="sourceStride">The source row stride, in samples.</param>
+    /// <param name="prediction">The first prediction, with three additional samples available at the right of each row.</param>
+    /// <param name="predictionStride">The prediction row stride, in samples.</param>
+    /// <param name="sums">The four results in increasing horizontal-offset order.</param>
+    public static void SumFourAbsoluteDifferences8x8(
+        ReadOnlySpan<byte> source, int sourceStride, ReadOnlySpan<byte> prediction, int predictionStride, Span<int> sums)
+        => SumFourAbsoluteDifferences8x8<byte, ByteOperator>(source, sourceStride, prediction, predictionStride, sums);
+
+    /// <summary>
+    /// Calculates the signed sum and squared sum of differences for an 8x8 block.
+    /// </summary>
+    /// <param name="source">The source samples starting at the block origin.</param>
+    /// <param name="sourceStride">The source row stride, in samples.</param>
+    /// <param name="prediction">The prediction samples starting at the block origin.</param>
+    /// <param name="predictionStride">The prediction row stride, in samples.</param>
+    /// <param name="sum">The signed sum of sample differences.</param>
+    /// <param name="sumOfSquares">The sum of squared sample differences.</param>
+    public static void GetMoments8x8(
+        ReadOnlySpan<byte> source, int sourceStride, ReadOnlySpan<byte> prediction, int predictionStride, out int sum, out int sumOfSquares)
+        => GetMoments8x8<byte, ByteOperator>(source, sourceStride, prediction, predictionStride, out sum, out sumOfSquares);
+
+    /// <summary>
+    /// Calculates the sum of absolute differences for an 8x8 block.
+    /// </summary>
+    /// <param name="source">The source samples starting at the block origin.</param>
+    /// <param name="sourceStride">The source row stride, in samples.</param>
+    /// <param name="prediction">The prediction samples starting at the block origin.</param>
+    /// <param name="predictionStride">The prediction row stride, in samples.</param>
+    /// <returns>The sum of absolute sample differences.</returns>
+    public static int SumAbsoluteDifferences8x8(ReadOnlySpan<ushort> source, int sourceStride, ReadOnlySpan<ushort> prediction, int predictionStride)
+        => SumAbsoluteDifferences8x8<ushort, UInt16Operator>(source, sourceStride, prediction, predictionStride);
+
+    /// <summary>
+    /// Measures four horizontally adjacent 8x8 predictions against one source block.
+    /// </summary>
+    /// <param name="source">The source samples starting at the block origin.</param>
+    /// <param name="sourceStride">The source row stride, in samples.</param>
+    /// <param name="prediction">The first prediction, with three additional samples available at the right of each row.</param>
+    /// <param name="predictionStride">The prediction row stride, in samples.</param>
+    /// <param name="sums">The four results in increasing horizontal-offset order.</param>
+    public static void SumFourAbsoluteDifferences8x8(
+        ReadOnlySpan<ushort> source, int sourceStride, ReadOnlySpan<ushort> prediction, int predictionStride, Span<int> sums)
+        => SumFourAbsoluteDifferences8x8<ushort, UInt16Operator>(source, sourceStride, prediction, predictionStride, sums);
+
+    /// <summary>
+    /// Calculates the signed sum and squared sum of differences for an 8x8 block.
+    /// </summary>
+    /// <param name="source">The source samples starting at the block origin.</param>
+    /// <param name="sourceStride">The source row stride, in samples.</param>
+    /// <param name="prediction">The prediction samples starting at the block origin.</param>
+    /// <param name="predictionStride">The prediction row stride, in samples.</param>
+    /// <param name="sum">The signed sum of sample differences.</param>
+    /// <param name="sumOfSquares">The sum of squared sample differences.</param>
+    public static void GetMoments8x8(
+        ReadOnlySpan<ushort> source, int sourceStride, ReadOnlySpan<ushort> prediction, int predictionStride, out int sum, out int sumOfSquares)
+        => GetMoments8x8<ushort, UInt16Operator>(source, sourceStride, prediction, predictionStride, out sum, out sumOfSquares);
+
+    /// <summary>
+    /// Traverses an 8x8 block while its closed operator calculates scalar or eight-sample row costs.
+    /// </summary>
+    private static int SumAbsoluteDifferences8x8<TSample, TOperator>(
+        ReadOnlySpan<TSample> source, int sourceStride, ReadOnlySpan<TSample> prediction, int predictionStride)
+        where TSample : unmanaged
+        where TOperator : struct, IResidualOperator<TSample>
+    {
+        int sum = 0;
+        if (Vector128.IsHardwareAccelerated)
+        {
+            // Eight widened AV1 samples exactly fill 128 bits. Wider loads would cross the row boundary;
+            // byte storage is loaded as eight bytes and ushort storage as eight native-order words.
+            for (int row = 0; row < SearchBlockDimension; row++)
+            {
+                Vector128<TSample> sourceRow = LoadSearchRow(source[(row * sourceStride)..]);
+                Vector128<TSample> predictionRow = LoadSearchRow(prediction[(row * predictionStride)..]);
+                sum += TOperator.SumAbsoluteDifferences(sourceRow, predictionRow);
+            }
+        }
+        else
+        {
+            for (int row = 0; row < SearchBlockDimension; row++)
+            {
+                ReadOnlySpan<TSample> sourceRow = source.Slice(row * sourceStride, SearchBlockDimension);
+                ReadOnlySpan<TSample> predictionRow = prediction.Slice(row * predictionStride, SearchBlockDimension);
+                for (int column = 0; column < SearchBlockDimension; column++)
+                {
+                    sum += TOperator.SumAbsoluteDifferences(sourceRow[column], predictionRow[column]);
+                }
+            }
+        }
+
+        return sum;
+    }
+
+    /// <summary>
+    /// Traverses four adjacent candidates together, retaining one source load per row or scalar sample.
+    /// </summary>
+    private static void SumFourAbsoluteDifferences8x8<TSample, TOperator>(
+        ReadOnlySpan<TSample> source, int sourceStride, ReadOnlySpan<TSample> prediction, int predictionStride, Span<int> sums)
+        where TSample : unmanaged
+        where TOperator : struct, IResidualOperator<TSample>
+    {
+        if (Vector128.IsHardwareAccelerated)
+        {
+            Vector128<int> totals = Vector128<int>.Zero;
+            for (int row = 0; row < SearchBlockDimension; row++)
+            {
+                ReadOnlySpan<TSample> predictionRow = prediction[(row * predictionStride)..];
+                Vector128<TSample> sourceRow = LoadSearchRow(source[(row * sourceStride)..]);
+
+                // The four prediction windows overlap, but each candidate owns one result lane. The operator
+                // widens the source only once and reuses it for all four independent absolute-difference sums.
+                totals += TOperator.SumFourAbsoluteDifferences(
+                    sourceRow,
+                    LoadSearchRow(predictionRow),
+                    LoadSearchRow(predictionRow[1..]),
+                    LoadSearchRow(predictionRow[2..]),
+                    LoadSearchRow(predictionRow[3..]));
+            }
+
+            totals.CopyTo(sums);
+        }
+        else
+        {
+            int sum0 = 0;
+            int sum1 = 0;
+            int sum2 = 0;
+            int sum3 = 0;
+            for (int row = 0; row < SearchBlockDimension; row++)
+            {
+                ReadOnlySpan<TSample> sourceRow = source.Slice(row * sourceStride, SearchBlockDimension);
+                ReadOnlySpan<TSample> predictionRow = prediction.Slice(row * predictionStride, SearchBlockDimension + 3);
+                for (int column = 0; column < SearchBlockDimension; column++)
+                {
+                    TSample sample = sourceRow[column];
+                    sum0 += TOperator.SumAbsoluteDifferences(sample, predictionRow[column]);
+                    sum1 += TOperator.SumAbsoluteDifferences(sample, predictionRow[column + 1]);
+                    sum2 += TOperator.SumAbsoluteDifferences(sample, predictionRow[column + 2]);
+                    sum3 += TOperator.SumAbsoluteDifferences(sample, predictionRow[column + 3]);
+                }
+            }
+
+            sums[0] = sum0;
+            sums[1] = sum1;
+            sums[2] = sum2;
+            sums[3] = sum3;
+        }
+    }
+
+    /// <summary>
+    /// Accumulates both residual moments in one traversal without materializing a residual buffer.
+    /// </summary>
+    private static void GetMoments8x8<TSample, TOperator>(
+        ReadOnlySpan<TSample> source, int sourceStride, ReadOnlySpan<TSample> prediction, int predictionStride, out int sum, out int sumOfSquares)
+        where TSample : unmanaged
+        where TOperator : struct, IResidualOperator<TSample>
+    {
+        sum = 0;
+        sumOfSquares = 0;
+
+        // Even 64 maximum twelve-bit residual squares fit in a signed int. Preserve the unnormalized
+        // moments here; the caller applies the frame's precision-dependent rounding before deriving variance.
+        if (Vector128.IsHardwareAccelerated)
+        {
+            for (int row = 0; row < SearchBlockDimension; row++)
+            {
+                Vector128<TSample> sourceRow = LoadSearchRow(source[(row * sourceStride)..]);
+                Vector128<TSample> predictionRow = LoadSearchRow(prediction[(row * predictionStride)..]);
+                sumOfSquares += TOperator.SumSquaredDifferences(sourceRow, predictionRow, out int rowSum);
+                sum += rowSum;
+            }
+        }
+        else
+        {
+            for (int row = 0; row < SearchBlockDimension; row++)
+            {
+                ReadOnlySpan<TSample> sourceRow = source.Slice(row * sourceStride, SearchBlockDimension);
+                ReadOnlySpan<TSample> predictionRow = prediction.Slice(row * predictionStride, SearchBlockDimension);
+                for (int column = 0; column < SearchBlockDimension; column++)
+                {
+                    sumOfSquares += TOperator.SumSquaredDifferences(sourceRow[column], predictionRow[column], out int difference);
+                    sum += difference;
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Loads exactly eight native-order samples; byte rows occupy the lower half of the returned vector.
+    /// </summary>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static Vector128<TSample> LoadSearchRow<TSample>(ReadOnlySpan<TSample> source)
+        where TSample : unmanaged
+    {
+        // The closed byte/ushort instantiation removes this storage-width choice. The eight-byte load
+        // never consumes padding or a following row; the operator widens only its eight populated lanes.
+        return Vector128<TSample>.Count == SearchBlockDimension
+            ? Vector128.Create(source)
+            : Vector128.Create(Vector64.Create(source), Vector64<TSample>.Zero);
+    }
+
     /// <summary>
     /// Subtracts an 8-bit prediction plane from its source plane.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1TileEncoder.cs

@@ -0,0 +1,383 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
+using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
+
+namespace SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
+
+/// <summary>
+/// Encodes one range-coded AV1 tile payload.
+/// </summary>
+internal readonly struct Av1TileEncoder : IAv1TileWriter
+{
+    private readonly ReadOnlyMemory<byte> tileData;
+    private readonly Av1PictureControlSet picture;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TileEncoder"/> struct for eight-bit samples.
+    /// </summary>
+    /// <param name="writer">The symbol encoder that retains tile output through the enclosing frame write.</param>
+    /// <param name="source">The coded source frame.</param>
+    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
+    /// <param name="picture">The frame coding and mode-information state.</param>
+    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
+    /// <param name="superblockWorkspace">The reusable partition and final-block decision workspace.</param>
+    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
+    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
+    public Av1TileEncoder(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<byte> source,
+        Av1EncoderFrame<byte> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderSuperblockWorkspace superblockWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+    {
+        this.picture = picture;
+        this.tileData = Encode<byte, Av1IntraSuperblockEncoder.ByteOperator>(
+            writer,
+            source,
+            reconstruction,
+            reconstruction,
+            picture,
+            coefficientBuffer,
+            new Av1EncoderTileWorkspace(picture.Parent.FrameHeader, superblockWorkspace),
+            blockWorkspace,
+            effort);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TileEncoder"/> struct for an eight-bit inter frame.
+    /// </summary>
+    /// <param name="writer">The symbol encoder that retains tile output through the enclosing frame write.</param>
+    /// <param name="source">The coded source frame.</param>
+    /// <param name="reference">The reconstructed reference frame.</param>
+    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
+    /// <param name="picture">The frame coding and mode-information state.</param>
+    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
+    /// <param name="superblockWorkspace">The reusable partition and final-block decision workspace.</param>
+    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
+    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
+    public Av1TileEncoder(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<byte> source,
+        Av1EncoderFrame<byte> reference,
+        Av1EncoderFrame<byte> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderSuperblockWorkspace superblockWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+    {
+        this.picture = picture;
+        this.tileData = Encode<byte, Av1IntraSuperblockEncoder.ByteOperator>(
+            writer,
+            source,
+            reference,
+            reconstruction,
+            picture,
+            coefficientBuffer,
+            new Av1EncoderTileWorkspace(picture.Parent.FrameHeader, superblockWorkspace),
+            blockWorkspace,
+            effort);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TileEncoder"/> struct for an eight-bit inter frame.
+    /// </summary>
+    /// <param name="writer">The symbol encoder that retains tile output through the enclosing frame write.</param>
+    /// <param name="source">The coded source frame.</param>
+    /// <param name="reference">The reconstructed reference frame.</param>
+    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
+    /// <param name="picture">The frame coding and mode-information state.</param>
+    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
+    /// <param name="tileWorkspace">The retained tile, superblock, and entropy cursor graph.</param>
+    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
+    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
+    public Av1TileEncoder(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<byte> source,
+        Av1EncoderFrame<byte> reference,
+        Av1EncoderFrame<byte> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderTileWorkspace tileWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+    {
+        this.picture = picture;
+        this.tileData = Encode<byte, Av1IntraSuperblockEncoder.ByteOperator>(
+            writer,
+            source,
+            reference,
+            reconstruction,
+            picture,
+            coefficientBuffer,
+            tileWorkspace,
+            blockWorkspace,
+            effort);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TileEncoder"/> struct for high-bit-depth samples.
+    /// </summary>
+    /// <param name="writer">The symbol encoder that retains tile output through the enclosing frame write.</param>
+    /// <param name="source">The coded source frame.</param>
+    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
+    /// <param name="picture">The frame coding and mode-information state.</param>
+    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
+    /// <param name="superblockWorkspace">The reusable partition and final-block decision workspace.</param>
+    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
+    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
+    public Av1TileEncoder(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<ushort> source,
+        Av1EncoderFrame<ushort> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderSuperblockWorkspace superblockWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+    {
+        this.picture = picture;
+        this.tileData = Encode<ushort, Av1IntraSuperblockEncoder.UInt16Operator>(
+            writer,
+            source,
+            reconstruction,
+            reconstruction,
+            picture,
+            coefficientBuffer,
+            new Av1EncoderTileWorkspace(picture.Parent.FrameHeader, superblockWorkspace),
+            blockWorkspace,
+            effort);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TileEncoder"/> struct for a high-bit-depth inter frame.
+    /// </summary>
+    /// <param name="writer">The symbol encoder that retains tile output through the enclosing frame write.</param>
+    /// <param name="source">The coded source frame.</param>
+    /// <param name="reference">The reconstructed reference frame.</param>
+    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
+    /// <param name="picture">The frame coding and mode-information state.</param>
+    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
+    /// <param name="superblockWorkspace">The reusable partition and final-block decision workspace.</param>
+    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
+    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
+    public Av1TileEncoder(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<ushort> source,
+        Av1EncoderFrame<ushort> reference,
+        Av1EncoderFrame<ushort> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderSuperblockWorkspace superblockWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+    {
+        this.picture = picture;
+        this.tileData = Encode<ushort, Av1IntraSuperblockEncoder.UInt16Operator>(
+            writer,
+            source,
+            reference,
+            reconstruction,
+            picture,
+            coefficientBuffer,
+            new Av1EncoderTileWorkspace(picture.Parent.FrameHeader, superblockWorkspace),
+            blockWorkspace,
+            effort);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TileEncoder"/> struct for a high-bit-depth inter frame.
+    /// </summary>
+    /// <param name="writer">The symbol encoder that retains tile output through the enclosing frame write.</param>
+    /// <param name="source">The coded source frame.</param>
+    /// <param name="reference">The reconstructed reference frame.</param>
+    /// <param name="reconstruction">The reconstructed frame updated during encoding.</param>
+    /// <param name="picture">The frame coding and mode-information state.</param>
+    /// <param name="coefficientBuffer">The frame-owned quantized coefficient and transform state.</param>
+    /// <param name="tileWorkspace">The retained tile, superblock, and entropy cursor graph.</param>
+    /// <param name="blockWorkspace">The reusable block arithmetic workspace.</param>
+    /// <param name="effort">The mode-search effort in the inclusive range zero through ten.</param>
+    public Av1TileEncoder(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<ushort> source,
+        Av1EncoderFrame<ushort> reference,
+        Av1EncoderFrame<ushort> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderTileWorkspace tileWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+    {
+        this.picture = picture;
+        this.tileData = Encode<ushort, Av1IntraSuperblockEncoder.UInt16Operator>(
+            writer,
+            source,
+            reference,
+            reconstruction,
+            picture,
+            coefficientBuffer,
+            tileWorkspace,
+            blockWorkspace,
+            effort);
+    }
+
+    /// <inheritdoc/>
+    public ReadOnlySpan<byte> GetTileData(int tileNum)
+    {
+        int offset = this.picture.TileDataOffsets.Span[tileNum];
+        int length = this.picture.TileDataLengths.Span[tileNum];
+        return this.tileData.Span.Slice(offset, length);
+    }
+
+    private static ReadOnlyMemory<byte> Encode<TSample, TOperator>(
+        Av1SymbolEncoder writer,
+        Av1EncoderFrame<TSample> source,
+        Av1EncoderFrame<TSample> reference,
+        Av1EncoderFrame<TSample> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficientBuffer,
+        Av1EncoderTileWorkspace tileWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace,
+        int effort)
+        where TSample : unmanaged
+        where TOperator : struct, Av1IntraSuperblockEncoder.IBlockEncodingOperator<TSample>
+    {
+        ObuFrameHeader frameHeader = picture.Parent.FrameHeader;
+        ObuSequenceHeader sequenceHeader = picture.Sequence.SequenceHeader;
+        Av1TileInfo tile = tileWorkspace.Tile;
+        Av1Superblock superblock = tileWorkspace.Superblock;
+        Av1TileWriter.Av1EntropyCodingContext entropyContext = tileWorkspace.EntropyContext;
+
+        int superblockModeInfoSize = sequenceHeader.SuperblockModeInfoSize;
+        int superblockShift = sequenceHeader.SuperblockSizeLog2 - Av1Constants.ModeInfoSizeLog2;
+        ObuTileGroupHeader tileLayout = frameHeader.TilesInfo;
+        Span<int> tileDataOffsets = picture.TileDataOffsets.Span;
+        Span<int> tileDataLengths = picture.TileDataLengths.Span;
+        if (frameHeader.AllowIntraBlockCopy)
+        {
+            // Hash the visible source once before reconstruction begins so candidate discovery never depends
+            // on coding order and the workspace can be reused as compact bucket links afterward.
+            picture.IntraBlockCopySearch.Initialize<TSample, TOperator>(
+                source.View.GetPlane(Av1Plane.Y));
+        }
+
+        int tileIndex = 0;
+        int tileDataEnd = 0;
+        for (int tileRow = 0; tileRow < tileLayout.TileRowCount; tileRow++)
+        {
+            tile.SetTileRow(tileLayout, frameHeader.ModeInfoRowCount, tileRow);
+            for (int tileColumn = 0; tileColumn < tileLayout.TileColumnCount; tileColumn++)
+            {
+                tile.SetTileColumn(tileLayout, frameHeader.ModeInfoColumnCount, tileColumn);
+                if (tileIndex > 0)
+                {
+                    // Every tile begins from the same frame probabilities, while its bytes follow the preceding
+                    // tile in the retained output allocation.
+                    writer.Reset(tileDataEnd);
+                }
+
+                Point firstModeInfoPosition = new(tile.ModeInfoColumnStart, tile.ModeInfoRowStart);
+                entropyContext.MacroBlockModeInfo = picture.GetMacroBlockModeInfo(firstModeInfoPosition);
+                for (int modeInfoRow = tile.ModeInfoRowStart;
+                    modeInfoRow < tile.ModeInfoRowEnd;
+                    modeInfoRow += superblockModeInfoSize)
+                {
+                    for (int modeInfoColumn = tile.ModeInfoColumnStart;
+                        modeInfoColumn < tile.ModeInfoColumnEnd;
+                        modeInfoColumn += superblockModeInfoSize)
+                    {
+                        int superblockRow = modeInfoRow >> superblockShift;
+                        int superblockColumn = modeInfoColumn >> superblockShift;
+                        superblock.Index = (superblockRow * coefficientBuffer.SuperblockColumnCount) + superblockColumn;
+                        entropyContext.SuperblockOrigin = new Point(
+                            modeInfoColumn << Av1Constants.ModeInfoSizeLog2,
+                            modeInfoRow << Av1Constants.ModeInfoSizeLog2);
+
+                        Av1IntraSuperblockEncoder.Prepare(
+                            picture,
+                            superblock,
+                            entropyContext.SuperblockOrigin);
+
+                        Av1IntraSuperblockEncoder.ModeDecision<TSample, TOperator> blockEncoder = new(
+                            source,
+                            reference,
+                            reconstruction,
+                            picture,
+                            superblock,
+                            coefficientBuffer,
+                            blockWorkspace,
+                            effort);
+
+                        Av1TileWriter.WriteSuperblock(
+                            picture,
+                            entropyContext,
+                            writer,
+                            superblock,
+                            coefficientBuffer,
+                            (ushort)tileIndex,
+                            ref blockEncoder);
+                    }
+                }
+
+                _ = writer.Exit(out int tileDataLength);
+                tileDataOffsets[tileIndex] = tileDataEnd;
+                tileDataLengths[tileIndex] = tileDataLength;
+                tileDataEnd += tileDataLength;
+                tileIndex++;
+            }
+        }
+
+        return writer.GetOutput(tileDataEnd);
+    }
+}
+
+/// <summary>
+/// Retains the mutable tile, superblock, and entropy cursor graph reused by serial frame encoding.
+/// </summary>
+internal readonly struct Av1EncoderTileWorkspace
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1EncoderTileWorkspace"/> struct.
+    /// </summary>
+    /// <param name="frameHeader">The fixed-geometry frame header defining tile boundaries.</param>
+    /// <param name="superblockWorkspace">The retained superblock decision storage.</param>
+    public Av1EncoderTileWorkspace(
+        ObuFrameHeader frameHeader,
+        Av1EncoderSuperblockWorkspace superblockWorkspace)
+    {
+        this.Tile = new Av1TileInfo(0, 0, frameHeader);
+        this.Superblock = new Av1Superblock
+        {
+            Workspace = superblockWorkspace,
+            TileInfo = this.Tile
+        };
+
+        this.EntropyContext = new Av1TileWriter.Av1EntropyCodingContext
+        {
+            MacroBlock = new Av1MacroBlockD { Tile = this.Tile },
+            MacroBlockModeInfo = default
+        };
+    }
+
+    /// <summary>
+    /// Gets the mutable tile boundaries selected during raster traversal.
+    /// </summary>
+    public Av1TileInfo Tile { get; }
+
+    /// <summary>
+    /// Gets the mutable superblock cursor connected to the retained decision workspace.
+    /// </summary>
+    public Av1Superblock Superblock { get; }
+
+    /// <summary>
+    /// Gets the mutable entropy cursor shared by successive superblocks.
+    /// </summary>
+    public Av1TileWriter.Av1EntropyCodingContext EntropyContext { get; }
+}

src/ImageSharp/Formats/Heif/Av1/Prediction/Inter/Av1TranslationalInterPredictor.cs

@@ -35,7 +35,7 @@ internal static partial class Av1TranslationalInterPredictor
     /// <summary>
     /// The maximum number of source rows added by an eight-tap vertical filter.
     /// </summary>
-    private const int MaximumExtraRows = FilterCoefficientCount - 1;
+    internal const int MaximumExtraRows = FilterCoefficientCount - 1;
 
     /// <summary>
     /// The minimum scratch stride that lets a 128-bit byte kernel handle four- and eight-sample blocks.

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1EncoderBlockModeInfo.cs

@@ -2,6 +2,7 @@
 // Licensed under the Six Labors Split License.
 
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
@@ -11,16 +12,36 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 /// </summary>
 internal struct Av1EncoderBlockModeInfo
 {
+    /// <summary>The residual-skip flag in the packed prediction state.</summary>
     private const byte SkipMask = 1 << 0;
+
+    /// <summary>The compound-skip flag in the packed prediction state.</summary>
     private const byte SkipModeMask = 1 << 1;
+
+    /// <summary>The intra-block-copy flag in the packed prediction state.</summary>
     private const byte IntraBlockCopyMask = 1 << 2;
 
-    // Every stored syntax value has an AV1-defined range below 256. Byte fields and one shared flag byte
-    // keep the frame-wide mode allocation compact without losing any representable encoder state.
+    /// <summary>The three low bits store the segment identifier, followed by the primary reference.</summary>
+    private const int ReferenceFrameShift = 3;
+
+    /// <summary>Both segment identifiers and primary references have eight possible values.</summary>
+    private const int SegmentAndReferenceMask = 7;
+
+    /// <summary>The vertical filter follows the three prediction flags.</summary>
+    private const int VerticalFilterShift = 3;
+
+    /// <summary>The horizontal filter follows the two-bit vertical filter.</summary>
+    private const int HorizontalFilterShift = 5;
+
+    /// <summary>Two bits represent each concrete interpolation filter, excluding the frame-level switchable sentinel.</summary>
+    private const int InterpolationFilterMask = 3;
+
+    // Primary references never use the absent-secondary sentinel. Pack their three bits beside the segment,
+    // and the concrete filters beside the flags, so adding inter syntax does not enlarge the frame-wide grid.
     private byte blockSize;
     private byte partitionType;
     private byte flags;
-    private byte segmentId;
+    private byte segmentAndReference;
     private byte transformSize;
     private byte mode;
     private byte uvMode;
@@ -75,8 +96,8 @@ internal struct Av1EncoderBlockModeInfo
     /// </summary>
     public int SegmentId
     {
-        readonly get => this.segmentId;
-        set => this.segmentId = (byte)value;
+        readonly get => this.segmentAndReference & SegmentAndReferenceMask;
+        set => this.segmentAndReference = (byte)((this.segmentAndReference & ~SegmentAndReferenceMask) | value);
     }
 
     /// <summary>
@@ -105,4 +126,31 @@ internal struct Av1EncoderBlockModeInfo
         readonly get => (Av1ChromaPredictionMode)this.uvMode;
         set => this.uvMode = (byte)value;
     }
+
+    /// <summary>
+    /// Gets or sets the primary prediction reference selected for the block.
+    /// </summary>
+    public Av1ReferenceFrameType ReferenceFrame
+    {
+        readonly get => (Av1ReferenceFrameType)(this.segmentAndReference >> ReferenceFrameShift);
+        set => this.segmentAndReference = (byte)((this.segmentAndReference & SegmentAndReferenceMask) | ((int)value << ReferenceFrameShift));
+    }
+
+    /// <summary>
+    /// Gets or sets the concrete vertical interpolation filter selected for the block.
+    /// </summary>
+    public Av1InterpolationFilter VerticalInterpolationFilter
+    {
+        readonly get => (Av1InterpolationFilter)((this.flags >> VerticalFilterShift) & InterpolationFilterMask);
+        set => this.flags = (byte)((this.flags & ~(InterpolationFilterMask << VerticalFilterShift)) | ((int)value << VerticalFilterShift));
+    }
+
+    /// <summary>
+    /// Gets or sets the concrete horizontal interpolation filter selected for the block.
+    /// </summary>
+    public Av1InterpolationFilter HorizontalInterpolationFilter
+    {
+        readonly get => (Av1InterpolationFilter)((this.flags >> HorizontalFilterShift) & InterpolationFilterMask);
+        set => this.flags = (byte)((this.flags & ~(InterpolationFilterMask << HorizontalFilterShift)) | ((int)value << HorizontalFilterShift));
+    }
 }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1EncoderBlockStruct.cs

@@ -65,6 +65,18 @@ internal struct Av1EncoderBlockStruct
         set => this.filterIntraMode = (byte)value;
     }
 
+    /// <summary>
+    /// Gets or sets the dynamic-reference-list index selected for an inter block.
+    /// </summary>
+    /// <remarks>
+    /// Filter-intra and inter prediction are mutually exclusive, so both syntax branches share one packed byte.
+    /// </remarks>
+    public int ReferenceMotionVectorIndex
+    {
+        readonly get => this.filterIntraMode;
+        set => this.filterIntraMode = (byte)value;
+    }
+
     /// <summary>
     /// Gets the encoder prediction-unit state for the block.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1EncoderPaletteMapBuffer.cs

@@ -1,13 +1,12 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-using System.Buffers;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 
 /// <summary>
-/// Owns the reusable luma and chroma palette color-index maps for encoder block decisions.
+/// Exposes reusable luma and chroma palette color-index maps over superblock-workspace storage.
 /// </summary>
 internal sealed class Av1EncoderPaletteMapBuffer : IDisposable
 {
@@ -21,21 +20,19 @@ internal sealed class Av1EncoderPaletteMapBuffer : IDisposable
     /// </summary>
     public const int StorageLength = 2 * MapLength * MapLength;
 
-    private readonly IMemoryOwner<byte> owner;
     private readonly Buffer2D<byte> luma;
     private readonly Buffer2D<byte> chroma;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="Av1EncoderPaletteMapBuffer"/> class.
     /// </summary>
-    /// <param name="configuration">The configuration providing the encoder allocator.</param>
-    public Av1EncoderPaletteMapBuffer(Configuration configuration)
+    /// <param name="storage">The superblock-owned backing storage.</param>
+    public Av1EncoderPaletteMapBuffer(Memory<byte> storage)
     {
-        this.owner = configuration.MemoryAllocator.Allocate<byte>(StorageLength);
         int mapArea = MapLength * MapLength;
-        Memory<byte> storage = this.owner.Memory[..StorageLength];
-        this.luma = Buffer2D<byte>.WrapMemory(storage[..mapArea], MapLength, MapLength);
-        this.chroma = Buffer2D<byte>.WrapMemory(storage[mapArea..], MapLength, MapLength);
+        Memory<byte> mapStorage = storage[..StorageLength];
+        this.luma = Buffer2D<byte>.WrapMemory(mapStorage[..mapArea], MapLength, MapLength);
+        this.chroma = Buffer2D<byte>.WrapMemory(mapStorage[mapArea..], MapLength, MapLength);
     }
 
     /// <summary>
@@ -51,12 +48,11 @@ internal sealed class Av1EncoderPaletteMapBuffer : IDisposable
             new Rectangle(0, 0, width, height));
 
     /// <summary>
-    /// Returns the shared palette-map owner to the configured allocator.
+    /// Releases the non-owning two-dimensional wrappers.
     /// </summary>
     public void Dispose()
     {
         this.luma.Dispose();
         this.chroma.Dispose();
-        this.owner.Dispose();
     }
 }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1EncoderPictureBuffer.cs

@@ -10,12 +10,18 @@ using SixLabors.ImageSharp.Memory;
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 
 /// <summary>
-/// Owns mode information, segmentation data, and tile-neighbor contexts for one encoded AV1 picture.
+/// Owns reusable mode information, segmentation data, and tile-neighbor contexts for fixed-geometry AV1 pictures.
 /// </summary>
 internal sealed class Av1EncoderPictureBuffer : IDisposable
 {
     private readonly Av1EncoderModeInfoBuffer modeInfo;
     private readonly IMemoryOwner<byte> stateStorage;
+
+    /// <summary>
+    /// The exact packed state region cleared between frames without touching excess pool capacity.
+    /// </summary>
+    private readonly Memory<byte> stateMemory;
+
     private readonly ByteMemoryManager<Av1PartitionContext> partitionContextMemory;
     private readonly Av1NeighborArrayUnit<Av1PartitionContext>[] partitionContexts;
     private readonly Av1NeighborArrayUnit<byte>[] lumaCoefficientContexts;
@@ -40,6 +46,42 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
         int width,
         int height,
         bool disallow4x4AllFrames)
+        : this(
+            configuration,
+            sequenceHeader,
+            frameHeader,
+            width,
+            height,
+            disallow4x4AllFrames,
+            frameHeader.AllowScreenContentTools,
+            frameHeader.AllowIntraBlockCopy || !frameHeader.IsIntra,
+            frameHeader.AllowIntraBlockCopy)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1EncoderPictureBuffer"/> class with the maximum state
+    /// required by a fixed-geometry sequence.
+    /// </summary>
+    /// <param name="configuration">The configuration providing picture-lifetime memory.</param>
+    /// <param name="sequenceHeader">The sequence header defining superblock and chroma geometry.</param>
+    /// <param name="frameHeader">The initial frame header defining dimensions and tiles.</param>
+    /// <param name="width">The visible luma width.</param>
+    /// <param name="height">The visible luma height.</param>
+    /// <param name="disallow4x4AllFrames">Whether each allocated mode-information value represents an 8x8 region.</param>
+    /// <param name="allocateScreenContentState">Whether palette neighbor state can be required by any frame.</param>
+    /// <param name="allocateMotionVectorState">Whether inter or intra-block-copy vectors can be required by any frame.</param>
+    /// <param name="allocateIntraBlockCopySearch">Whether intra-block-copy search state can be required by any frame.</param>
+    public Av1EncoderPictureBuffer(
+        Configuration configuration,
+        ObuSequenceHeader sequenceHeader,
+        ObuFrameHeader frameHeader,
+        int width,
+        int height,
+        bool disallow4x4AllFrames,
+        bool allocateScreenContentState,
+        bool allocateMotionVectorState,
+        bool allocateIntraBlockCopySearch)
     {
         const int ContextAlignmentLog2 = Av1Constants.MaxSuperBlockSizeLog2 - Av1Constants.ModeInfoSizeLog2;
         this.modeInfo = new Av1EncoderModeInfoBuffer(
@@ -79,17 +121,17 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
         int paletteLeftLength = alignedModeInfoRowCount;
         int paletteTopLength = this.modeInfo.ModeInfoStride;
         int paletteContextLength = checked(paletteLeftLength + paletteTopLength);
-        int paletteStorageOffset = frameHeader.AllowScreenContentTools
+        int paletteStorageOffset = allocateScreenContentState
             ? Av1Math.AlignPowerOf2(byteContextStorageEnd, 1)
             : byteContextStorageEnd;
 
-        int paletteStorageLength = frameHeader.AllowScreenContentTools
+        int paletteStorageLength = allocateScreenContentState
             ? checked(tileCount * paletteContextLength * Unsafe.SizeOf<Av1EncoderPaletteInfo>())
             : 0;
 
         int paletteStorageEnd = checked(paletteStorageOffset + paletteStorageLength);
-        int displacementVectorLength = frameHeader.AllowIntraBlockCopy ? this.modeInfo.Allocation.Length : 0;
-        int displacementVectorStorageOffset = frameHeader.AllowIntraBlockCopy
+        int displacementVectorLength = allocateMotionVectorState ? this.modeInfo.Allocation.Length : 0;
+        int displacementVectorStorageOffset = allocateMotionVectorState
             ? Av1Math.AlignPowerOf2(paletteStorageEnd, 1)
             : paletteStorageEnd;
 
@@ -97,23 +139,32 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
             displacementVectorLength * Unsafe.SizeOf<Av1EncoderDisplacementVector>());
 
         int displacementVectorStorageEnd = checked(displacementVectorStorageOffset + displacementVectorStorageLength);
-        int intraBlockCopySearchStorageOffset = frameHeader.AllowIntraBlockCopy
+        int intraBlockCopySearchStorageOffset = allocateIntraBlockCopySearch
             ? Av1Math.AlignPowerOf2(displacementVectorStorageEnd, 2)
             : displacementVectorStorageEnd;
 
-        int intraBlockCopySearchStorageLength = frameHeader.AllowIntraBlockCopy
+        int intraBlockCopySearchStorageLength = allocateIntraBlockCopySearch
             ? Av1IntraBlockCopySearchIndex.GetStorageLength(width, height)
             : 0;
 
-        int stateStorageLength = checked(intraBlockCopySearchStorageOffset + intraBlockCopySearchStorageLength);
+        int intraBlockCopySearchStorageEnd = checked(
+            intraBlockCopySearchStorageOffset + intraBlockCopySearchStorageLength);
+
+        int tileStateStorageOffset = Av1Math.AlignPowerOf2(intraBlockCopySearchStorageEnd, 2);
+        int cdefPresetLength = tileCount * Av1Constants.CdefUnitsPerSuperblock;
+        int tileStateLength = cdefPresetLength + (3 * tileCount);
+        int tileStateStorageLength = tileStateLength * sizeof(int);
+        int stateStorageLength = checked(tileStateStorageOffset + tileStateStorageLength);
 
         // Segmentation and every tile edge share one clean picture lifetime. The partition region begins at its
-        // native alignment, while typed views keep the entropy writer independent from the packed byte owner.
+        // native alignment. CDEF, quantizer, and encoded-tile bounds occupy one aligned trailing integer region
+        // instead of allocating separate managed arrays for every picture.
         this.stateStorage = configuration.MemoryAllocator.Allocate<byte>(
             stateStorageLength,
             AllocationOptions.Clean);
 
-        Memory<byte> stateStorage = this.stateStorage.Memory[..stateStorageLength];
+        this.stateMemory = this.stateStorage.Memory[..stateStorageLength];
+        Memory<byte> stateStorage = this.stateMemory;
         this.partitionContextMemory = new ByteMemoryManager<Av1PartitionContext>(
             stateStorage.Slice(partitionStorageOffset, partitionStorageLength));
 
@@ -125,7 +176,7 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
         this.redCoefficientContexts = new Av1NeighborArrayUnit<byte>[tileCount];
         this.transformContexts = new Av1NeighborArrayUnit<byte>[tileCount];
         Memory<Av1EncoderPaletteInfo> paletteStorage = Memory<Av1EncoderPaletteInfo>.Empty;
-        if (frameHeader.AllowScreenContentTools)
+        if (allocateScreenContentState)
         {
             // Palette entries contain 16-bit colors, so their packed typed region begins at an even byte offset.
             ByteMemoryManager<Av1EncoderPaletteInfo> paletteMemory = new(
@@ -140,10 +191,10 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
         }
 
         Memory<Av1EncoderDisplacementVector> displacementVectors = Memory<Av1EncoderDisplacementVector>.Empty;
-        if (frameHeader.AllowIntraBlockCopy)
+        if (allocateMotionVectorState)
         {
-            // Each component lies strictly inside plus or minus 16384, so two signed 16-bit fields preserve the
-            // complete syntax domain without expanding every frame's compact mode-information allocation.
+            // Each component lies strictly inside plus or minus 16384. Two signed 16-bit fields preserve both
+            // inter and intra-block-copy vectors without expanding every compact mode-information entry.
             ByteMemoryManager<Av1EncoderDisplacementVector> displacementVectorMemory = new(
                 stateStorage.Slice(displacementVectorStorageOffset, displacementVectorStorageLength));
 
@@ -151,7 +202,7 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
         }
 
         Av1IntraBlockCopySearchIndex intraBlockCopySearch = default;
-        if (frameHeader.AllowIntraBlockCopy)
+        if (allocateIntraBlockCopySearch)
         {
             // The search index casts its packed workspace to 32-bit links, so its non-owning region begins at
             // a four-byte boundary inside the existing picture-state rent.
@@ -161,8 +212,15 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
                 height);
         }
 
-        int[][] cdefPreset = new int[tileCount][];
-        int[] previousQIndex = new int[tileCount];
+        ByteMemoryManager<int> tileStateMemory = new(
+            stateStorage.Slice(tileStateStorageOffset, tileStateStorageLength));
+
+        Memory<int> tileState = tileStateMemory.Memory;
+        Memory<int> cdefPreset = tileState[..cdefPresetLength];
+        Memory<int> previousQIndex = tileState.Slice(cdefPresetLength, tileCount);
+        Memory<int> tileDataOffsets = tileState.Slice(cdefPresetLength + tileCount, tileCount);
+        Memory<int> tileDataLengths = tileState.Slice(cdefPresetLength + (2 * tileCount), tileCount);
+        cdefPreset.Span.Fill(-1);
         for (int tileIndex = 0; tileIndex < tileCount; tileIndex++)
         {
             this.partitionContexts[tileIndex] = new Av1NeighborArrayUnit<Av1PartitionContext>(
@@ -214,7 +272,7 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
             this.transformContexts[tileIndex].Left.Fill((byte)Av1Constants.MaxTransformSize);
             this.transformContexts[tileIndex].Top.Fill((byte)Av1Constants.MaxTransformSize);
 
-            if (frameHeader.AllowScreenContentTools)
+            if (allocateScreenContentState)
             {
                 this.paletteContexts[tileIndex] = new Av1NeighborArrayUnit<Av1EncoderPaletteInfo>(
                     paletteStorage.Slice(tileIndex * paletteContextLength, paletteContextLength),
@@ -225,8 +283,7 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
                 };
             }
 
-            cdefPreset[tileIndex] = [-1, -1, -1, -1];
-            previousQIndex[tileIndex] = frameHeader.QuantizationParameters.BaseQIndex;
+            previousQIndex.Span[tileIndex] = frameHeader.QuantizationParameters.BaseQIndex;
         }
 
         this.Picture = new Av1PictureControlSet
@@ -258,7 +315,9 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
             IntraBlockCopySearch = intraBlockCopySearch,
             ModeInfoStride = this.modeInfo.ModeInfoStride,
             Disallow4x4AllFrames = this.modeInfo.Disallow4x4AllFrames,
-            CdefPreset = cdefPreset
+            CdefPreset = cdefPreset,
+            TileDataOffsets = tileDataOffsets,
+            TileDataLengths = tileDataLengths
         };
     }
 
@@ -267,6 +326,31 @@ internal sealed class Av1EncoderPictureBuffer : IDisposable
     /// </summary>
     public Av1PictureControlSet Picture { get; }
 
+    /// <summary>
+    /// Restores clean per-frame state while retaining every fixed-geometry allocation.
+    /// </summary>
+    /// <param name="frameHeader">The frame header consumed by the next encoding pass.</param>
+    public void Reset(ObuFrameHeader frameHeader)
+    {
+        this.modeInfo.Grid.Span.Clear();
+        this.modeInfo.Allocation.Span.Clear();
+        this.stateMemory.Span.Clear();
+
+        // Transform contexts begin at the largest transform size until an encoded neighbor publishes its
+        // selected size. This sentinel must be restored after the packed state owner is cleared.
+        foreach (Av1NeighborArrayUnit<byte> context in this.transformContexts)
+        {
+            context.Left.Fill((byte)Av1Constants.MaxTransformSize);
+            context.Top.Fill((byte)Av1Constants.MaxTransformSize);
+        }
+
+        this.Picture.CdefPreset.Span.Fill(-1);
+        this.Picture.Parent.PreviousQIndex.Span.Fill(frameHeader.QuantizationParameters.BaseQIndex);
+        this.Picture.Parent.FrameHeader = frameHeader;
+        this.Picture.Parent.Common.FrameSize = frameHeader.FrameSize;
+        this.Picture.Parent.Common.TilesInfo = frameHeader.TilesInfo;
+    }
+
     /// <summary>
     /// Returns every picture-lifetime allocation to the configured allocator.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1EncoderSuperblockWorkspace.cs

@@ -3,6 +3,7 @@
 
 using System.Buffers;
 using System.Runtime.InteropServices;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 
@@ -22,14 +23,26 @@ internal sealed class Av1EncoderSuperblockWorkspace : IDisposable
     public const int MaximumPartitionCount = 1 + 4 + 16 + 64 + 256;
 
     /// <summary>
-    /// The complete workspace length in packed final-block storage elements.
+    /// The decision-region length in packed final-block storage elements.
     /// </summary>
-    public const int StorageLength = MaximumFinalBlockCount + ((MaximumPartitionCount + Av1EncoderBlockStruct.StorageSize - 1) / Av1EncoderBlockStruct.StorageSize);
+    public const int DecisionStorageLength = MaximumFinalBlockCount + ((MaximumPartitionCount + Av1EncoderBlockStruct.StorageSize - 1) / Av1EncoderBlockStruct.StorageSize);
 
-    private readonly Configuration configuration;
-    private readonly IMemoryOwner<Av1EncoderBlockStruct> owner;
-    private Av1EncoderPaletteMapBuffer? paletteMaps;
+    /// <summary>
+    /// The byte length of the aligned final-block and partition decision region.
+    /// </summary>
+    public const int DecisionStorageByteLength = DecisionStorageLength * Av1EncoderBlockStruct.StorageSize;
+
+    /// <summary>
+    /// The complete byte length of the decision and palette-map regions.
+    /// </summary>
+    public const int StorageByteLength = DecisionStorageByteLength + Av1EncoderPaletteMapBuffer.StorageLength;
+
+    private const int PartitionStorageOffset = MaximumFinalBlockCount * Av1EncoderBlockStruct.StorageSize;
+
+    private readonly IMemoryOwner<byte> owner;
+    private readonly Av1EncoderPaletteMapBuffer paletteMaps;
     private Av1EncoderPaletteInfo paletteInfo;
+    private Av1ReferenceMotionVectors referenceMotionVectors;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="Av1EncoderSuperblockWorkspace"/> class.
@@ -37,21 +50,27 @@ internal sealed class Av1EncoderSuperblockWorkspace : IDisposable
     /// <param name="configuration">The configuration providing the encoder allocator.</param>
     public Av1EncoderSuperblockWorkspace(Configuration configuration)
     {
-        this.configuration = configuration;
-        this.owner = configuration.MemoryAllocator.Allocate<Av1EncoderBlockStruct>(StorageLength);
+        this.owner = configuration.MemoryAllocator.Allocate<byte>(StorageByteLength);
+        Memory<byte> storage = this.owner.Memory[..StorageByteLength];
+
+        // Decisions and palette maps have the same serial superblock lifetime. Keeping both regions in one
+        // owner preserves their distinct layouts while removing a separate palette allocation and cleanup path.
+        this.paletteMaps = new Av1EncoderPaletteMapBuffer(
+            storage.Slice(DecisionStorageByteLength, Av1EncoderPaletteMapBuffer.StorageLength));
+
         this.Reset();
     }
 
     /// <summary>
     /// Gets the maximum-size final-block decision span in partition traversal order.
     /// </summary>
-    public Span<Av1EncoderBlockStruct> FinalBlocks => this.owner.Memory.Span[..MaximumFinalBlockCount];
+    public Span<Av1EncoderBlockStruct> FinalBlocks
+        => MemoryMarshal.Cast<byte, Av1EncoderBlockStruct>(this.owner.Memory.Span[..DecisionStorageByteLength])[..MaximumFinalBlockCount];
 
     /// <summary>
     /// Gets the maximum-size partition-type span in partition-tree preorder.
     /// </summary>
-    public Span<byte> PartitionTypes
-        => MemoryMarshal.AsBytes(this.owner.Memory.Span[MaximumFinalBlockCount..])[..MaximumPartitionCount];
+    public Span<byte> PartitionTypes => this.owner.Memory.Span.Slice(PartitionStorageOffset, MaximumPartitionCount);
 
     /// <summary>
     /// Gets the palette sizes and colors selected for the block currently being written.
@@ -59,20 +78,15 @@ internal sealed class Av1EncoderSuperblockWorkspace : IDisposable
     public ref Av1EncoderPaletteInfo PaletteInfo => ref this.paletteInfo;
 
     /// <summary>
-    /// Gets the reusable palette maps, allocating their shared owner only after a block enters palette search.
+    /// Gets the reusable reference-vector stack used while writing inter syntax.
     /// </summary>
-    /// <returns>The reusable luma and chroma palette maps.</returns>
-    public Av1EncoderPaletteMapBuffer GetPaletteMaps()
-    {
-        Av1EncoderPaletteMapBuffer? maps = this.paletteMaps;
-        if (maps is null)
-        {
-            maps = new Av1EncoderPaletteMapBuffer(this.configuration);
-            this.paletteMaps = maps;
-        }
+    public ref Av1ReferenceMotionVectors ReferenceMotionVectors => ref this.referenceMotionVectors;
 
-        return maps;
-    }
+    /// <summary>
+    /// Gets the reusable palette maps within the superblock-workspace owner.
+    /// </summary>
+    /// <returns>The reusable luma and chroma palette maps.</returns>
+    public Av1EncoderPaletteMapBuffer GetPaletteMaps() => this.paletteMaps;
 
     /// <summary>
     /// Clears all decisions before the workspace is reused for another superblock.
@@ -86,7 +100,7 @@ internal sealed class Av1EncoderSuperblockWorkspace : IDisposable
         };
 
         this.FinalBlocks.Fill(initialBlock);
-        MemoryMarshal.AsBytes(this.owner.Memory.Span[MaximumFinalBlockCount..]).Clear();
+        this.PartitionTypes.Clear();
         this.paletteInfo = default;
     }
 
@@ -95,7 +109,7 @@ internal sealed class Av1EncoderSuperblockWorkspace : IDisposable
     /// </summary>
     public void Dispose()
     {
-        this.paletteMaps?.Dispose();
+        this.paletteMaps.Dispose();
         this.owner.Dispose();
     }
 }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1PictureControlSet.cs

@@ -86,9 +86,20 @@ internal class Av1PictureControlSet
     public bool Disallow4x4AllFrames { get; set; }
 
     /// <summary>
-    /// Gets or sets the constrained directional enhancement filter presets for each filter block.
+    /// Gets or sets the constrained directional enhancement filter presets for each tile.
+    /// Each tile occupies <see cref="Av1Constants.CdefUnitsPerSuperblock"/> consecutive entries.
     /// </summary>
-    public required int[][] CdefPreset { get; set; }
+    public required Memory<int> CdefPreset { get; set; }
+
+    /// <summary>
+    /// Gets or sets the starting byte of each tile in the shared encoded output buffer.
+    /// </summary>
+    public required Memory<int> TileDataOffsets { get; set; }
+
+    /// <summary>
+    /// Gets or sets the encoded byte length of each tile.
+    /// </summary>
+    public required Memory<int> TileDataLengths { get; set; }
 
     /// <summary>
     /// Gets the mode-information entry mapped to a frame position.

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1PictureParentControlSet.cs

@@ -23,7 +23,7 @@ internal class Av1PictureParentControlSet
     /// <summary>
     /// Gets or sets the preceding quantizer index for each tile context.
     /// </summary>
-    public required int[] PreviousQIndex { get; set; }
+    public required Memory<int> PreviousQIndex { get; set; }
 
     /// <summary>
     /// Gets or sets the encoder palette-search level.

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileReader.cs

@@ -134,7 +134,7 @@ internal sealed class Av1TileReader : IAv1TileReader, IDisposable
     /// <summary>
     /// Reusable fixed-capacity storage for one block's weighted reference-motion-vector candidates.
     /// </summary>
-    private readonly Av1ReferenceMotionVectors referenceMotionVectors = new();
+    private Av1ReferenceMotionVectors referenceMotionVectors;
 
     /// <summary>
     /// Reusable fixed-capacity state for motion-mode eligibility and local warped-motion projection.
@@ -250,6 +250,7 @@ internal sealed class Av1TileReader : IAv1TileReader, IDisposable
         Av1ReferenceFrameStore? referenceFrames,
         PaletteColorIndexMaps? sharedPaletteColorIndexMaps)
     {
+        this.referenceMotionVectors = default;
         this.FrameHeader = frameHeader;
         this.configuration = configuration;
         this.SequenceHeader = sequenceHeader;
@@ -2044,7 +2045,7 @@ internal sealed class Av1TileReader : IAv1TileReader, IDisposable
             Av1ReferenceFrameType secondaryReferenceFrame = modeInfo.ReferenceFrames[1];
             bool isCompound = secondaryReferenceFrame > Av1ReferenceFrameType.Intra;
 
-            Av1ReferenceMotionVectors referenceMotionVectors = this.referenceMotionVectors;
+            ref Av1ReferenceMotionVectors referenceMotionVectors = ref this.referenceMotionVectors;
             referenceMotionVectors.Build(
                 ref partitionInfo,
                 tileInfo,
@@ -2110,9 +2111,7 @@ internal sealed class Av1TileReader : IAv1TileReader, IDisposable
                 }
             }
 
-            Av1MotionVectorPrecision precision = this.FrameHeader.ForceIntegerMotionVector
-                ? Av1MotionVectorPrecision.Integer
-                : this.FrameHeader.AllowHighPrecisionMotionVector ? Av1MotionVectorPrecision.EighthSample : Av1MotionVectorPrecision.QuarterSample;
+            Av1MotionVectorPrecision precision = this.FrameHeader.MotionVectorPrecision;
 
             Span<Av1MotionVector> motionVectors = modeInfo.MotionVectors;
             if (!isCompound)

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileWriter.cs

@@ -5,6 +5,7 @@ using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
 
@@ -832,18 +833,35 @@ internal partial class Av1TileWriter
 
         bool skipWritingCoefficients = macroBlockModeInfo.Block.Skip;
 
-        // This encoder path currently writes intra frames only, so every block follows the key-frame mode syntax.
+        // Segmentation, skip, filter, and quantizer syntax precede the prediction-domain branch in both
+        // intra and inter frames. Keeping this prefix shared preserves the decoder's symbol order.
         {
             if (pcs.Parent.FrameHeader.SegmentationParameters.Enabled && pcs.Parent.FrameHeader.SegmentationParameters.SegmentIdPrecedesSkip)
             {
-                WriteSegmentId(pcs, writer, blockSize, blockOrigin, macroBlock, ref blk_ptr, skipWritingCoefficients);
+                WriteSegmentId(
+                    pcs,
+                    writer,
+                    blockSize,
+                    blockOrigin,
+                    macroBlock,
+                    ref blk_ptr,
+                    skipWritingCoefficients,
+                    beforeSkip: true);
             }
 
             EncodeSkipCoefficients(writer, macroBlock, skipWritingCoefficients);
 
             if (pcs.Parent.FrameHeader.SegmentationParameters.Enabled && !pcs.Parent.FrameHeader.SegmentationParameters.SegmentIdPrecedesSkip)
             {
-                WriteSegmentId(pcs, writer, blockSize, blockOrigin, macroBlock, ref blk_ptr, skipWritingCoefficients);
+                WriteSegmentId(
+                    pcs,
+                    writer,
+                    blockSize,
+                    blockOrigin,
+                    macroBlock,
+                    ref blk_ptr,
+                    skipWritingCoefficients,
+                    beforeSkip: false);
             }
 
             WriteCdef(
@@ -862,17 +880,122 @@ internal partial class Av1TileWriter
                 if ((blockSize != scs.SequenceHeader.SuperblockSize || !skipWritingCoefficients) && super_block_upper_left)
                 {
                     Guard.MustBeGreaterThan(current_q_index, 0, nameof(current_q_index));
-                    int reduced_delta_qindex = (current_q_index - pcs.Parent.PreviousQIndex[tile_idx]) /
+                    int reduced_delta_qindex = (current_q_index - pcs.Parent.PreviousQIndex.Span[tile_idx]) /
                         frm_hdr.DeltaQParameters.Resolution;
 
                     writer.WriteDeltaQuantizerIndex(reduced_delta_qindex);
-                    pcs.Parent.PreviousQIndex[tile_idx] = current_q_index;
+                    pcs.Parent.PreviousQIndex.Span[tile_idx] = current_q_index;
                 }
             }
 
-            Av1PredictionMode intra_luma_mode = macroBlockModeInfo.Block.Mode;
+            bool isInterBlock = macroBlockModeInfo.Block.ReferenceFrame > Av1ReferenceFrameType.Intra;
+            bool isGlobalMotionForced = false;
+            bool isReferenceForced = false;
+            if (!frm_hdr.IsIntra)
+            {
+                ObuSegmentationParameters segmentation = frm_hdr.SegmentationParameters;
+                int segmentId = macroBlockModeInfo.Block.SegmentId;
+                isReferenceForced = segmentation.IsFeatureActive(
+                    segmentId,
+                    ObuSegmentationLevelFeature.ReferenceFrame);
+
+                isGlobalMotionForced = segmentation.IsFeatureActive(
+                    segmentId,
+                    ObuSegmentationLevelFeature.GlobalMotionVector);
+
+                if (!isReferenceForced && !isGlobalMotionForced)
+                {
+                    int intraInterContext = GetIntraInterContext(macroBlock);
+                    writer.WriteIsInter(isInterBlock, intraInterContext);
+                }
+            }
+
+            Av1PredictionMode lumaMode = macroBlockModeInfo.Block.Mode;
             Av1ChromaPredictionMode intra_chroma_mode = macroBlockModeInfo.Block.UvMode;
-            if (IsIntraBlockCopyAllowed(pcs.Parent.FrameHeader/*, pcs.Parent.SliceType*/))
+            if (isInterBlock)
+            {
+                if (!isReferenceForced && !isGlobalMotionForced)
+                {
+                    Span<byte> referenceCounts = stackalloc byte[Av1Constants.ReferenceFrameCount];
+                    CollectNeighborReferenceCounts(macroBlock, referenceCounts);
+                    writer.WriteSingleReference(
+                        macroBlockModeInfo.Block.ReferenceFrame,
+                        referenceCounts);
+                }
+
+                if (!isGlobalMotionForced)
+                {
+                    ref Av1ReferenceMotionVectors referenceMotionVectors = ref tb_ptr.Workspace.ReferenceMotionVectors;
+                    referenceMotionVectors.Build(
+                        pcs,
+                        macroBlock,
+                        modeInfoPosition,
+                        blockSize,
+                        macroBlockModeInfo.Block.PartitionType,
+                        scs.SequenceHeader,
+                        frm_hdr,
+                        macroBlockModeInfo.Block.ReferenceFrame);
+
+                    writer.WriteInterMode(lumaMode, referenceMotionVectors.ModeContext);
+                    int referenceMotionVectorIndex = blk_ptr.ReferenceMotionVectorIndex;
+                    if (lumaMode == Av1PredictionMode.NearMotionVector)
+                    {
+                        // NEARMV reserves stack entry zero for NEARESTMV, so its DRL decisions advance from
+                        // near entry zero to one and then from one to two.
+                        for (int index = 1; index < 3 && referenceMotionVectors.Count > index + 1; index++)
+                        {
+                            bool advance = referenceMotionVectorIndex >= index;
+                            int context = Av1SymbolContextHelper.GetDrlContext(referenceMotionVectors.Weights, index);
+                            writer.WriteDynamicReferenceList(advance, context);
+                            if (!advance)
+                            {
+                                break;
+                            }
+                        }
+                    }
+                    else if (lumaMode == Av1PredictionMode.NewMotionVector)
+                    {
+                        // NEWMV begins at stack entry zero and can advance through entries one and two.
+                        for (int index = 0; index < 2 && referenceMotionVectors.Count > index + 1; index++)
+                        {
+                            bool advance = referenceMotionVectorIndex > index;
+                            int context = Av1SymbolContextHelper.GetDrlContext(referenceMotionVectors.Weights, index);
+                            writer.WriteDynamicReferenceList(advance, context);
+                            if (!advance)
+                            {
+                                break;
+                            }
+                        }
+
+                        Av1MotionVector vector = pcs.GetDisplacementVector(modeInfoPosition);
+                        writer.WriteMotionVector(
+                            vector,
+                            referenceMotionVectors.GetNewReference(referenceMotionVectorIndex),
+                            frm_hdr.MotionVectorPrecision);
+                    }
+                }
+
+                if (UsesSwitchableInterpolation(frm_hdr, macroBlockModeInfo.Block))
+                {
+                    // The vertical symbol is first and supplies both axes unless the sequence enables dual filters.
+                    int verticalContext = Av1SymbolContextHelper.GetSwitchableInterpolationContext(
+                        macroBlockModeInfo.Block,
+                        macroBlock,
+                        direction: 0);
+
+                    writer.WriteSwitchableInterpolationFilter(macroBlockModeInfo.Block.VerticalInterpolationFilter, verticalContext);
+                    if (scs.SequenceHeader.EnableDualFilter)
+                    {
+                        int horizontalContext = Av1SymbolContextHelper.GetSwitchableInterpolationContext(
+                            macroBlockModeInfo.Block,
+                            macroBlock,
+                            direction: 1);
+
+                        writer.WriteSwitchableInterpolationFilter(macroBlockModeInfo.Block.HorizontalInterpolationFilter, horizontalContext);
+                    }
+                }
+            }
+            else if (IsIntraBlockCopyAllowed(pcs.Parent.FrameHeader/*, pcs.Parent.SliceType*/))
             {
                 WriteIntraBlockCopyInfo(
                     pcs,
@@ -882,12 +1005,19 @@ internal partial class Av1TileWriter
                     macroBlockModeInfo);
             }
 
-            if (!macroBlockModeInfo.Block.UseIntraBlockCopy)
+            if (!isInterBlock && !macroBlockModeInfo.Block.UseIntraBlockCopy)
             {
-                EncodeIntraLumaMode(writer, macroBlockModeInfo, macroBlock, ref blk_ptr, blockSize, intra_luma_mode);
+                EncodeIntraLumaMode(
+                    writer,
+                    frm_hdr,
+                    macroBlockModeInfo,
+                    macroBlock,
+                    ref blk_ptr,
+                    blockSize,
+                    lumaMode);
             }
 
-            if (!macroBlockModeInfo.Block.UseIntraBlockCopy)
+            if (!isInterBlock && !macroBlockModeInfo.Block.UseIntraBlockCopy)
             {
                 if (blk_ptr.HasChroma)
                 {
@@ -898,12 +1028,13 @@ internal partial class Av1TileWriter
                         macroBlockModeInfo,
                         ref blk_ptr,
                         blockSize,
-                        intra_luma_mode,
+                        lumaMode,
                         intra_chroma_mode);
                 }
             }
 
-            bool paletteAllowed = !macroBlockModeInfo.Block.UseIntraBlockCopy &&
+            bool paletteAllowed = !isInterBlock &&
+                !macroBlockModeInfo.Block.UseIntraBlockCopy &&
                 IsPaletteAllowed(frm_hdr.AllowScreenContentTools, blockSize);
 
             if (paletteAllowed)
@@ -921,12 +1052,13 @@ internal partial class Av1TileWriter
                     blk_ptr.HasChroma);
             }
 
-            if (!macroBlockModeInfo.Block.UseIntraBlockCopy &&
+            if (!isInterBlock &&
+                !macroBlockModeInfo.Block.UseIntraBlockCopy &&
                 IsFilterIntraAllowed(
                     scs.SequenceHeader.EnableFilterIntra,
                     blockSize,
                     paletteInfo.PaletteSizes[0],
-                    intra_luma_mode))
+                    lumaMode))
             {
                 writer.WriteFilterIntraMode(blk_ptr.FilterIntraMode, blockSize);
             }
@@ -983,7 +1115,7 @@ internal partial class Av1TileWriter
                     writer,
                     ref blk_ptr,
                     blockOrigin,
-                    intra_luma_mode,
+                    lumaMode,
                     blockSize,
                     coefficientBuffer,
                     tb_ptr.Index,
@@ -1035,7 +1167,8 @@ internal partial class Av1TileWriter
             ref Av1MacroBlockModeInfo aboveModeInfo =
                 ref macroBlock.GetRelativeModeInfo(-macroBlock.ModeInfoStride);
 
-            if (aboveModeInfo.Block.UseIntraBlockCopy)
+            if (aboveModeInfo.Block.ReferenceFrame > Av1ReferenceFrameType.Intra ||
+                aboveModeInfo.Block.UseIntraBlockCopy)
             {
                 above = aboveModeInfo.Block.BlockSize.GetWidth() >= maximumTransformSize.GetWidth() ? 1 : 0;
             }
@@ -1044,7 +1177,8 @@ internal partial class Av1TileWriter
         if (macroBlock.IsLeftAvailable)
         {
             ref Av1MacroBlockModeInfo leftModeInfo = ref macroBlock.GetRelativeModeInfo(-1);
-            if (leftModeInfo.Block.UseIntraBlockCopy)
+            if (leftModeInfo.Block.ReferenceFrame > Av1ReferenceFrameType.Intra ||
+                leftModeInfo.Block.UseIntraBlockCopy)
             {
                 left = leftModeInfo.Block.BlockSize.GetHeight() >= maximumTransformSize.GetHeight() ? 1 : 0;
             }
@@ -1076,7 +1210,8 @@ internal partial class Av1TileWriter
     {
         ObuFrameHeader frameHeader = pcs.Parent.FrameHeader;
         bool isLossless = frameHeader.LosslessArray[macroBlockModeInfo.Block.SegmentId];
-        bool isInter = macroBlockModeInfo.Block.UseIntraBlockCopy;
+        bool isInter = macroBlockModeInfo.Block.ReferenceFrame > Av1ReferenceFrameType.Intra ||
+            macroBlockModeInfo.Block.UseIntraBlockCopy;
         bool writesUniformTransformSize = !isLossless &&
             frameHeader.TransformMode == Av1TransformMode.Select &&
             !isInter &&
@@ -1110,7 +1245,7 @@ internal partial class Av1TileWriter
                 blockSize,
                 blockSize.GetMaximumTransformSize());
 
-            // Intra-block copy currently retains the maximum transform, so its variable-transform tree has one unsplit root.
+            // Current inter decisions retain the maximum transform, so their variable-transform tree has one unsplit root.
             writer.WriteTransformPartition(false, context);
         }
 
@@ -1247,23 +1382,34 @@ internal partial class Av1TileWriter
     }
 
     /// <summary>
-    /// Gets the key-frame luma mode rate against the current neighboring modes and tile probabilities.
+    /// Gets the luma mode rate from the frame-appropriate distribution.
     /// </summary>
     /// <param name="writer">The live tile symbol encoder.</param>
     /// <param name="macroBlock">The current block's mapped neighbor state.</param>
     /// <param name="blockSize">The selected block size.</param>
     /// <param name="mode">The candidate luma mode.</param>
     /// <param name="angleDelta">The signed directional-angle adjustment.</param>
+    /// <param name="isIntraFrame">Whether the frame uses key-frame neighbor-conditioned mode syntax.</param>
     /// <returns>The luma mode and directional-angle rate in 1/512-bit units.</returns>
     public static int GetLumaModeCost(
         Av1SymbolEncoder writer,
         Av1MacroBlockD macroBlock,
         Av1BlockSize blockSize,
         Av1PredictionMode mode,
-        int angleDelta)
+        int angleDelta,
+        bool isIntraFrame)
     {
-        GetYModeContext(macroBlock, out byte topContext, out byte leftContext);
-        int cost = writer.GetLumaModeCost(mode, topContext, leftContext);
+        int cost;
+        if (isIntraFrame)
+        {
+            GetYModeContext(macroBlock, out byte topContext, out byte leftContext);
+            cost = writer.GetLumaModeCost(mode, topContext, leftContext);
+        }
+        else
+        {
+            cost = writer.GetInterFrameLumaModeCost(mode, blockSize);
+        }
+
         if (blockSize >= Av1BlockSize.Block8x8 && mode.IsDirectional())
         {
             cost += writer.GetAngleDeltaCost(angleDelta + Av1Constants.MaxAngleDelta, mode);
@@ -1273,9 +1419,10 @@ internal partial class Av1TileWriter
     }
 
     /// <summary>
-    /// Writes the key-frame luma prediction mode and any directional angle adjustment.
+    /// Writes the frame-appropriate luma prediction mode and any directional angle adjustment.
     /// </summary>
     /// <param name="writer">The tile symbol encoder.</param>
+    /// <param name="frameHeader">The frame header that selects the luma-mode probability model.</param>
     /// <param name="macroBlockModeInfo">The selected block modes.</param>
     /// <param name="macroBlock">The reusable macroblock edge and neighbor state.</param>
     /// <param name="blk_ptr">The encoder prediction-unit state.</param>
@@ -1283,14 +1430,22 @@ internal partial class Av1TileWriter
     /// <param name="lumaMode">The selected luma prediction mode.</param>
     private static void EncodeIntraLumaMode(
         Av1SymbolEncoder writer,
+        ObuFrameHeader frameHeader,
         Av1MacroBlockModeInfo macroBlockModeInfo,
         Av1MacroBlockD macroBlock,
         ref Av1EncoderBlockStruct blk_ptr,
         Av1BlockSize blockSize,
         Av1PredictionMode lumaMode)
     {
-        GetYModeContext(macroBlock, out byte topContext, out byte leftContext);
-        writer.WriteLumaMode(lumaMode, topContext, leftContext);
+        if (frameHeader.IsIntra)
+        {
+            GetYModeContext(macroBlock, out byte topContext, out byte leftContext);
+            writer.WriteLumaMode(lumaMode, topContext, leftContext);
+        }
+        else
+        {
+            writer.WriteInterFrameLumaMode(lumaMode, blockSize);
+        }
 
         if (blockSize >= Av1BlockSize.Block8x8 && macroBlockModeInfo.Block.Mode.IsDirectional())
         {
@@ -1298,6 +1453,91 @@ internal partial class Av1TileWriter
         }
     }
 
+    /// <summary>
+    /// Gets the prediction-domain context from the immediately above and left encoder blocks.
+    /// </summary>
+    /// <param name="macroBlock">The current block's mapped neighbor state.</param>
+    /// <returns>The context in the inclusive range zero through three.</returns>
+    public static int GetIntraInterContext(Av1MacroBlockD macroBlock)
+    {
+        bool hasAbove = macroBlock.IsUpAvailable;
+        bool hasLeft = macroBlock.IsLeftAvailable;
+        if (hasAbove && hasLeft)
+        {
+            bool aboveIsIntra = macroBlock
+                .GetRelativeModeInfo(-macroBlock.ModeInfoStride)
+                .Block.ReferenceFrame <= Av1ReferenceFrameType.Intra;
+
+            bool leftIsIntra = macroBlock
+                .GetRelativeModeInfo(-1)
+                .Block.ReferenceFrame <= Av1ReferenceFrameType.Intra;
+
+            if (aboveIsIntra && leftIsIntra)
+            {
+                return 3;
+            }
+
+            return aboveIsIntra || leftIsIntra ? 1 : 0;
+        }
+
+        if (hasAbove)
+        {
+            return macroBlock
+                .GetRelativeModeInfo(-macroBlock.ModeInfoStride)
+                .Block.ReferenceFrame <= Av1ReferenceFrameType.Intra
+                    ? 2
+                    : 0;
+        }
+
+        if (hasLeft)
+        {
+            return macroBlock
+                .GetRelativeModeInfo(-1)
+                .Block.ReferenceFrame <= Av1ReferenceFrameType.Intra
+                    ? 2
+                    : 0;
+        }
+
+        return 0;
+    }
+
+    /// <summary>
+    /// Counts the single-reference labels used by the immediately above and left encoded blocks.
+    /// </summary>
+    /// <param name="macroBlock">The current block's mapped neighbor state.</param>
+    /// <param name="referenceCounts">The eight-entry destination indexed by reference-frame label.</param>
+    public static void CollectNeighborReferenceCounts(
+        Av1MacroBlockD macroBlock,
+        Span<byte> referenceCounts)
+    {
+        // The caller supplies short-lived fixed storage for one block. Clearing it here keeps unavailable
+        // neighbors from retaining votes collected for a preceding block.
+        referenceCounts.Clear();
+        if (macroBlock.IsUpAvailable)
+        {
+            Av1ReferenceFrameType referenceFrame = macroBlock
+                .GetRelativeModeInfo(-macroBlock.ModeInfoStride)
+                .Block.ReferenceFrame;
+
+            if (referenceFrame > Av1ReferenceFrameType.Intra)
+            {
+                referenceCounts[(int)referenceFrame]++;
+            }
+        }
+
+        if (macroBlock.IsLeftAvailable)
+        {
+            Av1ReferenceFrameType referenceFrame = macroBlock
+                .GetRelativeModeInfo(-1)
+                .Block.ReferenceFrame;
+
+            if (referenceFrame > Av1ReferenceFrameType.Intra)
+            {
+                referenceCounts[(int)referenceFrame]++;
+            }
+        }
+    }
+
     /// <summary>
     /// Writes luma and chroma palette-mode syntax for a block.
     /// </summary>
@@ -1498,6 +1738,26 @@ internal partial class Av1TileWriter
         }
     }
 
+    /// <summary>
+    /// Determines whether a single-reference encoder block carries switchable interpolation symbols.
+    /// </summary>
+    /// <param name="frameHeader">The current frame header.</param>
+    /// <param name="modeInfo">The selected block syntax.</param>
+    /// <returns>Whether the block writes a vertical filter and, when enabled, a horizontal filter.</returns>
+    public static bool UsesSwitchableInterpolation(ObuFrameHeader frameHeader, Av1EncoderBlockModeInfo modeInfo)
+    {
+        if (frameHeader.InterpolationFilter != Av1InterpolationFilter.Switchable || modeInfo.SkipMode)
+        {
+            return false;
+        }
+
+        // Global identity and affine models infer the regular filter on blocks at least 8x8. Translation still
+        // carries filter symbols, including integer translations. Residual skip does not suppress these symbols.
+        return modeInfo.Mode != Av1PredictionMode.GlobalMotionVector ||
+            Math.Min(modeInfo.BlockSize.GetWidth(), modeInfo.BlockSize.GetHeight()) < Av1BlockSize.Block8x8.GetWidth() ||
+            frameHeader.GetGlobalMotionParameters()[(int)modeInfo.ReferenceFrame - 1].Type == Av1GlobalMotionType.Translation;
+    }
+
     /// <summary>
     /// Determines whether the current frame permits intra block copy.
     /// </summary>
@@ -1603,14 +1863,15 @@ internal partial class Av1TileWriter
             return;
         }
 
+        Span<int> cdefPreset = pcs.CdefPreset.Span.Slice(
+            tileIndex * Av1Constants.CdefUnitsPerSuperblock,
+            Av1Constants.CdefUnitsPerSuperblock);
+
         // Each superblock begins with all contained 64x64 filter units unassigned.
         if ((modeInfoPosition.Y & (scs.SequenceHeader.SuperblockModeInfoSize - 1)) == 0 &&
             (modeInfoPosition.X & (scs.SequenceHeader.SuperblockModeInfoSize - 1)) == 0)
         {
-            pcs.CdefPreset[tileIndex][0] = -1;
-            pcs.CdefPreset[tileIndex][1] = -1;
-            pcs.CdefPreset[tileIndex][2] = -1;
-            pcs.CdefPreset[tileIndex][3] = -1;
+            cdefPreset.Fill(-1);
         }
 
         // The strength is coded once, at the first non-skipped block in each 64x64 CDEF filter unit.
@@ -1619,7 +1880,7 @@ internal partial class Av1TileWriter
         int unitRow = (modeInfoPosition.Y & cdefSize) != 0 ? 1 : 0;
         int index = scs.SequenceHeader.Use128x128Superblock ? unitColumn + (2 * unitRow) : 0;
 
-        if (pcs.CdefPreset[tileIndex][index] == -1 && !skip)
+        if (cdefPreset[index] == -1 && !skip)
         {
             int firstBlockMask = ~(cdefSize - 1);
             Point firstBlockPosition = new(
@@ -1630,7 +1891,7 @@ internal partial class Av1TileWriter
             // CDEF strength belongs to the first mode-info block in the 64x64 filter unit even when skipped
             // blocks delay transmission until a later coding block.
             writer.WriteCdefStrength(firstBlock.CdefStrength, frameHeader.CdefParameters.BitCount);
-            pcs.CdefPreset[tileIndex][index] = firstBlock.CdefStrength;
+            cdefPreset[index] = firstBlock.CdefStrength;
         }
     }
 
@@ -2036,7 +2297,9 @@ internal partial class Av1TileWriter
         int transformBlockHeight = transformSize.Get4x4HighCount();
         int transformWidth = transformSize.GetWidth();
         int transformHeight = transformSize.GetHeight();
-        bool usesInterTransformSet = entropyCodingContext.MacroBlockModeInfo.Block.UseIntraBlockCopy;
+        bool usesInterTransformSet =
+            entropyCodingContext.MacroBlockModeInfo.Block.ReferenceFrame > Av1ReferenceFrameType.Intra ||
+            entropyCodingContext.MacroBlockModeInfo.Block.UseIntraBlockCopy;
         Av1ComponentType componentType = isLuma
             ? Av1ComponentType.Luminance
             : Av1ComponentType.Chroma;
@@ -2240,6 +2503,7 @@ internal partial class Av1TileWriter
     /// <param name="macroBlock">The reusable macroblock edge and neighbor state.</param>
     /// <param name="block">The encoder block state.</param>
     /// <param name="skip">A value indicating whether residual coefficients are omitted.</param>
+    /// <param name="beforeSkip">Whether the segment identifier is written before the skip flag.</param>
     private static void WriteSegmentId(
         Av1PictureControlSet pcs,
         Av1SymbolEncoder writer,
@@ -2247,7 +2511,8 @@ internal partial class Av1TileWriter
         Point blockOrigin,
         Av1MacroBlockD macroBlock,
         ref Av1EncoderBlockStruct block,
-        bool skip)
+        bool skip,
+        bool beforeSkip)
     {
         ObuSegmentationParameters segmentation_params = pcs.Parent.FrameHeader.SegmentationParameters;
         if (!segmentation_params.Enabled)
@@ -2256,9 +2521,9 @@ internal partial class Av1TileWriter
         }
 
         int spatial_pred = GetSpatialSegmentationPrediction(pcs, macroBlock, blockOrigin, out int cdf_num);
-        if (skip)
+        if (!beforeSkip && skip)
         {
-            // With segment-id-before-skip syntax, a skipped block inherits the spatial predictor without coding a residual ID.
+            // Post-skip segment syntax can infer the spatial predictor once the decoder already knows the block is skipped.
             pcs.UpdateSegmentation(blockSize, blockOrigin, spatial_pred);
             block.SegmentId = spatial_pred;
             return;

src/ImageSharp/Formats/Heif/Components/Alpha/HeifPlanarAlphaEncoder.cs

@@ -15,6 +15,28 @@ namespace SixLabors.ImageSharp.Formats.Heif.Components.Alpha;
 /// </summary>
 internal static class HeifPlanarAlphaEncoder
 {
+    /// <summary>
+    /// The number of float elements sharing storage with one packed <see cref="Rgba64"/> value.
+    /// </summary>
+    private const int Rgba64FloatElementCount = 2;
+
+    /// <summary>
+    /// The additional float element receiving one extracted alpha value.
+    /// </summary>
+    private const int AlphaFloatElementCount = 1;
+
+    /// <summary>
+    /// The complete reusable row-storage length per source pixel.
+    /// </summary>
+    private const int RowFloatElementCount = Rgba64FloatElementCount + AlphaFloatElementCount;
+
+    /// <summary>
+    /// Gets the reusable row-storage length required for the specified source width.
+    /// </summary>
+    /// <param name="width">The source-row width.</param>
+    /// <returns>The required number of float elements.</returns>
+    public static int GetRowStorageLength(int width) => width * RowFloatElementCount;
+
     /// <summary>
     /// Converts one packed image frame into a full-range native alpha plane.
     /// </summary>
@@ -34,19 +56,85 @@ internal static class HeifPlanarAlphaEncoder
         where TSample : unmanaged
         where TStorer : struct, IHeifSampleConverter<TSample>
     {
-        int width = image.Width;
+        Rectangle sourceRectangle = new(0, 0, image.Width, image.Height);
+        Convert<TPixel, TBuffer, TSample, TStorer>(configuration, image, sourceRectangle, buffer);
+    }
+
+    /// <summary>
+    /// Converts one packed image region into a full-range native alpha plane.
+    /// </summary>
+    /// <typeparam name="TPixel">The packed source pixel type.</typeparam>
+    /// <typeparam name="TBuffer">The codec adapter exposing the destination plane.</typeparam>
+    /// <typeparam name="TSample">The native unsigned sample storage type.</typeparam>
+    /// <typeparam name="TStorer">The SIMD narrowing and storage operations for the sample type.</typeparam>
+    /// <param name="configuration">The configuration used for row allocation and pixel conversion.</param>
+    /// <param name="image">The packed source image frame.</param>
+    /// <param name="sourceRectangle">The source region mapped to the complete destination plane.</param>
+    /// <param name="buffer">The monochrome destination buffer.</param>
+    public static void Convert<TPixel, TBuffer, TSample, TStorer>(
+        Configuration configuration,
+        ImageFrame<TPixel> image,
+        Rectangle sourceRectangle,
+        TBuffer buffer)
+        where TPixel : unmanaged, IPixel<TPixel>
+        where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
+        where TSample : unmanaged
+        where TStorer : struct, IHeifSampleConverter<TSample>
+    {
+        int width = sourceRectangle.Width;
 
         // Rgba64 preserves the source pixel's normalized alpha precision before quantization to the requested AV1
         // depth. Both row views share one owner because their lifetimes never escape this conversion operation.
-        using IMemoryOwner<float> rowOwner = configuration.MemoryAllocator.Allocate<float>(width * 3);
-        Span<float> rowStorage = rowOwner.GetSpan();
-        Span<Rgba64> packed = MemoryMarshal.Cast<float, Rgba64>(rowStorage[..(width * 2)]);
-        Span<float> alpha = rowStorage.Slice(width * 2, width);
+        using IMemoryOwner<float> rowOwner = configuration.MemoryAllocator.Allocate<float>(
+            GetRowStorageLength(width));
+
+        Convert<TPixel, TBuffer, TSample, TStorer>(
+            configuration,
+            image,
+            sourceRectangle,
+            buffer,
+            rowOwner.GetSpan());
+    }
+
+    /// <summary>
+    /// Converts one packed image region using caller-owned reusable row storage.
+    /// </summary>
+    /// <typeparam name="TPixel">The packed source pixel type.</typeparam>
+    /// <typeparam name="TBuffer">The codec adapter exposing the destination plane.</typeparam>
+    /// <typeparam name="TSample">The native unsigned sample storage type.</typeparam>
+    /// <typeparam name="TStorer">The SIMD narrowing and storage operations for the sample type.</typeparam>
+    /// <param name="configuration">The configuration used for pixel conversion.</param>
+    /// <param name="image">The packed source image frame.</param>
+    /// <param name="sourceRectangle">The source region mapped to the complete destination plane.</param>
+    /// <param name="buffer">The monochrome destination buffer.</param>
+    /// <param name="rowStorage">Storage for one packed high-precision row and its extracted alpha values.</param>
+    public static void Convert<TPixel, TBuffer, TSample, TStorer>(
+        Configuration configuration,
+        ImageFrame<TPixel> image,
+        Rectangle sourceRectangle,
+        TBuffer buffer,
+        Span<float> rowStorage)
+        where TPixel : unmanaged, IPixel<TPixel>
+        where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
+        where TSample : unmanaged
+        where TStorer : struct, IHeifSampleConverter<TSample>
+    {
+        int width = sourceRectangle.Width;
+        Span<Rgba64> packed = MemoryMarshal.Cast<float, Rgba64>(
+            rowStorage[..(width * Rgba64FloatElementCount)]);
+
+        Span<float> alpha = rowStorage.Slice(
+            width * Rgba64FloatElementCount,
+            width * AlphaFloatElementCount);
+
         float maximum = (1 << buffer.LumaBitDepth) - 1;
         float scale = maximum / ushort.MaxValue;
-        for (int y = 0; y < image.Height; y++)
+        for (int y = 0; y < sourceRectangle.Height; y++)
         {
-            ReadOnlySpan<TPixel> source = image.PixelBuffer.DangerousGetRowSpan(y);
+            ReadOnlySpan<TPixel> source = image.PixelBuffer
+                .DangerousGetRowSpan(sourceRectangle.Y + y)
+                .Slice(sourceRectangle.X, sourceRectangle.Width);
+
             PixelOperations<TPixel>.Instance.ToRgba64(configuration, source, packed);
             ExtractAlpha(packed, alpha);
             HeifSampleConversion.WriteSamples<TSample, TStorer>(

src/ImageSharp/Formats/Heif/Components/ColorConverters/HeifPlanarColorConverter.cs

@@ -3,8 +3,6 @@
 
 using System.Buffers;
 using System.Runtime.InteropServices;
-using SixLabors.ImageSharp.Advanced;
-using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 
@@ -15,6 +13,16 @@ namespace SixLabors.ImageSharp.Formats.Heif.Components;
 /// </summary>
 internal static class HeifPlanarColorConverter
 {
+    /// <summary>
+    /// The number of planar color components retained for each source row.
+    /// </summary>
+    private const int ColorComponentCount = 3;
+
+    /// <summary>
+    /// The number of source rows consumed together by vertically subsampled chroma.
+    /// </summary>
+    private const int VerticallySubsampledRowCount = 2;
+
     /// <summary>
     /// The largest value represented by an eight-bit packed RGB component.
     /// </summary>
@@ -26,7 +34,20 @@ internal static class HeifPlanarColorConverter
     private const float UShortMaximum = ushort.MaxValue;
 
     /// <summary>
-    /// Converts native unsigned 16-bit component storage to packed pixels and selects eligible exact integer kernels.
+    /// Converts complete native unsigned 16-bit component planes to packed pixels.
+    /// </summary>
+    public static void ConvertToRgb<TPixel, TBuffer>(
+        Configuration configuration,
+        TBuffer buffer,
+        ImageFrame<TPixel> image,
+        in HeifColorConversionParameters parameters,
+        HeifColorConversionMode mode)
+        where TPixel : unmanaged, IPixel<TPixel>
+        where TBuffer : struct, IHeifPlanarSampleBuffer<ushort>
+        => ConvertToRgb(configuration, buffer, image, in parameters, mode, 0, 0);
+
+    /// <summary>
+    /// Converts a region of native unsigned 16-bit component storage to packed pixels and selects eligible exact integer kernels.
     /// </summary>
     /// <typeparam name="TPixel">The destination pixel type.</typeparam>
     /// <typeparam name="TBuffer">The codec adapter exposing the native component planes.</typeparam>
@@ -43,8 +64,8 @@ internal static class HeifPlanarColorConverter
         ImageFrame<TPixel> image,
         in HeifColorConversionParameters parameters,
         HeifColorConversionMode mode,
-        int sourceX = 0,
-        int sourceY = 0)
+        int sourceX,
+        int sourceY)
         where TPixel : unmanaged, IPixel<TPixel>
         where TBuffer : struct, IHeifPlanarSampleBuffer<ushort>
     {
@@ -74,7 +95,22 @@ internal static class HeifPlanarColorConverter
     }
 
     /// <summary>
-    /// Converts native component planes to packed pixels.
+    /// Converts complete native component planes to packed pixels.
+    /// </summary>
+    public static void ConvertToRgb<TPixel, TBuffer, TSample, TLoader>(
+        Configuration configuration,
+        TBuffer buffer,
+        ImageFrame<TPixel> image,
+        in HeifColorConversionParameters parameters,
+        HeifColorConversionMode mode)
+        where TPixel : unmanaged, IPixel<TPixel>
+        where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
+        where TSample : unmanaged
+        where TLoader : struct, IHeifSampleConverter<TSample>
+        => ConvertToRgb<TPixel, TBuffer, TSample, TLoader>(configuration, buffer, image, in parameters, mode, 0, 0);
+
+    /// <summary>
+    /// Converts a region of native component planes to packed pixels.
     /// </summary>
     /// <typeparam name="TPixel">The destination pixel type.</typeparam>
     /// <typeparam name="TBuffer">The codec adapter exposing the native component planes.</typeparam>
@@ -93,8 +129,8 @@ internal static class HeifPlanarColorConverter
         ImageFrame<TPixel> image,
         in HeifColorConversionParameters parameters,
         HeifColorConversionMode mode,
-        int sourceX = 0,
-        int sourceY = 0)
+        int sourceX,
+        int sourceY)
         where TPixel : unmanaged, IPixel<TPixel>
         where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
         where TSample : unmanaged
@@ -139,12 +175,48 @@ internal static class HeifPlanarColorConverter
         where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
         where TSample : unmanaged
         where TStorer : struct, IHeifSampleConverter<TSample>
+    {
+        Rectangle sourceRectangle = new(0, 0, image.Width, image.Height);
+        ConvertFromRgb<TPixel, TBuffer, TSample, TStorer>(
+            configuration,
+            image,
+            sourceRectangle,
+            buffer,
+            in parameters,
+            mode);
+    }
+
+    /// <summary>
+    /// Converts a rectangular packed-pixel region to native component planes.
+    /// </summary>
+    /// <typeparam name="TPixel">The source pixel type.</typeparam>
+    /// <typeparam name="TBuffer">The codec adapter exposing the native component planes.</typeparam>
+    /// <typeparam name="TSample">The native unsigned sample storage type.</typeparam>
+    /// <typeparam name="TStorer">The SIMD narrowing and storage operations for the sample type.</typeparam>
+    /// <param name="configuration">The configuration used for allocation and pixel conversion.</param>
+    /// <param name="image">The source image frame.</param>
+    /// <param name="sourceRectangle">The source region mapped to the complete destination buffer.</param>
+    /// <param name="buffer">The destination component-plane buffer.</param>
+    /// <param name="parameters">The resolved H.273 conversion parameters.</param>
+    /// <param name="mode">The resolved H.273 conversion mode.</param>
+    public static void ConvertFromRgb<TPixel, TBuffer, TSample, TStorer>(
+        Configuration configuration,
+        ImageFrame<TPixel> image,
+        Rectangle sourceRectangle,
+        TBuffer buffer,
+        in HeifColorConversionParameters parameters,
+        HeifColorConversionMode mode)
+        where TPixel : unmanaged, IPixel<TPixel>
+        where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
+        where TSample : unmanaged
+        where TStorer : struct, IHeifSampleConverter<TSample>
     {
         HeifColorConverterBase colorConverter = HeifColorConverterBase.Create(mode, in parameters, buffer.IsMonochrome);
         RgbToYuvRowConverter<TPixel, TBuffer, TSample, TStorer> converter = new(
             configuration,
             buffer,
             image,
+            sourceRectangle,
             colorConverter,
             in parameters);
 
@@ -156,10 +228,60 @@ internal static class HeifPlanarColorConverter
             return;
         }
 
-        using IMemoryOwner<Rgb48> packedOwner = configuration.MemoryAllocator.Allocate<Rgb48>(image.Width);
-        converter.Convert(packedOwner.GetSpan()[..image.Width], components);
+        using IMemoryOwner<Rgb48> packedOwner = configuration.MemoryAllocator.Allocate<Rgb48>(sourceRectangle.Width);
+        converter.Convert(packedOwner.GetSpan()[..sourceRectangle.Width], components);
     }
 
+    /// <summary>
+    /// Converts a rectangular packed-pixel region using a retained color converter and caller-owned row storage.
+    /// </summary>
+    /// <typeparam name="TPixel">The source pixel type.</typeparam>
+    /// <typeparam name="TBuffer">The codec adapter exposing the native component planes.</typeparam>
+    /// <typeparam name="TSample">The native unsigned sample storage type.</typeparam>
+    /// <typeparam name="TStorer">The SIMD narrowing and storage operations for the sample type.</typeparam>
+    /// <param name="configuration">The configuration used for pixel conversion.</param>
+    /// <param name="image">The source image frame.</param>
+    /// <param name="sourceRectangle">The source region mapped to the complete destination buffer.</param>
+    /// <param name="buffer">The destination component-plane buffer.</param>
+    /// <param name="parameters">The resolved H.273 conversion parameters.</param>
+    /// <param name="colorConverter">The retained converter matching <paramref name="parameters"/>.</param>
+    /// <param name="packed">The reusable high-bit-depth packed RGB row, or an empty span for eight-bit input.</param>
+    /// <param name="components">The reusable planar component rows.</param>
+    public static void ConvertFromRgb<TPixel, TBuffer, TSample, TStorer>(
+        Configuration configuration,
+        ImageFrame<TPixel> image,
+        Rectangle sourceRectangle,
+        TBuffer buffer,
+        in HeifColorConversionParameters parameters,
+        HeifColorConverterBase colorConverter,
+        Span<Rgb48> packed,
+        Span<float> components)
+        where TPixel : unmanaged, IPixel<TPixel>
+        where TBuffer : struct, IHeifPlanarSampleBuffer<TSample>
+        where TSample : unmanaged
+        where TStorer : struct, IHeifSampleConverter<TSample>
+    {
+        RgbToYuvRowConverter<TPixel, TBuffer, TSample, TStorer> converter = new(
+            configuration,
+            buffer,
+            image,
+            sourceRectangle,
+            colorConverter,
+            in parameters);
+
+        converter.Convert(packed, components);
+    }
+
+    /// <summary>
+    /// Gets the planar float storage required to convert one row, or one vertically subsampled row pair.
+    /// </summary>
+    /// <param name="width">The source-row width.</param>
+    /// <param name="isMonochrome">Whether only luma is written.</param>
+    /// <param name="subsamplingY">The vertical chroma-subsampling shift.</param>
+    /// <returns>The required number of float elements.</returns>
+    public static int GetRgbToYuvComponentBufferLength(int width, bool isMonochrome, int subsamplingY)
+        => width * ColorComponentCount * (subsamplingY == 0 || isMonochrome ? 1 : VerticallySubsampledRowCount);
+
     /// <summary>
     /// Resolves the two chroma rows and quarter-sample weight surrounding a luma row.
     /// </summary>
@@ -484,6 +606,11 @@ internal static class HeifPlanarColorConverter
         /// </summary>
         private readonly ImageFrame<TPixel> image;
 
+        /// <summary>
+        /// The source region mapped to the complete destination planes.
+        /// </summary>
+        private readonly Rectangle sourceRectangle;
+
         /// <summary>
         /// The selected H.273 color converter.
         /// </summary>
@@ -535,18 +662,21 @@ internal static class HeifPlanarColorConverter
         /// <param name="configuration">The configuration used for pixel conversion.</param>
         /// <param name="buffer">The codec adapter exposing the destination component planes.</param>
         /// <param name="image">The source image frame.</param>
+        /// <param name="sourceRectangle">The source region mapped to the complete destination planes.</param>
         /// <param name="colorConverter">The selected H.273 color converter.</param>
         /// <param name="parameters">The resolved H.273 component ranges.</param>
         public RgbToYuvRowConverter(
             Configuration configuration,
             TBuffer buffer,
             ImageFrame<TPixel> image,
+            Rectangle sourceRectangle,
             HeifColorConverterBase colorConverter,
             in HeifColorConversionParameters parameters)
         {
             this.configuration = configuration;
             this.buffer = buffer;
             this.image = image;
+            this.sourceRectangle = sourceRectangle;
             this.colorConverter = colorConverter;
             this.lumaMaximum = parameters.LumaSampleMaximum;
             this.chromaMaximum = parameters.ChromaSampleMaximum;
@@ -566,7 +696,8 @@ internal static class HeifPlanarColorConverter
         /// <summary>
         /// Gets the number of float elements required by the reusable component buffer.
         /// </summary>
-        public readonly int ComponentBufferLength => this.image.Width * (this.subsamplingY == 0 || this.isMonochrome ? 3 : 6);
+        public readonly int ComponentBufferLength
+            => GetRgbToYuvComponentBufferLength(this.sourceRectangle.Width, this.isMonochrome, this.subsamplingY);
 
         /// <summary>
         /// Converts every packed source row to the destination component planes.
@@ -575,13 +706,13 @@ internal static class HeifPlanarColorConverter
         /// <param name="components">The reusable planar component buffer.</param>
         public void Convert(Span<Rgb48> packed, Span<float> components)
         {
-            int width = this.image.Width;
+            int width = this.sourceRectangle.Width;
             Span<float> luma0 = components[..width];
             Span<float> blue0 = components.Slice(width, width);
             Span<float> red0 = components.Slice(width * 2, width);
             if (this.subsamplingY == 0)
             {
-                for (int y = 0; y < this.image.Height; y++)
+                for (int y = 0; y < this.sourceRectangle.Height; y++)
                 {
                     this.ConvertSourceRow(y, packed, luma0, blue0, red0);
                     HeifSampleConversion.WriteSamples<TSample, TStorer>(
@@ -603,7 +734,7 @@ internal static class HeifPlanarColorConverter
             Span<float> luma1 = this.isMonochrome ? luma0 : components.Slice(width * 3, width);
             Span<float> blue1 = this.isMonochrome ? blue0 : components.Slice(width * 4, width);
             Span<float> red1 = this.isMonochrome ? red0 : components.Slice(width * 5, width);
-            int chromaHeight = (this.image.Height + 1) >> 1;
+            int chromaHeight = (this.sourceRectangle.Height + 1) >> 1;
             for (int destinationY = 0; destinationY < chromaHeight; destinationY++)
             {
                 // A vertically subsampled chroma row is owned by one two-row luma cell. Processing that cell as a
@@ -617,7 +748,7 @@ internal static class HeifPlanarColorConverter
                     this.colorConverter.LumaBias,
                     this.lumaMaximum);
 
-                bool hasSecondRow = sourceY + 1 < this.image.Height;
+                bool hasSecondRow = sourceY + 1 < this.sourceRectangle.Height;
                 if (hasSecondRow)
                 {
                     this.ConvertSourceRow(sourceY + 1, packed, luma1, blue1, red1);
@@ -659,7 +790,10 @@ internal static class HeifPlanarColorConverter
         /// <param name="chromaRed">The destination red-difference or third component values.</param>
         private void ConvertSourceRow(int y, Span<Rgb48> packed, Span<float> luma, Span<float> chromaBlue, Span<float> chromaRed)
         {
-            ReadOnlySpan<TPixel> source = this.image.PixelBuffer.DangerousGetRowSpan(y);
+            ReadOnlySpan<TPixel> source = this.image.PixelBuffer
+                .DangerousGetRowSpan(this.sourceRectangle.Y + y)
+                .Slice(this.sourceRectangle.X, this.sourceRectangle.Width);
+
             if (this.UsesByteInput)
             {
                 // JPEG's planar unpack contract reaches the existing pixel-specific SIMD implementation before

src/ImageSharp/Formats/Heif/GridHeifItemDecoder.cs

@@ -3,7 +3,6 @@
 
 using System.Buffers;
 using System.Buffers.Binary;
-using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Components.Alpha;
 using SixLabors.ImageSharp.Memory;
@@ -19,6 +18,31 @@ namespace SixLabors.ImageSharp.Formats.Heif;
 internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IHeifAlphaItemDecoder<TPixel>
     where TPixel : unmanaged, IPixel<TPixel>
 {
+    /// <summary>
+    /// The image-grid descriptor version defined by HEIF.
+    /// </summary>
+    private const byte GridDescriptorVersion = 0;
+
+    /// <summary>
+    /// The descriptor flag that selects 32-bit output dimensions instead of 16-bit dimensions.
+    /// </summary>
+    private const byte LargeDimensionsFlag = 1;
+
+    /// <summary>
+    /// The descriptor length when output dimensions use 16-bit fields.
+    /// </summary>
+    private const int ShortGridDescriptorLength = 8;
+
+    /// <summary>
+    /// The descriptor length when output dimensions use 32-bit fields.
+    /// </summary>
+    private const int LongGridDescriptorLength = 12;
+
+    /// <summary>
+    /// The minimum width and height of the first cell in a MIAF image grid.
+    /// </summary>
+    private const int MinimumGridCellDimension = 64;
+
     /// <summary>
     /// The item definitions available to the grid, indexed by item identifier.
     /// </summary>
@@ -122,9 +146,16 @@ internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IH
                     ref av1GridConfiguration,
                     cancellationToken);
 
-                if (tile.Width != tileWidth || tile.Height != tileHeight)
+                Size copySize = GetGridTileCopySize(
+                    descriptor,
+                    tileWidth,
+                    tileHeight,
+                    tileIndex);
+
+                if (!IsGridTileExtentValid(tile.Size, copySize, tileWidth, tileHeight))
                 {
-                    throw new InvalidImageContentException("The HEIF image grid contains tiles with mismatched dimensions.");
+                    throw new InvalidImageContentException(
+                        $"HEIF image grid tile {item.Id} has dimensions {tile.Size}, which cannot cover its {copySize} grid region.");
                 }
 
                 CopyGridTile(tile, result, descriptor, tileIndex, tileWidth, tileHeight);
@@ -165,16 +196,13 @@ internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IH
 
         tileWidth = tile.Width;
         tileHeight = tile.Height;
-        if (((long)tileWidth * descriptor.Columns) < descriptor.OutputSize.Width ||
-            ((long)tileHeight * descriptor.Rows) < descriptor.OutputSize.Height)
-        {
-            throw new InvalidImageContentException("The HEIF image grid tiles do not cover the output canvas.");
-        }
-
-        if (((long)tileWidth * (descriptor.Columns - 1)) >= descriptor.OutputSize.Width ||
-            ((long)tileHeight * (descriptor.Rows - 1)) >= descriptor.OutputSize.Height)
+        ValidateGridCoverage(descriptor, tileWidth, tileHeight);
+        ValidateGridDimensions(descriptor, tile.Size, av1GridConfiguration);
+        Size copySize = GetGridTileCopySize(descriptor, tileWidth, tileHeight, 0);
+        if (!IsGridTileExtentValid(tile.Size, copySize, tileWidth, tileHeight))
         {
-            throw new InvalidImageContentException("The HEIF image grid edge tiles do not overlap the output canvas.");
+            throw new InvalidImageContentException(
+                $"HEIF image grid tile {item.Id} has dimensions {tile.Size}, which cannot cover its {copySize} grid region.");
         }
 
         Image<TPixel> result = new(
@@ -279,12 +307,20 @@ internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IH
         IReadOnlyList<uint> linked = this.GetLinkedTileIds(gridItem, descriptor);
         Heif4CharCode tileType = default;
         Av1CodecConfiguration? av1GridConfiguration = null;
-        Size tileSize = default;
+        HeifItem firstItem = this.items[linked[0]];
+        if (firstItem.Extent == default)
+        {
+            throw new InvalidImageContentException($"HEIF alpha grid tile {firstItem.Id} has no spatial extent.");
+        }
+
+        Size tileSize = firstItem.Extent;
+        ValidateGridCoverage(descriptor, tileSize.Width, tileSize.Height);
 
         // Validate the complete grid before mutating the color frame. IgnoreImageData can then omit a failed alpha
         // grid without leaving a partially composed prefix in the returned image.
-        foreach (uint id in linked)
+        for (int tileIndex = 0; tileIndex < linked.Count; tileIndex++)
         {
+            uint id = linked[tileIndex];
             HeifItem item = this.items[id];
             ValidateTileConfiguration(item, ref tileType, ref av1GridConfiguration);
             if (HeifCompressionFactory.GetDecoder<TPixel>(item.Type) is not IHeifAlphaItemDecoder<TPixel>)
@@ -297,29 +333,23 @@ internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IH
                 throw new InvalidImageContentException($"HEIF alpha grid tile {item.Id} has no spatial extent.");
             }
 
-            if (tileSize == default)
-            {
-                tileSize = item.Extent;
-            }
-            else if (item.Extent != tileSize)
+            Size copySize = GetGridTileCopySize(
+                descriptor,
+                tileSize.Width,
+                tileSize.Height,
+                tileIndex);
+
+            if (!IsGridTileExtentValid(item.Extent, copySize, tileSize.Width, tileSize.Height))
             {
-                throw new InvalidImageContentException("The HEIF alpha grid contains tiles with mismatched dimensions.");
+                throw new InvalidImageContentException(
+                    $"HEIF alpha grid tile {item.Id} has dimensions {item.Extent}, which cannot cover its {copySize} grid region.");
             }
         }
 
+        ValidateGridDimensions(descriptor, tileSize, av1GridConfiguration);
+
         int gridWidth = descriptor.OutputSize.Width;
         int gridHeight = descriptor.OutputSize.Height;
-        if (((long)tileSize.Width * descriptor.Columns) < gridWidth || ((long)tileSize.Height * descriptor.Rows) < gridHeight)
-        {
-            throw new InvalidImageContentException("The HEIF alpha grid tiles do not cover the output canvas.");
-        }
-
-        if (((long)tileSize.Width * (descriptor.Columns - 1)) >= gridWidth ||
-            ((long)tileSize.Height * (descriptor.Rows - 1)) >= gridHeight)
-        {
-            throw new InvalidImageContentException("The HEIF alpha grid edge tiles do not overlap the output canvas.");
-        }
-
         if (descriptor.OutputSize != outputSize || destinationRectangle.Size != outputSize)
         {
             throw new InvalidImageContentException("The HEIF alpha grid dimensions do not match the color grid dimensions.");
@@ -340,22 +370,109 @@ internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IH
             int row = tileIndex / descriptor.Columns;
             int destinationX = destinationRectangle.X + (column * tileSize.Width);
             int destinationY = destinationRectangle.Y + (row * tileSize.Height);
-            int copyWidth = Math.Min(tileSize.Width, destinationRectangle.Right - destinationX);
-            int copyHeight = Math.Min(tileSize.Height, destinationRectangle.Bottom - destinationY);
-            Rectangle tileDestination = new(destinationX, destinationY, copyWidth, copyHeight);
+            Size copySize = GetGridTileCopySize(
+                descriptor,
+                tileSize.Width,
+                tileSize.Height,
+                tileIndex);
+
+            Rectangle tileDestination = new(destinationX, destinationY, copySize.Width, copySize.Height);
 
             decoder.DecodeAlphaItemData(
                 options,
                 item,
                 itemMemory.GetSpan(),
                 destination,
-                tileSize,
+                item.Extent,
                 tileDestination,
                 premultiplied,
                 cancellationToken);
         }
     }
 
+    /// <summary>
+    /// Validates that the first cell dimensions cover the grid while leaving a nonempty final row and column.
+    /// </summary>
+    private static void ValidateGridCoverage(in GridDescriptor descriptor, int tileWidth, int tileHeight)
+    {
+        if (((long)tileWidth * descriptor.Columns) < descriptor.OutputSize.Width ||
+            ((long)tileHeight * descriptor.Rows) < descriptor.OutputSize.Height)
+        {
+            throw new InvalidImageContentException("The HEIF image grid tiles do not cover the output canvas.");
+        }
+
+        if (((long)tileWidth * (descriptor.Columns - 1)) >= descriptor.OutputSize.Width ||
+            ((long)tileHeight * (descriptor.Rows - 1)) >= descriptor.OutputSize.Height)
+        {
+            throw new InvalidImageContentException("The HEIF image grid edge tiles do not overlap the output canvas.");
+        }
+    }
+
+    /// <summary>
+    /// Gets the portion of one cell that overlaps the output canvas.
+    /// </summary>
+    private static Size GetGridTileCopySize(
+        in GridDescriptor descriptor,
+        int tileWidth,
+        int tileHeight,
+        int tileIndex)
+    {
+        int column = tileIndex % descriptor.Columns;
+        int row = tileIndex / descriptor.Columns;
+        int copyWidth = column == descriptor.Columns - 1
+            ? descriptor.OutputSize.Width - (tileWidth * (descriptor.Columns - 1))
+            : tileWidth;
+
+        int copyHeight = row == descriptor.Rows - 1
+            ? descriptor.OutputSize.Height - (tileHeight * (descriptor.Rows - 1))
+            : tileHeight;
+
+        return new Size(copyWidth, copyHeight);
+    }
+
+    /// <summary>
+    /// Determines whether a cell can cover its output region without exceeding the first cell's dimensions.
+    /// </summary>
+    private static bool IsGridTileExtentValid(Size extent, Size copySize, int tileWidth, int tileHeight)
+        => extent.Width >= copySize.Width
+            && extent.Width <= tileWidth
+            && extent.Height >= copySize.Height
+            && extent.Height <= tileHeight;
+
+    /// <summary>
+    /// Validates the MIAF cell-size and chroma-alignment rules established by the first grid cell.
+    /// </summary>
+    private static void ValidateGridDimensions(
+        in GridDescriptor descriptor,
+        Size tileSize,
+        Av1CodecConfiguration? av1GridConfiguration)
+    {
+        if (tileSize.Width < MinimumGridCellDimension || tileSize.Height < MinimumGridCellDimension)
+        {
+            throw new InvalidImageContentException(
+                $"HEIF image grid cells must be at least {MinimumGridCellDimension} samples wide and high.");
+        }
+
+        if (av1GridConfiguration is null || av1GridConfiguration.IsMonochrome)
+        {
+            return;
+        }
+
+        if (av1GridConfiguration.ChromaSubsamplingX &&
+            (((descriptor.OutputSize.Width & 1) != 0) || ((tileSize.Width & 1) != 0)))
+        {
+            throw new InvalidImageContentException(
+                "HEIF image grid widths must be even when AV1 chroma is horizontally subsampled.");
+        }
+
+        if (av1GridConfiguration.ChromaSubsamplingY &&
+            (((descriptor.OutputSize.Height & 1) != 0) || ((tileSize.Height & 1) != 0)))
+        {
+            throw new InvalidImageContentException(
+                "HEIF image grid heights must be even when AV1 chroma is vertically subsampled.");
+        }
+    }
+
     /// <summary>
     /// Parses and validates the fixed HEIF image-grid descriptor fields used by both color and alpha composition.
     /// </summary>
@@ -363,38 +480,43 @@ internal sealed class GridHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>, IH
     /// <returns>The validated row, column, and output dimensions.</returns>
     private static GridDescriptor ParseGridDescriptor(ReadOnlySpan<byte> data)
     {
-        if (data.Length < 8)
+        if (data.Length < ShortGridDescriptorLength)
         {
             throw new InvalidImageContentException("The HEIF image grid descriptor is truncated.");
         }
 
-        byte version = data[0];
-        if (version != 0)
+        int offset = 0;
+        byte version = data[offset++];
+        if (version != GridDescriptorVersion)
         {
             throw new InvalidImageContentException($"The HEIF image grid descriptor has unsupported version {version}.");
         }
 
-        bool usesLargeDimensions = (data[1] & 1) != 0;
-        int descriptorLength = usesLargeDimensions ? 12 : 8;
+        byte flags = data[offset++];
+        bool usesLargeDimensions = (flags & LargeDimensionsFlag) != 0;
+        int descriptorLength = usesLargeDimensions ? LongGridDescriptorLength : ShortGridDescriptorLength;
         if (data.Length != descriptorLength)
         {
             throw new InvalidImageContentException("The HEIF image grid descriptor has an invalid length.");
         }
 
+        int rows = data[offset++] + 1;
+        int columns = data[offset++] + 1;
         uint outputWidth = usesLargeDimensions
-            ? BinaryPrimitives.ReadUInt32BigEndian(data[4..])
-            : BinaryPrimitives.ReadUInt16BigEndian(data[4..]);
+            ? BinaryPrimitives.ReadUInt32BigEndian(data[offset..])
+            : BinaryPrimitives.ReadUInt16BigEndian(data[offset..]);
 
+        offset += usesLargeDimensions ? sizeof(uint) : sizeof(ushort);
         uint outputHeight = usesLargeDimensions
-            ? BinaryPrimitives.ReadUInt32BigEndian(data[8..])
-            : BinaryPrimitives.ReadUInt16BigEndian(data[6..]);
+            ? BinaryPrimitives.ReadUInt32BigEndian(data[offset..])
+            : BinaryPrimitives.ReadUInt16BigEndian(data[offset..]);
 
         if (outputWidth is 0 or > int.MaxValue || outputHeight is 0 or > int.MaxValue)
         {
             throw new InvalidImageContentException("The HEIF image grid descriptor has invalid output dimensions.");
         }
 
-        return new GridDescriptor(data[2] + 1, data[3] + 1, new Size((int)outputWidth, (int)outputHeight));
+        return new GridDescriptor(rows, columns, new Size((int)outputWidth, (int)outputHeight));
     }
 
     /// <summary>

src/ImageSharp/Formats/Heif/HeifDecoderCore.cs

@@ -16,6 +16,7 @@ using SixLabors.ImageSharp.Metadata.Profiles.Icc;
 using SixLabors.ImageSharp.Metadata.Profiles.Xmp;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Transforms;
 
 namespace SixLabors.ImageSharp.Formats.Heif;
 
@@ -266,23 +267,32 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
     /// <returns>The bounded selected image-sequence model.</returns>
     private HeifSequence ParseImageSequence(BufferedReadStream stream)
     {
+        this.items.Clear();
+        this.itemLinks.Clear();
+        this.itemDataOffset = -1;
+        this.itemDataLength = 0;
+
         HeifSequence? sequence = null;
         while (stream.Position < stream.Length)
         {
             long boxLength = HeifBoxReader.ReadHeader(stream, stream.Length, this.boxHeaderScratch, out Heif4CharCode boxType, true);
-            if (boxType == Heif4CharCode.Moov)
+            switch (boxType)
             {
-                if (sequence is not null)
-                {
-                    throw new InvalidImageContentException("The HEIF image sequence contains more than one movie box.");
-                }
+                case Heif4CharCode.Meta:
+                    this.ParseMetadata(stream, boxLength);
+                    break;
+                case Heif4CharCode.Moov:
+                    if (sequence is not null)
+                    {
+                        throw new InvalidImageContentException("The HEIF image sequence contains more than one movie box.");
+                    }
 
-                sequence = this.sequenceParser.Parse(stream, boxLength, this.fileStartOffset);
-            }
-            else
-            {
-                // Sequence samples use absolute file offsets, so unrelated top-level payloads never need buffering.
-                HeifBoxReader.Skip(stream, boxLength);
+                    sequence = this.sequenceParser.Parse(stream, boxLength, this.fileStartOffset);
+                    break;
+                default:
+                    // Sequence samples and image items use file-relative offsets, so payload boxes never need buffering.
+                    HeifBoxReader.Skip(stream, boxLength);
+                    break;
             }
         }
 
@@ -297,9 +307,18 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
     private ImageInfo IdentifyImageSequence(HeifSequence sequence)
     {
         HeifSequenceTrack colorTrack = sequence.ColorTrack;
-        this.UpdateSequenceMetadata(this.metadata, sequence);
-        ImageFrameMetadata[] frameMetadata = CreateSequenceFrameMetadata(colorTrack);
-        this.Dimensions = GetSequencePresentationExtent(colorTrack);
+        HeifItem primaryItem = this.FindSequencePrimaryItem();
+        bool animateRootFrame = IsPrimaryItemFirstSequenceSample(primaryItem, colorTrack);
+        Size sequenceExtent = GetSequencePresentationExtent(colorTrack);
+        this.Dimensions = animateRootFrame ? sequenceExtent : GetPresentationExtent(primaryItem);
+        if (!animateRootFrame && this.Dimensions != sequenceExtent)
+        {
+            throw new InvalidImageContentException("The primary image and image sequence have different presentation dimensions.");
+        }
+
+        this.UpdateMetadata(this.metadata, primaryItem);
+        this.UpdateSequenceMetadata(this.metadata, sequence, animateRootFrame);
+        ImageFrameMetadata[] frameMetadata = CreateSequenceFrameMetadata(colorTrack, animateRootFrame);
         return new ImageInfo(this.Dimensions, this.metadata, frameMetadata);
     }
 
@@ -318,170 +337,241 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
         where TPixel : unmanaged, IPixel<TPixel>
     {
         HeifSequenceTrack colorTrack = sequence.ColorTrack;
-        this.UpdateSequenceMetadata(this.metadata, sequence);
-        ImageFrame<TPixel>[] colorFrames = this.DecodeVisibleSequenceFrames<TPixel>(
-            stream,
-            colorTrack,
-            cancellationToken,
-            out int[] sampleIndices);
+        HeifItem primaryItem = this.FindSequencePrimaryItem();
+        bool animateRootFrame = IsPrimaryItemFirstSequenceSample(primaryItem, colorTrack);
+        this.UpdateSequenceMetadata(this.metadata, sequence, animateRootFrame);
+        Size codedSize = new(colorTrack.CodedWidth, colorTrack.CodedHeight);
+        Rectangle sourceRectangle = colorTrack.CleanAperture is not null
+            ? colorTrack.CleanAperture.Value.ToRectangle(codedSize)
+            : new Rectangle(Point.Empty, codedSize);
+
+        // HEIF stores counter-clockwise quarter turns; ImageSharp's exact modes are clockwise.
+        RotateMode rotation = colorTrack.RotationAngle switch
+        {
+            1 => RotateMode.Rotate270,
+            2 => RotateMode.Rotate180,
+            3 => RotateMode.Rotate90,
+            _ => RotateMode.None
+        };
+
+        Size presentationSize = rotation is RotateMode.Rotate90 or RotateMode.Rotate270
+            ? new Size(sourceRectangle.Height, sourceRectangle.Width)
+            : sourceRectangle.Size;
+
+        // The returned image owns every presented frame from the outset. A separate primary becomes its root;
+        // otherwise the first successfully decoded timed sample fills the root allocated here.
+        Image<TPixel> image = animateRootFrame
+            ? new Image<TPixel>(
+                this.configuration,
+                presentationSize.Width,
+                presentationSize.Height,
+                this.metadata)
+            : this.DecodePrimaryItem<TPixel>(stream, cancellationToken);
 
-        Image<TPixel>? image = null;
         try
         {
-            // Each codec frame owns its pixel buffer. The multi-frame image adopts those buffers directly instead
-            // of cloning a complete decoded frame on every append.
-            image = new Image<TPixel>(this.configuration, this.metadata, colorFrames);
+            if (image.Size != presentationSize)
+            {
+                throw new InvalidImageContentException(
+                    "The primary image and image sequence have different presentation dimensions.");
+            }
+
+            using Av1Decoder colorDecoder = new(this.configuration);
+
             HeifSequenceTrack? alphaTrack = sequence.AlphaTrack;
-            if (alphaTrack is not null)
+            (HeifSequenceTrack Track, Av1Decoder Decoder)? alphaState = alphaTrack is null
+                ? null
+                : (alphaTrack, new Av1Decoder(this.configuration));
+
+            using Av1Decoder? alphaDecoder = alphaState?.Decoder;
+
+            // Quarter turns need source and destination frames with opposite dimensions. Reuse one source frame
+            // across the sequence, then rotate each completed color-and-alpha sample into its final owned frame.
+            using ImageFrame<TPixel>? rotationSource = rotation == RotateMode.None
+                ? null
+                : new ImageFrame<TPixel>(
+                    this.configuration,
+                    sourceRectangle.Width,
+                    sourceRectangle.Height);
+
+            int decodedFrameCount = 0;
+            for (int sampleIndex = 0; sampleIndex < colorTrack.Samples.Length; sampleIndex++)
             {
-                int frameIndex = 0;
-                using Av1Decoder alphaDecoder = new(this.configuration);
-                for (int sampleIndex = 0; sampleIndex < alphaTrack.Samples.Length; sampleIndex++)
+                cancellationToken.ThrowIfCancellationRequested();
+                HeifSequenceSample colorSample = colorTrack.Samples[sampleIndex];
+                if (colorSample.IsHidden)
                 {
-                    cancellationToken.ThrowIfCancellationRequested();
-                    HeifSequenceSample alphaSample = alphaTrack.Samples[sampleIndex];
-                    if (alphaSample.IsHidden ||
-                        frameIndex >= colorFrames.Length ||
-                        sampleIndices[frameIndex] != sampleIndex)
+                    this.ExecuteImageDataSegmentAction(
+                        () => this.DecodeSequenceReference(stream, colorTrack, colorSample, colorDecoder));
+
+                    if (alphaState is not null)
                     {
+                        (HeifSequenceTrack Track, Av1Decoder Decoder) currentAlphaState = alphaState.Value;
+                        HeifSequenceSample alphaSample = currentAlphaState.Track.Samples[sampleIndex];
                         this.ExecuteImageDataSegmentAction(
-                            () => this.DecodeSequenceReference(stream, alphaTrack, alphaSample, alphaDecoder));
-
-                        continue;
+                            () => this.DecodeSequenceReference(
+                                stream,
+                                currentAlphaState.Track,
+                                alphaSample,
+                                currentAlphaState.Decoder));
                     }
 
-                    this.ExecuteImageDataSegmentAction(
-                        () => this.DecodeSequenceAlphaFrame(
-                            stream,
-                            alphaTrack,
-                            alphaSample,
-                            alphaDecoder,
-                            colorFrames[frameIndex],
-                            colorTrack.IsPremultiplied));
+                    continue;
+                }
 
-                    frameIndex++;
+                bool appendedDestination = false;
+                ImageFrame<TPixel> decodedFrame;
+                if (rotationSource is not null)
+                {
+                    decodedFrame = rotationSource;
+                }
+                else if (animateRootFrame && decodedFrameCount == 0)
+                {
+                    decodedFrame = image.Frames.RootFrame;
+                }
+                else
+                {
+                    decodedFrame = image.Frames.CreateFrame();
+                    appendedDestination = true;
                 }
-            }
 
-            ApplyPresentationTransforms(
-                image,
-                colorTrack.CleanAperture,
-                colorTrack.RotationAngle,
-                colorTrack.MirrorAxis);
+                bool colorDecoded = false;
+                this.ExecuteImageDataSegmentAction(
+                    () =>
+                    {
+                        this.DecodeSequenceFrame(
+                            stream,
+                            colorTrack,
+                            colorSample,
+                            colorDecoder,
+                            sourceRectangle,
+                            decodedFrame);
 
-            if (!this.Options.SkipMetadata)
-            {
-                image.Metadata.CicpProfile ??= image.Frames.RootFrame.Metadata.CicpProfile?.DeepClone();
-                _ = this.TryConvertIccProfile(image);
-            }
-            else
-            {
-                foreach (ImageFrame<TPixel> frame in image.Frames)
+                        colorDecoded = true;
+                    });
+
+                if (!colorDecoded)
                 {
-                    frame.Metadata.CicpProfile = null;
+                    if (alphaState is not null)
+                    {
+                        (HeifSequenceTrack Track, Av1Decoder Decoder) currentAlphaState = alphaState.Value;
+                        HeifSequenceSample alphaSample = currentAlphaState.Track.Samples[sampleIndex];
+                        this.ExecuteImageDataSegmentAction(
+                            () => this.DecodeSequenceReference(
+                                stream,
+                                currentAlphaState.Track,
+                                alphaSample,
+                                currentAlphaState.Decoder));
+                    }
+
+                    if (appendedDestination)
+                    {
+                        image.Frames.RemoveFrame(image.Frames.Count - 1);
+                    }
+
+                    continue;
                 }
-            }
 
-            this.Dimensions = image.Size;
-            return image;
-        }
-        catch
-        {
-            if (image is not null)
-            {
-                image.Dispose();
-            }
-            else
-            {
-                // Ownership transfers to Image only after its constructor validates every decoded frame.
-                foreach (ImageFrame<TPixel> frame in colorFrames)
+                bool alphaDecoded = true;
+                if (alphaState is not null)
                 {
-                    frame.Dispose();
+                    alphaDecoded = false;
+                    (HeifSequenceTrack Track, Av1Decoder Decoder) currentAlphaState = alphaState.Value;
+                    HeifSequenceSample alphaSample = currentAlphaState.Track.Samples[sampleIndex];
+                    this.ExecuteImageDataSegmentAction(
+                        () =>
+                        {
+                            this.DecodeSequenceAlphaFrame(
+                                stream,
+                                currentAlphaState.Track,
+                                alphaSample,
+                                currentAlphaState.Decoder,
+                                sourceRectangle,
+                                decodedFrame,
+                                colorTrack.IsPremultiplied);
+
+                            alphaDecoded = true;
+                        });
                 }
-            }
 
-            throw;
-        }
-    }
+                if (!alphaDecoded)
+                {
+                    if (appendedDestination)
+                    {
+                        image.Frames.RemoveFrame(image.Frames.Count - 1);
+                    }
 
-    /// <summary>
-    /// Decodes visible samples while preserving their source indices for frame-aligned alpha lookup.
-    /// </summary>
-    /// <typeparam name="TPixel">The destination pixel format.</typeparam>
-    /// <param name="stream">The complete seekable HEIF stream.</param>
-    /// <param name="track">The selected coded-image track.</param>
-    /// <param name="cancellationToken">The token used to cancel work between coded samples.</param>
-    /// <param name="sampleIndices">Receives the decode-order sample index for each returned visible frame.</param>
-    /// <returns>The exact array of successfully decoded visible frames.</returns>
-    private ImageFrame<TPixel>[] DecodeVisibleSequenceFrames<TPixel>(
-        BufferedReadStream stream,
-        HeifSequenceTrack track,
-        CancellationToken cancellationToken,
-        out int[] sampleIndices)
-        where TPixel : unmanaged, IPixel<TPixel>
-    {
-        int visibleFrameCount = 0;
-        foreach (HeifSequenceSample sample in track.Samples)
-        {
-            visibleFrameCount += sample.IsHidden ? 0 : 1;
-        }
+                    continue;
+                }
 
-        ImageFrame<TPixel>[] frames = new ImageFrame<TPixel>[visibleFrameCount];
-        sampleIndices = new int[visibleFrameCount];
-        int decodedFrameCount = 0;
-        using Av1Decoder decoder = new(this.configuration);
-        try
-        {
-            for (int sampleIndex = 0; sampleIndex < track.Samples.Length; sampleIndex++)
-            {
-                HeifSequenceSample sample = track.Samples[sampleIndex];
-                if (sample.IsHidden)
+                ImageFrame<TPixel> presentedFrame = decodedFrame;
+                if (rotationSource is not null)
                 {
-                    this.ExecuteImageDataSegmentAction(
-                        () => this.DecodeSequenceReference(stream, track, sample, decoder));
+                    presentedFrame = animateRootFrame && decodedFrameCount == 0
+                        ? image.Frames.RootFrame
+                        : image.Frames.CreateFrame();
 
-                    continue;
+                    RotateProcessor<TPixel>.ApplyQuarterTurn(
+                        rotation,
+                        rotationSource,
+                        presentedFrame,
+                        this.configuration);
+
+                    presentedFrame.Metadata.CicpProfile = rotationSource.Metadata.CicpProfile;
                 }
 
-                cancellationToken.ThrowIfCancellationRequested();
-                ImageFrame<TPixel>? frame = null;
-                this.ExecuteImageDataSegmentAction(
-                    () => frame = this.DecodeSequenceFrame<TPixel>(stream, track, sample, decoder));
+                if (colorTrack.MirrorAxis is not null)
+                {
+                    // Axis zero reflects top-to-bottom around the horizontal axis; axis one reflects left-to-right.
+                    FlipMode flip = colorTrack.MirrorAxis.Value == 0 ? FlipMode.Vertical : FlipMode.Horizontal;
+                    FlipProcessor<TPixel>.Apply(flip, presentedFrame, this.configuration);
+                }
+
+                presentedFrame.Metadata.GetHeifMetadata().FrameDelay = new Rational(
+                    colorSample.Duration,
+                    colorTrack.MediaTimescale);
 
-                if (frame is null)
+                if (!animateRootFrame && !this.Options.SkipMetadata)
                 {
-                    continue;
+                    presentedFrame.Metadata.IccProfile = colorTrack.IccProfile;
+                    _ = this.TryConvertIccProfile(presentedFrame);
                 }
 
-                frame.Metadata.GetHeifMetadata().FrameDelay = new Rational(sample.Duration, track.MediaTimescale);
-                frames[decodedFrameCount] = frame;
-                sampleIndices[decodedFrameCount] = sampleIndex;
                 decodedFrameCount++;
             }
 
             if (decodedFrameCount == 0)
             {
-                throw new InvalidImageContentException("The HEIF image sequence contains no decodable visible samples.");
+                throw new InvalidImageContentException(
+                    "The HEIF image sequence contains no decodable visible samples.");
             }
 
-            if (decodedFrameCount != frames.Length)
+            if (!this.Options.SkipMetadata)
+            {
+                if (animateRootFrame)
+                {
+                    image.Metadata.CicpProfile ??= image.Frames.RootFrame.Metadata.CicpProfile?.DeepClone();
+                    _ = this.TryConvertIccProfile(image);
+                }
+            }
+            else
             {
-                // Compaction occurs only in IgnoreImageData mode after a recoverable coded-sample failure.
-                Array.Resize(ref frames, decodedFrameCount);
-                Array.Resize(ref sampleIndices, decodedFrameCount);
+                foreach (ImageFrame<TPixel> frame in image.Frames)
+                {
+                    frame.Metadata.CicpProfile = null;
+                }
             }
 
-            return frames;
+            HeifMetadata resultMetadata = image.Metadata.GetHeifMetadata();
+            resultMetadata.RepeatCount = colorTrack.RepeatCount;
+            resultMetadata.AnimateRootFrame = animateRootFrame;
+            resultMetadata.HasAlpha |= alphaState is not null;
+            this.Dimensions = image.Size;
+            return image;
         }
         catch
         {
-            // Frames are independently allocated before the final Image adopts them. Retain ownership until this
-            // method returns so a later sample failure cannot leak the successfully decoded prefix.
-            for (int frameIndex = 0; frameIndex < decodedFrameCount; frameIndex++)
-            {
-                frames[frameIndex].Dispose();
-            }
-
+            image.Dispose();
             throw;
         }
     }
@@ -494,12 +584,15 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
     /// <param name="track">The track supplying the codec configuration and color description.</param>
     /// <param name="sample">The validated sample range.</param>
     /// <param name="decoder">The decoder retaining earlier sequence references.</param>
-    /// <returns>The independently owned decoded frame.</returns>
-    private ImageFrame<TPixel> DecodeSequenceFrame<TPixel>(
+    /// <param name="sourceRectangle">The clean-aperture region mapped to the destination frame.</param>
+    /// <param name="destination">The caller-owned frame receiving the presented sample.</param>
+    private void DecodeSequenceFrame<TPixel>(
         BufferedReadStream stream,
         HeifSequenceTrack track,
         HeifSequenceSample sample,
-        Av1Decoder decoder)
+        Av1Decoder decoder,
+        Rectangle sourceRectangle,
+        ImageFrame<TPixel> destination)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         if (track.CodecType != Heif4CharCode.Av01)
@@ -513,18 +606,13 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
         using IMemoryOwner<byte> sampleOwner = this.ReadSequenceSample(stream, track, sample);
         Span<byte> sampleData = sampleOwner.GetSpan()[..sample.Length];
 
-        ImageFrame<TPixel> frame = decoder.DecodeSequenceFrame<TPixel>(
+        decoder.DecodeSequenceFrame(
             sampleData,
             track.CicpProfile,
-            codecConfiguration);
-
-        if (frame.Width != track.CodedWidth || frame.Height != track.CodedHeight)
-        {
-            frame.Dispose();
-            throw new InvalidImageContentException("The decoded image-sequence sample dimensions do not match its visual sample entry.");
-        }
-
-        return frame;
+            codecConfiguration,
+            new Size(track.CodedWidth, track.CodedHeight),
+            sourceRectangle,
+            destination);
     }
 
     /// <summary>
@@ -535,6 +623,7 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
     /// <param name="track">The alpha track supplying the codec configuration and color description.</param>
     /// <param name="sample">The validated alpha sample range.</param>
     /// <param name="decoder">The decoder retaining earlier alpha-sequence references.</param>
+    /// <param name="sourceRectangle">The clean-aperture region mapped to the destination frame.</param>
     /// <param name="destination">The decoded color frame receiving alpha values.</param>
     /// <param name="premultiplied">Whether stored color samples must be converted to unassociated alpha.</param>
     private void DecodeSequenceAlphaFrame<TPixel>(
@@ -542,6 +631,7 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
         HeifSequenceTrack track,
         HeifSequenceSample sample,
         Av1Decoder decoder,
+        Rectangle sourceRectangle,
         ImageFrame<TPixel> destination,
         bool premultiplied)
         where TPixel : unmanaged, IPixel<TPixel>
@@ -566,6 +656,7 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
             track.CicpProfile,
             codecConfiguration,
             new Size(track.CodedWidth, track.CodedHeight),
+            sourceRectangle,
             destination,
             destination.Size,
             destination.Bounds,
@@ -642,28 +733,33 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
     /// </summary>
     /// <param name="metadata">The image metadata receiving the sequence description.</param>
     /// <param name="sequence">The parsed selected image sequence.</param>
-    private void UpdateSequenceMetadata(ImageMetadata metadata, HeifSequence sequence)
+    /// <param name="animateRootFrame">Whether the primary image is also the first timed sample.</param>
+    private void UpdateSequenceMetadata(ImageMetadata metadata, HeifSequence sequence, bool animateRootFrame)
     {
         HeifSequenceTrack colorTrack = sequence.ColorTrack;
         HeifMetadata heifMetadata = metadata.GetHeifMetadata();
         heifMetadata.RepeatCount = colorTrack.RepeatCount;
-        heifMetadata.AnimateRootFrame = true;
-        heifMetadata.HasAlpha = sequence.AlphaTrack is not null;
+        heifMetadata.AnimateRootFrame = animateRootFrame;
+        heifMetadata.HasAlpha |= sequence.AlphaTrack is not null;
         switch (colorTrack.CodecType)
         {
             case Heif4CharCode.Av01:
                 Av1CodecConfiguration av1Configuration = colorTrack.Av1CodecConfiguration
                     ?? throw new InvalidImageContentException("The AV1 image-sequence track has no codec configuration.");
 
-                heifMetadata.CompressionMethod = HeifCompressionMethod.Av1;
-                heifMetadata.BitDepth = av1Configuration.BitDepth;
-                heifMetadata.IsMonochrome = av1Configuration.IsMonochrome;
+                if (animateRootFrame)
+                {
+                    heifMetadata.CompressionMethod = HeifCompressionMethod.Av1;
+                    heifMetadata.BitDepth = av1Configuration.BitDepth;
+                    heifMetadata.IsMonochrome = av1Configuration.IsMonochrome;
+                }
+
                 break;
             default:
                 throw new InvalidImageContentException($"The image-sequence sample entry '{colorTrack.CodecType}' is not supported.");
         }
 
-        if (this.Options.SkipMetadata)
+        if (this.Options.SkipMetadata || !animateRootFrame)
         {
             return;
         }
@@ -696,8 +792,11 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
     /// Creates one HEIF frame-metadata entry for each visible retained sequence sample.
     /// </summary>
     /// <param name="track">The selected color track supplying sample durations.</param>
+    /// <param name="animateRootFrame">Whether the first sequence sample occupies the root-frame slot.</param>
     /// <returns>The exact visible-frame metadata array in presentation order.</returns>
-    private static ImageFrameMetadata[] CreateSequenceFrameMetadata(HeifSequenceTrack track)
+    private static ImageFrameMetadata[] CreateSequenceFrameMetadata(
+        HeifSequenceTrack track,
+        bool animateRootFrame)
     {
         int visibleFrameCount = 0;
         foreach (HeifSequenceSample sample in track.Samples)
@@ -705,8 +804,14 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
             visibleFrameCount += sample.IsHidden ? 0 : 1;
         }
 
-        ImageFrameMetadata[] result = new ImageFrameMetadata[visibleFrameCount];
-        int frameIndex = 0;
+        int firstSequenceFrameIndex = animateRootFrame ? 0 : 1;
+        ImageFrameMetadata[] result = new ImageFrameMetadata[visibleFrameCount + firstSequenceFrameIndex];
+        if (!animateRootFrame)
+        {
+            result[0] = new ImageFrameMetadata();
+        }
+
+        int frameIndex = firstSequenceFrameIndex;
         foreach (HeifSequenceSample sample in track.Samples)
         {
             if (sample.IsHidden)
@@ -722,6 +827,39 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
         return result;
     }
 
+    /// <summary>
+    /// Gets the primary image item required alongside an AVIF image sequence.
+    /// </summary>
+    private HeifItem FindSequencePrimaryItem()
+        => this.FindItemById(this.primaryItem)
+            ?? throw new InvalidImageContentException("The HEIF image sequence contains no primary image item.");
+
+    /// <summary>
+    /// Determines whether the primary image item reuses the first presented sequence sample.
+    /// </summary>
+    private static bool IsPrimaryItemFirstSequenceSample(HeifItem primaryItem, HeifSequenceTrack colorTrack)
+    {
+        if (primaryItem.DataLocations.Count != 1)
+        {
+            return false;
+        }
+
+        HeifLocation location = primaryItem.DataLocations[0];
+        foreach (HeifSequenceSample sample in colorTrack.Samples)
+        {
+            if (sample.IsHidden)
+            {
+                continue;
+            }
+
+            return location.Origin == HeifLocationOffsetOrigin.FileOffset
+                && location.BaseOffset + location.Offset == sample.Offset
+                && location.Length == sample.Length;
+        }
+
+        return false;
+    }
+
     /// <summary>
     /// Updates identification metadata from the primary item or its decodable thumbnail fallback.
     /// </summary>

src/ImageSharp/Formats/Heif/HeifEncoder.cs

@@ -92,20 +92,22 @@ public sealed class HeifEncoder : AnimatedImageEncoder
     /// <summary>
     /// Gets a value indicating whether the primary and auxiliary alpha images are encoded without loss. When
     /// <see langword="true"/>, <see cref="Quality"/> and <see cref="AlphaQuality"/> do not affect the encoded image.
-    /// Legacy JPEG image items do not support lossless encoding. The default is <see langword="false"/>.
+    /// This option has no effect on legacy JPEG image items. The default is <see langword="false"/>.
     /// </summary>
     public bool Lossless { get; init; }
 
     /// <summary>
     /// Gets the encoded precision of each image component, or <see langword="null"/> to use the HEIF metadata bit
     /// depth. Metadata that does not specify a bit depth defaults to <see cref="HeifBitDepth.Bit8"/>. Legacy JPEG
-    /// image items support only <see cref="HeifBitDepth.Bit8"/>.
+    /// image items are always encoded with <see cref="HeifBitDepth.Bit8"/>.
     /// </summary>
     public HeifBitDepth? BitDepth { get; init; }
 
     /// <summary>
     /// Gets the encoded chroma sampling, or <see langword="null"/> to use <see cref="HeifChromaSubsampling.Yuv420"/>
-    /// for lossy encoding and <see cref="HeifChromaSubsampling.Yuv444"/> for lossless encoding.
+    /// for lossy encoding and <see cref="HeifChromaSubsampling.Yuv444"/> for lossless encoding. Oversized still
+    /// images use <see cref="HeifChromaSubsampling.Yuv444"/> when a subsampled AVIF grid cannot represent an odd
+    /// output dimension.
     /// </summary>
     public HeifChromaSubsampling? ChromaSubsampling { get; init; }
 

src/ImageSharp/Formats/Heif/HeifEncoderCore.Sequence.cs

@@ -16,16 +16,59 @@ namespace SixLabors.ImageSharp.Formats.Heif;
 
 internal sealed partial class HeifEncoderCore
 {
+    /// <summary>
+    /// The millisecond media timescale used when every frame delay can be represented exactly.
+    /// </summary>
     private const uint DefaultSequenceTimescale = 1000;
+
+    /// <summary>
+    /// The microsecond fallback used when the exact common frame-delay timescale exceeds 32 bits.
+    /// </summary>
     private const uint FallbackSequenceTimescale = 1000000;
+
+    /// <summary>
+    /// The identity value for signed 16.16 movie and track matrix entries.
+    /// </summary>
     private const uint UnityFixed16Point16 = 1U << 16;
+
+    /// <summary>
+    /// The identity value for the signed 2.30 homogeneous movie and track matrix entry.
+    /// </summary>
     private const uint UnityFixed2Point30 = 1U << 30;
+
+    /// <summary>
+    /// The identity value for unsigned 8.8 track volume.
+    /// </summary>
     private const ushort UnityFixed8Point8 = 1 << 8;
+
+    /// <summary>
+    /// The packed ISO 639-2/T language code for undetermined content.
+    /// </summary>
     private const ushort PackedUndeterminedLanguage = 0x55C4;
+
+    /// <summary>
+    /// The coding-constraints flag stating that every reference picture is intra.
+    /// </summary>
     private const uint AllReferencePicturesIntraMask = 1U << 31;
+
+    /// <summary>
+    /// The coding-constraints flag stating that intra prediction is used.
+    /// </summary>
     private const uint IntraPicturePredictionUsedMask = 1U << 30;
+
+    /// <summary>
+    /// The conventional 72-dpi horizontal and vertical resolution stored as unsigned 16.16.
+    /// </summary>
     private const uint DefaultVisualSampleResolution = 72U << 16;
+
+    /// <summary>
+    /// The fixed visual-sample-entry compressor-name field length.
+    /// </summary>
     private const int VisualSampleCompressorNameLength = 32;
+
+    /// <summary>
+    /// The visual-sample-entry depth used for color pictures.
+    /// </summary>
     private const ushort VisualSampleDepth = 24;
 
     private Av1EncodingSettings ResolveAv1Encoding<TPixel>(Image<TPixel> image)
@@ -48,6 +91,18 @@ internal sealed partial class HeifEncoderCore
         HeifChromaSubsampling chromaSubsampling = this.encoder.ChromaSubsampling ??
             (metadata.IsMonochrome ? HeifChromaSubsampling.Monochrome : defaultChromaSubsampling);
 
+        if (this.encoder.ChromaSubsampling is null
+            && image.Frames.Count == 1
+            && (image.Width > Av1Constants.MaxFrameDimension || image.Height > Av1Constants.MaxFrameDimension)
+            && ((chromaSubsampling == HeifChromaSubsampling.Yuv420
+                    && (((image.Width & 1) != 0) || ((image.Height & 1) != 0)))
+                || (chromaSubsampling == HeifChromaSubsampling.Yuv422 && (image.Width & 1) != 0)))
+        {
+            // A derived grid requires even output dimensions on every subsampled axis. When sampling was not
+            // explicitly requested, retain the complete image dimensions by selecting full-resolution chroma.
+            chromaSubsampling = HeifChromaSubsampling.Yuv444;
+        }
+
         (bool isMonochrome, bool subsamplingX, bool subsamplingY) = chromaSubsampling switch
         {
             HeifChromaSubsampling.Monochrome => (true, true, true),
@@ -143,14 +198,10 @@ internal sealed partial class HeifEncoderCore
         ChunkedMemoryStream stream,
         Av1EncodingSettings settings,
         Memory<HeifSequenceSampleInfo> samples,
+        int firstFrameIndex,
         CancellationToken cancellationToken)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        if (image.Width > ushort.MaxValue || image.Height > ushort.MaxValue)
-        {
-            throw new NotSupportedException("AV1 image-sequence dimensions cannot exceed 65535 pixels.");
-        }
-
         byte[]? exifData = null;
         uint tiffHeaderOffset = 0;
         byte[]? xmpData = null;
@@ -164,47 +215,58 @@ internal sealed partial class HeifEncoderCore
             }
         }
 
-        int frameCount = image.Frames.Count;
-        uint timescale = GetSequenceTimescale(image);
+        int frameCount = image.Frames.Count - firstFrameIndex;
+        uint timescale = GetSequenceTimescale(image, firstFrameIndex);
 
         // The container needs only offset, length, and duration after each frame is streamed. Color and alpha
         // share one allocator-owned table, with each track occupying one contiguous slice until moov is written.
         Span<HeifSequenceSampleInfo> colorSamples = samples.Span[..frameCount];
-        ImageFrame<TPixel> rootFrame = image.Frames.RootFrame;
-        uint duration = GetSequenceSampleDuration(rootFrame.Metadata.GetHeifMetadata().FrameDelay, timescale);
-        cancellationToken.ThrowIfCancellationRequested();
-        long colorOffset = stream.Length;
-        ObuSequenceHeader colorHeader = Av1FrameEncoder.Encode(
+        ImageFrame<TPixel> firstFrame = image.Frames[firstFrameIndex];
+        ObuSequenceHeader colorHeader;
+        bool colorUsesInterPrediction = settings.ColorQIndex != 0;
+        using (Av1FrameEncoder.SequenceEncoder colorEncoder = Av1FrameEncoder.CreateColorSequenceEncoder(
             this.configuration,
-            rootFrame,
-            stream,
+            image.Width,
+            image.Height,
             settings.ColorConfig,
             settings.ColorQIndex,
-            this.encoder.Effort);
-
-        colorSamples[0] = new HeifSequenceSampleInfo(
-            colorOffset,
-            checked((int)(stream.Length - colorOffset)),
-            duration);
-
-        for (int frameIndex = 1; frameIndex < frameCount; frameIndex++)
+            this.encoder.Effort))
         {
             cancellationToken.ThrowIfCancellationRequested();
-            ImageFrame<TPixel> frame = image.Frames[frameIndex];
-            duration = GetSequenceSampleDuration(frame.Metadata.GetHeifMetadata().FrameDelay, timescale);
-            colorOffset = stream.Length;
-            _ = Av1FrameEncoder.Encode(
-                this.configuration,
-                frame,
-                stream,
-                settings.ColorConfig,
-                settings.ColorQIndex,
-                this.encoder.Effort);
+            long colorOffset = stream.Length;
+            colorEncoder.EncodeKeyFrame(firstFrame, stream);
+            colorHeader = colorEncoder.SequenceHeader;
 
-            colorSamples[frameIndex] = new HeifSequenceSampleInfo(
+            colorSamples[0] = new HeifSequenceSampleInfo(
                 colorOffset,
                 checked((int)(stream.Length - colorOffset)),
-                duration);
+                GetSequenceSampleDuration(firstFrame.Metadata.GetHeifMetadata().FrameDelay, timescale),
+                isSyncSample: true);
+
+            for (int sampleIndex = 1; sampleIndex < frameCount; sampleIndex++)
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+                int frameIndex = firstFrameIndex + sampleIndex;
+                ImageFrame<TPixel> frame = image.Frames[frameIndex];
+                uint duration = GetSequenceSampleDuration(frame.Metadata.GetHeifMetadata().FrameDelay, timescale);
+                colorOffset = stream.Length;
+                if (colorUsesInterPrediction)
+                {
+                    colorEncoder.EncodeInterFrame(frame, stream);
+                }
+                else
+                {
+                    // Lossless AV1 requires 4x4 transforms. Until the inter path supports that reversible size,
+                    // continuation samples remain independent key frames instead of weakening losslessness.
+                    colorEncoder.EncodeKeyFrame(frame, stream);
+                }
+
+                colorSamples[sampleIndex] = new HeifSequenceSampleInfo(
+                    colorOffset,
+                    checked((int)(stream.Length - colorOffset)),
+                    duration,
+                    isSyncSample: !colorUsesInterPrediction);
+            }
         }
 
         HeifSequenceTrackEncoding colorTrack = new(
@@ -217,37 +279,47 @@ internal sealed partial class HeifEncoderCore
         {
             Memory<HeifSequenceSampleInfo> alphaSampleMemory = samples.Slice(frameCount, frameCount);
             Span<HeifSequenceSampleInfo> alphaSamples = alphaSampleMemory.Span;
-            cancellationToken.ThrowIfCancellationRequested();
-            long alphaOffset = stream.Length;
-            ObuSequenceHeader alphaHeader = Av1FrameEncoder.EncodeAlpha(
+            ObuSequenceHeader alphaHeader;
+            bool alphaUsesInterPrediction = settings.AlphaQIndex != 0;
+            using (Av1FrameEncoder.SequenceEncoder alphaEncoder = Av1FrameEncoder.CreateAlphaSequenceEncoder(
                 this.configuration,
-                rootFrame,
-                stream,
+                image.Width,
+                image.Height,
                 settings.AlphaConfig,
                 settings.AlphaQIndex,
-                this.encoder.Effort);
-
-            alphaSamples[0] = new HeifSequenceSampleInfo(
-                alphaOffset,
-                checked((int)(stream.Length - alphaOffset)),
-                colorSamples[0].Duration);
-
-            for (int frameIndex = 1; frameIndex < frameCount; frameIndex++)
+                this.encoder.Effort))
             {
                 cancellationToken.ThrowIfCancellationRequested();
-                alphaOffset = stream.Length;
-                _ = Av1FrameEncoder.EncodeAlpha(
-                    this.configuration,
-                    image.Frames[frameIndex],
-                    stream,
-                    settings.AlphaConfig,
-                    settings.AlphaQIndex,
-                    this.encoder.Effort);
-
-                alphaSamples[frameIndex] = new HeifSequenceSampleInfo(
+                long alphaOffset = stream.Length;
+                alphaEncoder.EncodeKeyFrame(firstFrame, stream);
+                alphaHeader = alphaEncoder.SequenceHeader;
+
+                alphaSamples[0] = new HeifSequenceSampleInfo(
                     alphaOffset,
                     checked((int)(stream.Length - alphaOffset)),
-                    colorSamples[frameIndex].Duration);
+                    colorSamples[0].Duration,
+                    isSyncSample: true);
+
+                for (int sampleIndex = 1; sampleIndex < frameCount; sampleIndex++)
+                {
+                    cancellationToken.ThrowIfCancellationRequested();
+                    int frameIndex = firstFrameIndex + sampleIndex;
+                    alphaOffset = stream.Length;
+                    if (alphaUsesInterPrediction)
+                    {
+                        alphaEncoder.EncodeInterFrame(image.Frames[frameIndex], stream);
+                    }
+                    else
+                    {
+                        alphaEncoder.EncodeKeyFrame(image.Frames[frameIndex], stream);
+                    }
+
+                    alphaSamples[sampleIndex] = new HeifSequenceSampleInfo(
+                        alphaOffset,
+                        checked((int)(stream.Length - alphaOffset)),
+                        colorSamples[sampleIndex].Duration,
+                        isSyncSample: !alphaUsesInterPrediction);
+                }
             }
 
             alphaTrack = new HeifSequenceTrackEncoding(
@@ -266,7 +338,7 @@ internal sealed partial class HeifEncoderCore
         return new HeifSequenceEncoding(
             image.Width,
             image.Height,
-            image.Metadata.GetHeifMetadata().RepeatCount,
+            this.encoder.RepeatCount ?? image.Metadata.GetHeifMetadata().RepeatCount,
             timescale,
             colorTrack,
             alphaTrack,
@@ -279,7 +351,7 @@ internal sealed partial class HeifEncoderCore
 
     private int WriteSequenceFileTypeBox(Stream stream)
     {
-        Span<byte> buffer = stackalloc byte[32];
+        Span<byte> buffer = stackalloc byte[44];
         int bytesWritten = WriteBoxHeader(buffer, Heif4CharCode.Ftyp);
         BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Avis);
         bytesWritten += sizeof(uint);
@@ -287,18 +359,24 @@ internal sealed partial class HeifEncoderCore
         bytesWritten += sizeof(uint);
         BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Avif);
         bytesWritten += sizeof(uint);
+        BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Avio);
+        bytesWritten += sizeof(uint);
+        BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Avis);
+        bytesWritten += sizeof(uint);
         BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Msf1);
         bytesWritten += sizeof(uint);
         BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Iso8);
         bytesWritten += sizeof(uint);
-        BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Avio);
+        BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Mif1);
+        bytesWritten += sizeof(uint);
+        BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)Heif4CharCode.Miaf);
         bytesWritten += sizeof(uint);
         BinaryPrimitives.WriteUInt32BigEndian(buffer, (uint)bytesWritten);
         stream.Write(buffer[..bytesWritten]);
         return bytesWritten;
     }
 
-    private void WriteSequenceMovieBox(HeifSequenceEncoding sequence, int fileTypeLength, Stream stream)
+    private void WriteSequenceMovieBox(HeifSequenceEncoding sequence, int precedingBoxLength, Stream stream)
     {
         int movieLength = GetSequenceMovieBoxLength(sequence);
         using IMemoryOwner<byte> movieOwner = this.configuration.MemoryAllocator.Allocate<byte>(movieLength);
@@ -343,7 +421,7 @@ internal sealed partial class HeifEncoderCore
         }
 
         EndSequenceBox(memory, movieStart, offset);
-        ulong mediaDataOffset = checked((ulong)fileTypeLength + (uint)offset + 8U);
+        ulong mediaDataOffset = checked((ulong)precedingBoxLength + (uint)offset + 8U);
         BinaryPrimitives.WriteUInt64BigEndian(
             memory[colorChunkOffsetPosition..],
             checked(mediaDataOffset + (ulong)sequence.ColorTrack.Samples[0].Offset));
@@ -377,7 +455,8 @@ internal sealed partial class HeifEncoderCore
         const int chunkOffsetBoxLength = 24;
         const int syncSampleBoxFixedLength = 16;
         const int timingRunLength = 8;
-        const int sampleSizeAndSyncEntryLength = 8;
+        const int sampleSizeEntryLength = sizeof(uint);
+        const int syncSampleEntryLength = sizeof(uint);
         const int sampleTableFixedLength =
             sampleTableBoxHeaderLength
             + sampleDescriptionBoxLength
@@ -398,10 +477,12 @@ internal sealed partial class HeifEncoderCore
 
         int repeatBoxLength = sequence.RepeatCount == 1 ? 0 : editListBoxLength;
         int colorRunCount = GetSequenceTimingRunCount(sequence.ColorTrack.Samples);
+        int colorSyncSampleCount = GetSequenceSyncSampleCount(sequence.ColorTrack.Samples);
         long colorSampleTableLength =
             (long)sampleTableFixedLength
             + (colorRunCount * timingRunLength)
-            + (sequence.ColorTrack.Samples.Length * sampleSizeAndSyncEntryLength)
+            + (sequence.ColorTrack.Samples.Length * sampleSizeEntryLength)
+            + (colorSyncSampleCount * syncSampleEntryLength)
             + colorInformationBoxLength;
 
         if (!sequence.IccProfileData.IsEmpty)
@@ -436,6 +517,7 @@ internal sealed partial class HeifEncoderCore
         {
             HeifSequenceTrackEncoding alphaTrack = sequence.AlphaTrack.GetValueOrDefault();
             int alphaRunCount = GetSequenceTimingRunCount(alphaTrack.Samples);
+            int alphaSyncSampleCount = GetSequenceSyncSampleCount(alphaTrack.Samples);
             int auxiliaryTypeBoxLength =
                 FullBoxHeaderLength
                 + Encoding.UTF8.GetByteCount(HeifConstants.AlphaAuxiliaryType)
@@ -444,7 +526,8 @@ internal sealed partial class HeifEncoderCore
             long alphaSampleTableLength =
                 (long)sampleTableFixedLength
                 + (alphaRunCount * timingRunLength)
-                + (alphaTrack.Samples.Length * sampleSizeAndSyncEntryLength)
+                + (alphaTrack.Samples.Length * sampleSizeEntryLength)
+                + (alphaSyncSampleCount * syncSampleEntryLength)
                 + auxiliaryTypeBoxLength;
 
             alphaTrackLength =
@@ -774,13 +857,19 @@ internal sealed partial class HeifEncoderCore
         WriteSequenceUInt64(memory, ref offset, 0);
         EndSequenceBox(memory, chunkOffsetsStart, offset);
 
-        // The current bounded sequence encoder emits independent all-intra pictures; every sample is seekable.
+        int syncSampleCount = GetSequenceSyncSampleCount(track.Samples);
         int syncSamplesStart = BeginSequenceBox(memory, ref offset, Heif4CharCode.Stss);
         WriteSequenceFullBoxHeader(memory, ref offset, 0, 0);
-        WriteSequenceUInt32(memory, ref offset, (uint)track.Samples.Length);
-        for (uint sampleIndex = 1; sampleIndex <= track.Samples.Length; sampleIndex++)
+        WriteSequenceUInt32(memory, ref offset, (uint)syncSampleCount);
+        uint sampleNumber = 1;
+        foreach (HeifSequenceSampleInfo sample in track.Samples)
         {
-            WriteSequenceUInt32(memory, ref offset, sampleIndex);
+            if (sample.IsSyncSample)
+            {
+                WriteSequenceUInt32(memory, ref offset, sampleNumber);
+            }
+
+            sampleNumber++;
         }
 
         EndSequenceBox(memory, syncSamplesStart, offset);
@@ -839,9 +928,15 @@ internal sealed partial class HeifEncoderCore
         int codingConstraintsStart = BeginSequenceBox(memory, ref offset, Heif4CharCode.Ccst);
         WriteSequenceFullBoxHeader(memory, ref offset, 0, 0);
 
-        // Every emitted sequence sample is independently decodable, while intra prediction remains available inside
-        // each picture. No inter-picture reference slot is therefore advertised.
-        WriteSequenceUInt32(memory, ref offset, AllReferencePicturesIntraMask | IntraPicturePredictionUsedMask);
+        uint codingConstraints = IntraPicturePredictionUsedMask;
+        if (GetSequenceSyncSampleCount(track.Samples) == track.Samples.Length)
+        {
+            codingConstraints |= AllReferencePicturesIntraMask;
+        }
+
+        // Sync samples are key frames in this encoder. The all-intra flag is therefore valid when every sample
+        // is independently decodable, including lossless sequences that deliberately avoid inter transforms.
+        WriteSequenceUInt32(memory, ref offset, codingConstraints);
         EndSequenceBox(memory, codingConstraintsStart, offset);
         EndSequenceBox(memory, sampleEntryStart, offset);
         EndSequenceBox(memory, descriptionStart, offset);
@@ -892,6 +987,20 @@ internal sealed partial class HeifEncoderCore
         return runCount;
     }
 
+    private static int GetSequenceSyncSampleCount(ReadOnlySpan<HeifSequenceSampleInfo> samples)
+    {
+        int count = 0;
+        foreach (HeifSequenceSampleInfo sample in samples)
+        {
+            if (sample.IsSyncSample)
+            {
+                count++;
+            }
+        }
+
+        return count;
+    }
+
     private static uint GetSequenceSampleDuration(Rational delay, uint timescale)
     {
         // HEIF metadata uses either a zero numerator or a zero denominator for an unspecified duration.
@@ -905,12 +1014,13 @@ internal sealed partial class HeifEncoderCore
         return checked((uint)Math.Max(1UL, scaledDuration / delay.Denominator));
     }
 
-    private static uint GetSequenceTimescale<TPixel>(Image<TPixel> image)
+    private static uint GetSequenceTimescale<TPixel>(Image<TPixel> image, int firstFrameIndex)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         uint timescale = DefaultSequenceTimescale;
-        foreach (ImageFrame<TPixel> frame in image.Frames)
+        for (int frameIndex = firstFrameIndex; frameIndex < image.Frames.Count; frameIndex++)
         {
+            ImageFrame<TPixel> frame = image.Frames[frameIndex];
             Rational delay = frame.Metadata.GetHeifMetadata().FrameDelay;
             if (delay.Numerator == 0 || delay.Denominator == 0)
             {
@@ -1070,11 +1180,12 @@ internal sealed partial class HeifEncoderCore
 
     private readonly struct HeifSequenceSampleInfo
     {
-        public HeifSequenceSampleInfo(long offset, int length, uint duration)
+        public HeifSequenceSampleInfo(long offset, int length, uint duration, bool isSyncSample)
         {
             this.Offset = offset;
             this.Length = length;
             this.Duration = duration;
+            this.IsSyncSample = isSyncSample;
         }
 
         public long Offset { get; }
@@ -1082,6 +1193,8 @@ internal sealed partial class HeifEncoderCore
         public int Length { get; }
 
         public uint Duration { get; }
+
+        public bool IsSyncSample { get; }
     }
 
     private readonly struct HeifSequenceEncoding

src/ImageSharp/Formats/Heif/HeifEncoderCore.cs

@@ -10,7 +10,6 @@ using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline.Quantizers;
 using SixLabors.ImageSharp.Formats.Jpeg;
 using SixLabors.ImageSharp.IO;
-using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.Metadata.Profiles.Cicp;
 using SixLabors.ImageSharp.Metadata.Profiles.Icc;
@@ -46,6 +45,32 @@ internal sealed partial class HeifEncoderCore
     private const int MaximumCompactPropertyIndex = 0x7F;
     private const ushort EssentialPropertyFlag = 0x8000;
     private const byte CompactEssentialPropertyFlag = 0x80;
+    private const uint HiddenImageItemFlag = 1;
+
+    /// <summary>
+    /// The version defined for the AVIF grid item payload.
+    /// </summary>
+    private const byte GridDescriptorVersion = 0;
+
+    /// <summary>
+    /// The largest row or column count representable by a grid descriptor.
+    /// </summary>
+    private const int MaximumGridAxisCellCount = byte.MaxValue + 1;
+
+    /// <summary>
+    /// The minimum width and height permitted for the first cell of an AVIF grid.
+    /// </summary>
+    private const int MinimumGridCellDimension = 64;
+
+    /// <summary>
+    /// The grid descriptor length when output dimensions use 32-bit fields.
+    /// </summary>
+    private const int LongGridDescriptorLength = 12;
+
+    /// <summary>
+    /// Selects 32-bit output dimensions in a grid descriptor.
+    /// </summary>
+    private const byte LargeGridDimensionsFlag = 1;
 
     /// <summary>
     /// The global configuration.
@@ -81,24 +106,93 @@ internal sealed partial class HeifEncoderCore
         Guard.NotNull(image, nameof(image));
         Guard.NotNull(stream, nameof(stream));
 
+        switch (this.encoder.CompressionMethod)
+        {
+            case HeifCompressionMethod.LegacyJpeg:
+                break;
+            case HeifCompressionMethod.Av1:
+                if (image.Frames.Count > 1)
+                {
+                    if (image.Width > ushort.MaxValue || image.Height > ushort.MaxValue)
+                    {
+                        throw new NotSupportedException("AV1 image-sequence dimensions cannot exceed 65535 pixels.");
+                    }
+                }
+
+                break;
+            default:
+                throw new NotSupportedException($"HEIF compression method '{this.encoder.CompressionMethod}' is not supported.");
+        }
+
         using ChunkedMemoryStream compressedPixels = new(this.configuration.MemoryAllocator);
         if (this.encoder.CompressionMethod == HeifCompressionMethod.Av1 && image.Frames.Count > 1)
         {
             Av1EncodingSettings settings = this.ResolveAv1Encoding(image);
-            int sampleCount = image.Frames.Count * (settings.HasAlpha ? 2 : 1);
+            bool animateRootFrame = this.encoder.AnimateRootFrame
+                ?? image.Metadata.GetHeifMetadata().AnimateRootFrame;
+
+            int firstFrameIndex = animateRootFrame ? 0 : 1;
+            int sequenceFrameCount = image.Frames.Count - firstFrameIndex;
+            int sampleCount = sequenceFrameCount * (settings.HasAlpha ? 2 : 1);
             using IMemoryOwner<HeifSequenceSampleInfo> samplesOwner =
                 this.configuration.MemoryAllocator.Allocate<HeifSequenceSampleInfo>(sampleCount);
 
+            List<HeifItem> sequenceItems = new();
+            List<HeifItemLink> sequenceLinks = new();
+            if (!animateRootFrame)
+            {
+                Av1ImageItemEncoding primaryImage = this.CompressAv1ImageItem(
+                    image.Frames.RootFrame,
+                    compressedPixels,
+                    settings,
+                    cancellationToken);
+
+                this.WriteAv1ImageItems(
+                    image,
+                    compressedPixels,
+                    settings,
+                    primaryImage,
+                    sequenceItems,
+                    sequenceLinks);
+            }
+
             Memory<HeifSequenceSampleInfo> samples = samplesOwner.Memory[..sampleCount];
             HeifSequenceEncoding sequence = this.CompressAv1Sequence(
                 image,
                 compressedPixels,
                 settings,
                 samples,
+                firstFrameIndex,
                 cancellationToken);
 
+            if (animateRootFrame)
+            {
+                HeifSequenceSampleInfo colorSample = sequence.ColorTrack.Samples[0];
+                HeifSequenceTrackEncoding? alphaTrack = sequence.AlphaTrack;
+                Av1ImageItemEncoding primaryImage = new(
+                    sequence.ColorTrack.Configuration,
+                    colorSample.Offset,
+                    colorSample.Length,
+                    alphaTrack?.Configuration,
+                    alphaTrack?.Samples[0].Offset ?? 0,
+                    alphaTrack?.Samples[0].Length ?? 0);
+
+                // The primary image item and the first track sample describe the same sync sample. Sharing its
+                // extent matches libavif and avoids encoding or storing the root frame twice.
+                this.WriteAv1ImageItems(
+                    image,
+                    compressedPixels,
+                    settings,
+                    primaryImage,
+                    sequenceItems,
+                    sequenceLinks);
+            }
+
             int fileTypeLength = this.WriteSequenceFileTypeBox(stream);
-            this.WriteSequenceMovieBox(sequence, fileTypeLength, stream);
+            int metadataLength = GetMetadataBoxLength(sequenceItems, sequenceLinks);
+            int movieLength = GetSequenceMovieBoxLength(sequence);
+            this.WriteMetadataBox(sequenceItems, sequenceLinks, fileTypeLength, movieLength, stream);
+            this.WriteSequenceMovieBox(sequence, fileTypeLength + metadataLength, stream);
             this.WriteMediaDataBox(compressedPixels, stream);
             stream.Flush();
             return;
@@ -115,13 +209,11 @@ internal sealed partial class HeifEncoderCore
             case HeifCompressionMethod.Av1:
                 this.CompressAv1Pixels(image, compressedPixels, items, links, cancellationToken);
                 break;
-            default:
-                throw new NotSupportedException($"HEIF compression method '{this.encoder.CompressionMethod}' is not supported.");
         }
 
         // Write out the generated header and pixels.
         long metadataBoxOffset = this.WriteFileTypeBox(stream);
-        this.WriteMetadataBox(items, links, metadataBoxOffset, stream);
+        this.WriteMetadataBox(items, links, metadataBoxOffset, 0, stream);
         this.WriteMediaDataBox(compressedPixels, stream);
         stream.Flush();
     }
@@ -229,8 +321,14 @@ internal sealed partial class HeifEncoderCore
     /// <param name="items">The declared image and metadata items.</param>
     /// <param name="links">The typed relationships between items.</param>
     /// <param name="metadataBoxOffset">The metadata box offset from the start of the encoded file.</param>
+    /// <param name="followingBoxLength">The number of bytes between this box and the media-data box.</param>
     /// <param name="stream">The destination stream positioned after the file-type box.</param>
-    private void WriteMetadataBox(List<HeifItem> items, List<HeifItemLink> links, long metadataBoxOffset, Stream stream)
+    private void WriteMetadataBox(
+        List<HeifItem> items,
+        List<HeifItemLink> links,
+        long metadataBoxOffset,
+        int followingBoxLength,
+        Stream stream)
     {
         int metadataLength = GetMetadataBoxLength(items, links);
         using IMemoryOwner<byte> metadataOwner = this.configuration.MemoryAllocator.Allocate<byte>(metadataLength);
@@ -254,7 +352,7 @@ internal sealed partial class HeifEncoderCore
         bytesWritten += WriteItemLocationBox(memory, bytesWritten, items, 0);
 
         // The mdat payload immediately follows the completed meta box and its own eight-byte header.
-        long mediaDataOffset = checked(metadataBoxOffset + bytesWritten + BasicBoxHeaderLength);
+        long mediaDataOffset = checked(metadataBoxOffset + bytesWritten + followingBoxLength + BasicBoxHeaderLength);
         WriteItemLocationBox(memory, itemLocationOffset, items, mediaDataOffset);
 
         buffer = memory[..bytesWritten];
@@ -315,13 +413,21 @@ internal sealed partial class HeifEncoderCore
     {
         long propertyCount = 0;
         long associationItemCount = 0;
+        long associationPropertyCount = 0;
         long propertyBytes = 0;
         foreach (HeifItem item in items)
         {
-            int itemPropertyCount = GetPropertyCount(item);
-            propertyCount += itemPropertyCount;
+            HeifItem propertyItem = item.PropertySource ?? item;
+            int itemPropertyCount = GetPropertyCount(propertyItem);
             associationItemCount += itemPropertyCount == 0 ? 0 : 1;
+            associationPropertyCount += itemPropertyCount;
+
+            if (item.PropertySource is not null)
+            {
+                continue;
+            }
 
+            propertyCount += itemPropertyCount;
             propertyBytes += item.Extent == default ? 0 : SpatialExtentPropertyBoxLength;
             if (item.ChannelBitDepths is not null)
             {
@@ -348,7 +454,7 @@ internal sealed partial class HeifEncoderCore
         long length = ItemPropertiesBoxFixedLength
             + propertyBytes
             + (associationItemCount * PropertyAssociationEntryFixedLength)
-            + (propertyCount * associationSize);
+            + (associationPropertyCount * associationSize);
 
         return checked((int)length);
     }
@@ -425,7 +531,12 @@ internal sealed partial class HeifEncoderCore
         foreach (HeifItem item in items)
         {
             int itemLengthOffset = bytesWritten;
-            bytesWritten += WriteBoxHeader(buffer[bytesWritten..], Heif4CharCode.Infe, 2, 0);
+            bytesWritten += WriteBoxHeader(
+                buffer[bytesWritten..],
+                Heif4CharCode.Infe,
+                2,
+                item.IsHidden ? HiddenImageItemFlag : 0);
+
             BinaryPrimitives.WriteUInt16BigEndian(buffer[bytesWritten..], (ushort)item.Id);
             bytesWritten += 2;
             BinaryPrimitives.WriteUInt16BigEndian(buffer[bytesWritten..], 0);
@@ -499,17 +610,26 @@ internal sealed partial class HeifEncoderCore
         // ipco order defines the one-based property indices written later in ipma.
         int ipcoLengthOffset = bytesWritten;
         bytesWritten += WriteBoxHeader(buffer[bytesWritten..], Heif4CharCode.Ipco);
+        ushort nextPropertyIndex = 1;
         foreach (HeifItem item in items)
         {
+            if (item.PropertySource is not null)
+            {
+                continue;
+            }
+
+            item.FirstPropertyIndex = nextPropertyIndex;
             if (item.Extent != default)
             {
                 bytesWritten += WriteSpatialExtentPropertyBox(memory, memoryOffset + bytesWritten, item);
+                nextPropertyIndex++;
             }
 
             byte[]? channelBitDepths = item.ChannelBitDepths;
             if (channelBitDepths is not null)
             {
                 bytesWritten += WritePixelInformationPropertyBox(memory, memoryOffset + bytesWritten, channelBitDepths);
+                nextPropertyIndex++;
             }
             else
             {
@@ -521,6 +641,8 @@ internal sealed partial class HeifEncoderCore
                         memoryOffset + bytesWritten,
                         item.ChannelCount,
                         uniformChannelBitDepth.Value);
+
+                    nextPropertyIndex++;
                 }
             }
 
@@ -528,39 +650,43 @@ internal sealed partial class HeifEncoderCore
             if (codecConfiguration is not null)
             {
                 bytesWritten += WriteAv1CodecConfigurationPropertyBox(memory, memoryOffset + bytesWritten, codecConfiguration);
+                nextPropertyIndex++;
             }
 
             string? auxiliaryType = item.AuxiliaryType;
             if (auxiliaryType is not null)
             {
                 bytesWritten += WriteAuxiliaryTypePropertyBox(memory, memoryOffset + bytesWritten, auxiliaryType);
+                nextPropertyIndex++;
             }
 
             IccProfile? iccProfile = item.IccProfile;
             if (iccProfile is not null)
             {
                 bytesWritten += WriteIccColorInformationPropertyBox(memory, memoryOffset + bytesWritten, item.GetIccProfileDataForWriting());
+                nextPropertyIndex++;
             }
 
             CicpProfile? cicpProfile = item.CicpProfile;
             if (cicpProfile is not null)
             {
                 bytesWritten += WriteColorInformationPropertyBox(memory, memoryOffset + bytesWritten, cicpProfile);
+                nextPropertyIndex++;
             }
         }
 
         BinaryPrimitives.WriteUInt32BigEndian(buffer[ipcoLengthOffset..], (uint)(bytesWritten - ipcoLengthOffset));
-        int propertyCount = 0;
+        int propertyCount = nextPropertyIndex - 1;
         int associationItemCount = 0;
         foreach (HeifItem item in items)
         {
-            int itemPropertyCount = GetPropertyCount(item);
+            HeifItem propertyItem = item.PropertySource ?? item;
+            int itemPropertyCount = GetPropertyCount(propertyItem);
             if (itemPropertyCount == 0)
             {
                 continue;
             }
 
-            propertyCount += itemPropertyCount;
             associationItemCount++;
         }
 
@@ -571,10 +697,10 @@ internal sealed partial class HeifEncoderCore
         bytesWritten += WriteBoxHeader(buffer[bytesWritten..], Heif4CharCode.Ipma, 0, largePropertyIndex ? 1U : 0U);
         BinaryPrimitives.WriteUInt32BigEndian(buffer[bytesWritten..], (uint)associationItemCount);
         bytesWritten += 4;
-        ushort propertyIndex = 1;
         foreach (HeifItem item in items)
         {
-            int itemPropertyCount = GetPropertyCount(item);
+            HeifItem propertyItem = item.PropertySource ?? item;
+            int itemPropertyCount = GetPropertyCount(propertyItem);
             if (itemPropertyCount == 0)
             {
                 continue;
@@ -584,32 +710,33 @@ internal sealed partial class HeifEncoderCore
             bytesWritten += 2;
 
             buffer[bytesWritten++] = (byte)itemPropertyCount;
-            if (item.Extent != default)
+            ushort propertyIndex = propertyItem.FirstPropertyIndex;
+            if (propertyItem.Extent != default)
             {
                 WritePropertyAssociation(buffer, ref bytesWritten, propertyIndex++, largePropertyIndex, false);
             }
 
-            if (item.ChannelBitDepths is not null || item.UniformChannelBitDepth is not null)
+            if (propertyItem.ChannelBitDepths is not null || propertyItem.UniformChannelBitDepth is not null)
             {
                 WritePropertyAssociation(buffer, ref bytesWritten, propertyIndex++, largePropertyIndex, false);
             }
 
-            if (item.Av1CodecConfiguration is not null)
+            if (propertyItem.Av1CodecConfiguration is not null)
             {
                 WritePropertyAssociation(buffer, ref bytesWritten, propertyIndex++, largePropertyIndex, true);
             }
 
-            if (item.AuxiliaryType is not null)
+            if (propertyItem.AuxiliaryType is not null)
             {
                 WritePropertyAssociation(buffer, ref bytesWritten, propertyIndex++, largePropertyIndex, false);
             }
 
-            if (item.IccProfile is not null)
+            if (propertyItem.IccProfile is not null)
             {
                 WritePropertyAssociation(buffer, ref bytesWritten, propertyIndex++, largePropertyIndex, false);
             }
 
-            if (item.CicpProfile is not null)
+            if (propertyItem.CicpProfile is not null)
             {
                 WritePropertyAssociation(buffer, ref bytesWritten, propertyIndex++, largePropertyIndex, false);
             }
@@ -918,68 +1045,396 @@ internal sealed partial class HeifEncoderCore
         CancellationToken cancellationToken)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        byte[]? exifData = null;
-        uint tiffHeaderOffset = 0;
-        byte[]? xmpData = null;
+        Av1EncodingSettings settings = this.ResolveAv1Encoding(image);
+        if (image.Width > Av1Constants.MaxFrameDimension || image.Height > Av1Constants.MaxFrameDimension)
+        {
+            this.CompressAv1GridPixels(image, stream, settings, items, links, cancellationToken);
+            return;
+        }
+
+        Av1ImageItemEncoding encoding = this.CompressAv1ImageItem(
+            image.Frames.RootFrame,
+            stream,
+            settings,
+            cancellationToken);
+
+        this.WriteAv1ImageItems(image, stream, settings, encoding, items, links);
+    }
+
+    /// <summary>
+    /// Encodes a still image as independently coded AV1 cells referenced by one derived grid item.
+    /// </summary>
+    private void CompressAv1GridPixels<TPixel>(
+        Image<TPixel> image,
+        ChunkedMemoryStream stream,
+        Av1EncodingSettings settings,
+        List<HeifItem> items,
+        List<HeifItemLink> links,
+        CancellationToken cancellationToken)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        bool isSubsampledX = !settings.ColorConfig.IsMonochrome && settings.ColorConfig.SubSamplingX;
+        bool isSubsampledY = !settings.ColorConfig.IsMonochrome && settings.ColorConfig.SubSamplingY;
+        if ((isSubsampledX && (image.Width & 1) != 0) || (isSubsampledY && (image.Height & 1) != 0))
+        {
+            throw new NotSupportedException("AVIF grid output dimensions must be even along each subsampled chroma axis.");
+        }
+
+        int columns = GetGridCellCount(image.Width, Av1Constants.MaxFrameDimension);
+        int rows = GetGridCellCount(image.Height, Av1Constants.MaxFrameDimension);
+        if (columns > MaximumGridAxisCellCount || rows > MaximumGridAxisCellCount)
+        {
+            throw new NotSupportedException(
+                $"AVIF grids support at most {MaximumGridAxisCellCount} columns and rows.");
+        }
+
+        int cellWidth = GetGridCellSize(image.Width, columns, isSubsampledX);
+        int cellHeight = GetGridCellSize(image.Height, rows, isSubsampledY);
+        Size encodedCellSize = new(
+            Math.Max(cellWidth, MinimumGridCellDimension),
+            Math.Max(cellHeight, MinimumGridCellDimension));
+
+        long cellCount = (long)columns * rows;
+        long itemCount = 1 + cellCount;
+        if (settings.HasAlpha)
+        {
+            itemCount += 1 + cellCount;
+        }
+
         if (!this.encoder.SkipMetadata)
         {
-            exifData = GetExifData(image.Metadata, out tiffHeaderOffset);
-            byte[]? sourceXmpData = image.Metadata.XmpProfile?.Data;
-            if (sourceXmpData is not null && sourceXmpData.Length > 0)
+            itemCount += image.Metadata.ExifProfile is null ? 0 : 1;
+            itemCount += image.Metadata.XmpProfile is null ? 0 : 1;
+        }
+
+        if (itemCount > ushort.MaxValue)
+        {
+            throw new NotSupportedException(
+                $"The encoded AVIF grid requires {itemCount} items, but this container supports at most {ushort.MaxValue}.");
+        }
+
+        byte channelBitDepth = (byte)settings.BitDepth;
+        long descriptorOffset = stream.Length;
+        int descriptorLength = WriteGridDescriptor(stream, rows, columns, image.Size);
+        HeifItem colorGrid = new(Heif4CharCode.Grid, 1)
+        {
+            ChannelCount = settings.ColorConfig.IsMonochrome ? 1 : 3,
+            UniformChannelBitDepth = channelBitDepth,
+            BitsPerPixel = channelBitDepth * (settings.ColorConfig.IsMonochrome ? 1 : 3),
+            IccProfile = this.encoder.SkipMetadata ? null : image.Metadata.IccProfile,
+            CicpProfile = settings.ColorProfile
+        };
+
+        colorGrid.DataLocations.Add(
+            new HeifLocation(
+                HeifLocationOffsetOrigin.FileOffset,
+                0L,
+                descriptorOffset,
+                descriptorLength));
+
+        colorGrid.SetExtent(image.Size);
+        items.Add(colorGrid);
+        HeifItemLink colorGridLink = new(Heif4CharCode.Dimg, colorGrid.Id);
+        links.Add(colorGridLink);
+        HeifItem? colorPropertySource = null;
+        ImageFrame<TPixel> rootFrame = image.Frames.RootFrame;
+        for (int row = 0; row < rows; row++)
+        {
+            int y = row * cellHeight;
+            int height = Math.Min(cellHeight, image.Height - y);
+            for (int column = 0; column < columns; column++)
             {
-                xmpData = sourceXmpData;
+                cancellationToken.ThrowIfCancellationRequested();
+                int x = column * cellWidth;
+                int width = Math.Min(cellWidth, image.Width - x);
+                Rectangle sourceRectangle = new(x, y, width, height);
+                long colorOffset = stream.Length;
+                ObuSequenceHeader colorHeader = Av1FrameEncoder.EncodeGridCell(
+                    this.configuration,
+                    rootFrame,
+                    sourceRectangle,
+                    encodedCellSize,
+                    stream,
+                    settings.ColorConfig,
+                    settings.ColorQIndex,
+                    this.encoder.Effort);
+
+                long colorLength = stream.Length - colorOffset;
+                HeifItem colorCell = new(Heif4CharCode.Av01, (uint)items.Count + 1)
+                {
+                    IsHidden = true,
+                    ChannelCount = colorGrid.ChannelCount,
+                    UniformChannelBitDepth = channelBitDepth,
+                    BitsPerPixel = colorGrid.BitsPerPixel,
+                    Av1CodecConfiguration = new Av1CodecConfiguration(colorHeader),
+                    IccProfile = colorGrid.IccProfile,
+                    CicpProfile = settings.ColorProfile
+                };
+
+                colorCell.DataLocations.Add(
+                    new HeifLocation(
+                        HeifLocationOffsetOrigin.FileOffset,
+                        0L,
+                        colorOffset,
+                        colorLength));
+
+                colorCell.SetExtent(encodedCellSize);
+                ShareGridCellProperties(colorCell, ref colorPropertySource);
+
+                items.Add(colorCell);
+                colorGridLink.DestinationIds.Add(colorCell.Id);
             }
         }
 
-        Av1EncodingSettings settings = this.ResolveAv1Encoding(image);
+        if (settings.HasAlpha)
+        {
+            descriptorOffset = stream.Length;
+            descriptorLength = WriteGridDescriptor(stream, rows, columns, image.Size);
+            HeifItem alphaGrid = new(Heif4CharCode.Grid, (uint)items.Count + 1)
+            {
+                ChannelCount = 1,
+                UniformChannelBitDepth = channelBitDepth,
+                BitsPerPixel = channelBitDepth,
+                AuxiliaryType = HeifConstants.AlphaAuxiliaryType
+            };
+
+            alphaGrid.DataLocations.Add(
+                new HeifLocation(
+                    HeifLocationOffsetOrigin.FileOffset,
+                    0L,
+                    descriptorOffset,
+                    descriptorLength));
+
+            alphaGrid.SetExtent(image.Size);
+            items.Add(alphaGrid);
+            HeifItemLink alphaGridLink = new(Heif4CharCode.Dimg, alphaGrid.Id);
+            links.Add(alphaGridLink);
+            HeifItemLink alphaLink = new(Heif4CharCode.Auxl, alphaGrid.Id);
+            alphaLink.DestinationIds.Add(colorGrid.Id);
+            links.Add(alphaLink);
+            HeifItem? alphaPropertySource = null;
+            for (int row = 0; row < rows; row++)
+            {
+                int y = row * cellHeight;
+                int height = Math.Min(cellHeight, image.Height - y);
+                for (int column = 0; column < columns; column++)
+                {
+                    cancellationToken.ThrowIfCancellationRequested();
+                    int x = column * cellWidth;
+                    int width = Math.Min(cellWidth, image.Width - x);
+                    Rectangle sourceRectangle = new(x, y, width, height);
+                    long alphaOffset = stream.Length;
+                    ObuSequenceHeader alphaHeader = Av1FrameEncoder.EncodeAlphaGridCell(
+                        this.configuration,
+                        rootFrame,
+                        sourceRectangle,
+                        encodedCellSize,
+                        stream,
+                        settings.AlphaConfig,
+                        settings.AlphaQIndex,
+                        this.encoder.Effort);
+
+                    long alphaLength = stream.Length - alphaOffset;
+                    HeifItem alphaCell = new(Heif4CharCode.Av01, (uint)items.Count + 1)
+                    {
+                        IsHidden = true,
+                        ChannelCount = 1,
+                        UniformChannelBitDepth = channelBitDepth,
+                        BitsPerPixel = channelBitDepth,
+                        Av1CodecConfiguration = new Av1CodecConfiguration(alphaHeader),
+                        AuxiliaryType = HeifConstants.AlphaAuxiliaryType
+                    };
+
+                    alphaCell.DataLocations.Add(
+                        new HeifLocation(
+                            HeifLocationOffsetOrigin.FileOffset,
+                            0L,
+                            alphaOffset,
+                            alphaLength));
+
+                    alphaCell.SetExtent(encodedCellSize);
+                    ShareGridCellProperties(alphaCell, ref alphaPropertySource);
+
+                    items.Add(alphaCell);
+                    alphaGridLink.DestinationIds.Add(alphaCell.Id);
+                }
+            }
+        }
+
+        this.WriteMetadataItems(image, stream, colorGrid, items, links);
+    }
+
+    /// <summary>
+    /// Gets the minimum number of independently coded cells needed along one grid axis.
+    /// </summary>
+    /// <param name="dimension">The complete output dimension along the axis.</param>
+    /// <param name="maximumCellDimension">The largest permitted nominal cell dimension.</param>
+    private static int GetGridCellCount(int dimension, int maximumCellDimension)
+        => (int)(((long)dimension + maximumCellDimension - 1) / maximumCellDimension);
+
+    /// <summary>
+    /// Gets the nominal cell size while preserving chroma alignment for every non-edge cell.
+    /// </summary>
+    private static int GetGridCellSize(int dimension, int cellCount, bool isSubsampled)
+    {
+        int cellSize = (int)(((long)dimension + cellCount - 1) / cellCount);
+        if (isSubsampled && (cellSize & 1) != 0)
+        {
+            cellSize++;
+        }
+
+        return cellSize;
+    }
+
+    /// <summary>
+    /// Writes the fixed grid item payload and returns its exact length.
+    /// </summary>
+    private static int WriteGridDescriptor(Stream stream, int rows, int columns, Size outputSize)
+    {
+        bool usesLargeDimensions = outputSize.Width > ushort.MaxValue || outputSize.Height > ushort.MaxValue;
+        Span<byte> descriptor = stackalloc byte[LongGridDescriptorLength];
+        int descriptorLength = 0;
+        descriptor[descriptorLength++] = GridDescriptorVersion;
+        descriptor[descriptorLength++] = usesLargeDimensions ? LargeGridDimensionsFlag : (byte)0;
+        descriptor[descriptorLength++] = (byte)(rows - 1);
+        descriptor[descriptorLength++] = (byte)(columns - 1);
+        if (usesLargeDimensions)
+        {
+            BinaryPrimitives.WriteUInt32BigEndian(descriptor[descriptorLength..], (uint)outputSize.Width);
+            descriptorLength += sizeof(uint);
+            BinaryPrimitives.WriteUInt32BigEndian(descriptor[descriptorLength..], (uint)outputSize.Height);
+            descriptorLength += sizeof(uint);
+        }
+        else
+        {
+            BinaryPrimitives.WriteUInt16BigEndian(descriptor[descriptorLength..], (ushort)outputSize.Width);
+            descriptorLength += sizeof(ushort);
+            BinaryPrimitives.WriteUInt16BigEndian(descriptor[descriptorLength..], (ushort)outputSize.Height);
+            descriptorLength += sizeof(ushort);
+        }
+
+        stream.Write(descriptor[..descriptorLength]);
+        return descriptorLength;
+    }
+
+    /// <summary>
+    /// Reuses the common property set emitted for the first cell in one grid plane.
+    /// </summary>
+    private static void ShareGridCellProperties(HeifItem item, ref HeifItem? source)
+    {
+        if (source is null)
+        {
+            source = item;
+            return;
+        }
+
+        // Every cell in one plane is coded to the same extent and configuration so current AVIF readers can
+        // share one property set. Only the source rectangle differs for cells clipped by the output canvas.
+        item.PropertySource = source;
+    }
+
+    /// <summary>
+    /// Encodes one frame as the color and optional alpha payloads used by a primary AV1 image item.
+    /// </summary>
+    private Av1ImageItemEncoding CompressAv1ImageItem<TPixel>(
+        ImageFrame<TPixel> frame,
+        ChunkedMemoryStream stream,
+        Av1EncodingSettings settings,
+        CancellationToken cancellationToken)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
         cancellationToken.ThrowIfCancellationRequested();
+        long colorOffset = stream.Length;
         ObuSequenceHeader colorHeader = Av1FrameEncoder.Encode(
             this.configuration,
-            image.Frames.RootFrame,
+            frame,
             stream,
             settings.ColorConfig,
             settings.ColorQIndex,
             this.encoder.Effort);
 
-        long colorLength = stream.Length;
+        long colorLength = stream.Length - colorOffset;
+        Av1CodecConfiguration? alphaConfiguration = null;
+        long alphaOffset = 0;
+        long alphaLength = 0;
+
+        if (settings.HasAlpha)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            alphaOffset = stream.Length;
+            ObuSequenceHeader alphaHeader = Av1FrameEncoder.EncodeAlpha(
+                this.configuration,
+                frame,
+                stream,
+                settings.AlphaConfig,
+                settings.AlphaQIndex,
+                this.encoder.Effort);
+
+            alphaLength = stream.Length - alphaOffset;
+            alphaConfiguration = new Av1CodecConfiguration(alphaHeader);
+        }
+
+        return new Av1ImageItemEncoding(
+            new Av1CodecConfiguration(colorHeader),
+            colorOffset,
+            colorLength,
+            alphaConfiguration,
+            alphaOffset,
+            alphaLength);
+    }
+
+    /// <summary>
+    /// Declares a primary AV1 image item over existing payload extents and appends its associated metadata payloads.
+    /// </summary>
+    private void WriteAv1ImageItems<TPixel>(
+        Image<TPixel> image,
+        ChunkedMemoryStream stream,
+        Av1EncodingSettings settings,
+        Av1ImageItemEncoding encoding,
+        List<HeifItem> items,
+        List<HeifItemLink> links)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
         byte channelBitDepth = (byte)settings.BitDepth;
         HeifItem colorItem = new(Heif4CharCode.Av01, 1)
         {
             ChannelCount = settings.ColorConfig.IsMonochrome ? 1 : 3,
             UniformChannelBitDepth = channelBitDepth,
             BitsPerPixel = channelBitDepth * (settings.ColorConfig.IsMonochrome ? 1 : 3),
-            Av1CodecConfiguration = new Av1CodecConfiguration(colorHeader),
+            Av1CodecConfiguration = encoding.ColorConfiguration,
             IccProfile = this.encoder.SkipMetadata ? null : image.Metadata.IccProfile,
             CicpProfile = settings.ColorProfile
         };
 
-        colorItem.DataLocations.Add(new HeifLocation(HeifLocationOffsetOrigin.FileOffset, 0L, 0L, colorLength));
+        colorItem.DataLocations.Add(
+            new HeifLocation(
+                HeifLocationOffsetOrigin.FileOffset,
+                0L,
+                encoding.ColorOffset,
+                encoding.ColorLength));
+
         colorItem.SetExtent(image.Size);
         items.Add(colorItem);
 
-        if (settings.HasAlpha)
+        Av1CodecConfiguration? alphaConfiguration = encoding.AlphaConfiguration;
+        if (alphaConfiguration is not null)
         {
-            cancellationToken.ThrowIfCancellationRequested();
-            long alphaOffset = stream.Length;
-            ObuSequenceHeader alphaHeader = Av1FrameEncoder.EncodeAlpha(
-                this.configuration,
-                image.Frames.RootFrame,
-                stream,
-                settings.AlphaConfig,
-                settings.AlphaQIndex,
-                this.encoder.Effort);
-
-            long alphaLength = stream.Length - alphaOffset;
             HeifItem alphaItem = new(Heif4CharCode.Av01, 2)
             {
                 ChannelCount = 1,
                 UniformChannelBitDepth = channelBitDepth,
                 BitsPerPixel = channelBitDepth,
-                Av1CodecConfiguration = new Av1CodecConfiguration(alphaHeader),
+                Av1CodecConfiguration = alphaConfiguration,
                 AuxiliaryType = HeifConstants.AlphaAuxiliaryType
             };
 
-            alphaItem.DataLocations.Add(new HeifLocation(HeifLocationOffsetOrigin.FileOffset, 0L, alphaOffset, alphaLength));
+            alphaItem.DataLocations.Add(
+                new HeifLocation(
+                    HeifLocationOffsetOrigin.FileOffset,
+                    0L,
+                    encoding.AlphaOffset,
+                    encoding.AlphaLength));
+
             alphaItem.SetExtent(image.Size);
             items.Add(alphaItem);
             HeifItemLink alphaLink = new(Heif4CharCode.Auxl, alphaItem.Id);
@@ -987,15 +1442,30 @@ internal sealed partial class HeifEncoderCore
             links.Add(alphaLink);
         }
 
+        this.WriteMetadataItems(image, stream, colorItem, items, links);
+    }
+
+    /// <summary>
+    /// Appends Exif and XMP payload items associated with the primary presentation item.
+    /// </summary>
+    private void WriteMetadataItems<TPixel>(
+        Image<TPixel> image,
+        ChunkedMemoryStream stream,
+        HeifItem primaryItem,
+        List<HeifItem> items,
+        List<HeifItemLink> links)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
         if (this.encoder.SkipMetadata)
         {
             return;
         }
 
+        byte[]? exifData = GetExifData(image.Metadata, out uint tiffHeaderOffset);
         if (exifData is not null)
         {
             long exifOffset = stream.Length;
-            Span<byte> offsetBuffer = stackalloc byte[4];
+            Span<byte> offsetBuffer = stackalloc byte[sizeof(uint)];
             BinaryPrimitives.WriteUInt32BigEndian(offsetBuffer, tiffHeaderOffset);
             stream.Write(offsetBuffer);
             stream.Write(exifData);
@@ -1010,15 +1480,16 @@ internal sealed partial class HeifEncoderCore
                     HeifLocationOffsetOrigin.FileOffset,
                     0L,
                     exifOffset,
-                    4L + exifData.Length));
+                    sizeof(uint) + (long)exifData.Length));
 
             items.Add(exifItem);
             HeifItemLink exifLink = new(Heif4CharCode.Cdsc, exifItem.Id);
-            exifLink.DestinationIds.Add(colorItem.Id);
+            exifLink.DestinationIds.Add(primaryItem.Id);
             links.Add(exifLink);
         }
 
-        if (xmpData is not null)
+        byte[]? xmpData = image.Metadata.XmpProfile?.Data;
+        if (xmpData is not null && xmpData.Length > 0)
         {
             long xmpOffset = stream.Length;
             stream.Write(xmpData);
@@ -1037,7 +1508,7 @@ internal sealed partial class HeifEncoderCore
 
             items.Add(xmpItem);
             HeifItemLink xmpLink = new(Heif4CharCode.Cdsc, xmpItem.Id);
-            xmpLink.DestinationIds.Add(colorItem.Id);
+            xmpLink.DestinationIds.Add(primaryItem.Id);
             links.Add(xmpLink);
         }
     }
@@ -1093,16 +1564,6 @@ internal sealed partial class HeifEncoderCore
         CancellationToken cancellationToken)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        if (this.encoder.Lossless)
-        {
-            throw new NotSupportedException("Legacy JPEG image items do not support lossless encoding.");
-        }
-
-        if (this.encoder.BitDepth is not null and not HeifBitDepth.Bit8)
-        {
-            throw new NotSupportedException("Legacy JPEG image items support only 8-bit component encoding.");
-        }
-
         JpegColorType colorType = this.encoder.ChromaSubsampling switch
         {
             null or HeifChromaSubsampling.Yuv420 => JpegColorType.YCbCrRatio420,
@@ -1125,4 +1586,38 @@ internal sealed partial class HeifEncoderCore
         // cannot return while its pooled item payload is still being produced.
         image.SaveAsJpegAsync(stream, encoder, cancellationToken).GetAwaiter().GetResult();
     }
+
+    /// <summary>
+    /// Describes the already-written color and optional alpha extents backing one AV1 image item.
+    /// </summary>
+    private readonly struct Av1ImageItemEncoding
+    {
+        public Av1ImageItemEncoding(
+            Av1CodecConfiguration colorConfiguration,
+            long colorOffset,
+            long colorLength,
+            Av1CodecConfiguration? alphaConfiguration,
+            long alphaOffset,
+            long alphaLength)
+        {
+            this.ColorConfiguration = colorConfiguration;
+            this.ColorOffset = colorOffset;
+            this.ColorLength = colorLength;
+            this.AlphaConfiguration = alphaConfiguration;
+            this.AlphaOffset = alphaOffset;
+            this.AlphaLength = alphaLength;
+        }
+
+        public Av1CodecConfiguration ColorConfiguration { get; }
+
+        public long ColorOffset { get; }
+
+        public long ColorLength { get; }
+
+        public Av1CodecConfiguration? AlphaConfiguration { get; }
+
+        public long AlphaOffset { get; }
+
+        public long AlphaLength { get; }
+    }
 }

src/ImageSharp/Formats/Heif/HeifItem.cs

@@ -28,6 +28,21 @@ internal sealed class HeifItem(Heif4CharCode type, uint id)
     /// </summary>
     public Heif4CharCode Type { get; } = type;
 
+    /// <summary>
+    /// Gets or sets a value indicating whether this item is excluded from primary-item discovery.
+    /// </summary>
+    public bool IsHidden { get; set; }
+
+    /// <summary>
+    /// Gets or sets an earlier item whose identical property associations are reused by this item.
+    /// </summary>
+    public HeifItem? PropertySource { get; set; }
+
+    /// <summary>
+    /// Gets or sets the first one-based property index assigned while writing the property container.
+    /// </summary>
+    public ushort FirstPropertyIndex { get; set; }
+
     /// <summary>
     /// Gets or sets the name of this item.
     /// </summary>

src/ImageSharp/Processing/Processors/Transforms/Linear/FlipProcessor{TPixel}.cs

@@ -60,15 +60,23 @@ internal class FlipProcessor<TPixel> : ImageProcessor<TPixel>
 
     /// <inheritdoc/>
     protected override void OnFrameApply(ImageFrame<TPixel> source)
+        => Apply(this.definition.FlipMode, source, this.Configuration);
+
+    /// <summary>
+    /// Applies an exact axis-aligned reflection to an existing frame.
+    /// </summary>
+    /// <param name="flipMode">The reflection direction.</param>
+    /// <param name="source">The frame modified in place.</param>
+    /// <param name="configuration">The configuration controlling row parallelism and scratch allocation.</param>
+    internal static void Apply(FlipMode flipMode, ImageFrame<TPixel> source, Configuration configuration)
     {
-        switch (this.definition.FlipMode)
+        switch (flipMode)
         {
-            // No default needed as we have already set the pixels.
             case FlipMode.Vertical:
-                FlipX(source.PixelBuffer, this.Configuration);
+                FlipX(source.PixelBuffer, configuration);
                 break;
             case FlipMode.Horizontal:
-                FlipY(source, this.Configuration);
+                FlipY(source, configuration);
                 break;
         }
     }

src/ImageSharp/Processing/Processors/Transforms/Linear/RotateProcessor{TPixel}.cs

@@ -103,25 +103,52 @@ internal class RotateProcessor<TPixel> : AffineTransformProcessor<TPixel>
 
         if (MathF.Abs(degrees - 90) < Constants.Epsilon)
         {
-            Rotate90(source, destination, configuration);
+            ApplyQuarterTurn(RotateMode.Rotate90, source, destination, configuration);
             return true;
         }
 
         if (MathF.Abs(degrees - 180) < Constants.Epsilon)
         {
-            Rotate180(source, destination, configuration);
+            ApplyQuarterTurn(RotateMode.Rotate180, source, destination, configuration);
             return true;
         }
 
         if (MathF.Abs(degrees - 270) < Constants.Epsilon)
         {
-            Rotate270(source, destination, configuration);
+            ApplyQuarterTurn(RotateMode.Rotate270, source, destination, configuration);
             return true;
         }
 
         return false;
     }
 
+    /// <summary>
+    /// Applies an exact quarter-turn rotation between already allocated frames.
+    /// </summary>
+    /// <param name="rotation">The clockwise quarter-turn rotation.</param>
+    /// <param name="source">The source frame.</param>
+    /// <param name="destination">The destination frame with the rotated dimensions.</param>
+    /// <param name="configuration">The configuration controlling row parallelism.</param>
+    internal static void ApplyQuarterTurn(
+        RotateMode rotation,
+        ImageFrame<TPixel> source,
+        ImageFrame<TPixel> destination,
+        Configuration configuration)
+    {
+        switch (rotation)
+        {
+            case RotateMode.Rotate90:
+                Rotate90(source, destination, configuration);
+                break;
+            case RotateMode.Rotate180:
+                Rotate180(source, destination, configuration);
+                break;
+            case RotateMode.Rotate270:
+                Rotate270(source, destination, configuration);
+                break;
+        }
+    }
+
     /// <summary>
     /// Rotates the image 180 degrees clockwise at the centre point.
     /// </summary>

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1BitStreamTests.cs

@@ -204,6 +204,26 @@ public class Av1BitStreamTests
         Assert.Equal(values, actuals);
     }
 
+    [Fact]
+    public void SignedReferenceSubexponentialMatchesFiniteRecentering()
+    {
+        const int ValueMagnitude = 5;
+        const int GroupBitCount = 3;
+        byte[] buffer = new byte[2];
+        Av1BitStreamWriter writer = new(buffer);
+        writer.WriteSignedReferenceSubexponential(-4, ValueMagnitude, GroupBitCount, -4);
+        writer.WriteSignedReferenceSubexponential(4, ValueMagnitude, GroupBitCount, -4);
+        writer.WriteSignedReferenceSubexponential(0, ValueMagnitude, GroupBitCount, 0);
+        writer.Flush();
+
+        Assert.Equal([0x1e, 0x00], buffer);
+
+        Av1BitStreamReader reader = new(buffer);
+        Assert.Equal(-4, reader.ReadSignedReferenceSubexponential(ValueMagnitude, GroupBitCount, -4));
+        Assert.Equal(4, reader.ReadSignedReferenceSubexponential(ValueMagnitude, GroupBitCount, -4));
+        Assert.Equal(0, reader.ReadSignedReferenceSubexponential(ValueMagnitude, GroupBitCount, 0));
+    }
+
     [Theory]
     [InlineData(3)]
     [InlineData(4)]

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1CoefficientsEntropyTests.cs

@@ -5,8 +5,10 @@ using System.Buffers;
 using System.Runtime.CompilerServices;
 using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -133,14 +135,16 @@ public class Av1CoefficientsEntropyTests
                     TilesInfo = new ObuTileGroupHeader()
                 },
                 FrameHeader = new ObuFrameHeader(),
-                PreviousQIndex = []
+                PreviousQIndex = Memory<int>.Empty
             },
             SegmentationNeighborMap = Memory<byte>.Empty,
             ModeInfoGrid = grid,
             ModeInfoAllocation = allocation,
             ModeInfoStride = 4,
             Disallow4x4AllFrames = disallow4x4,
-            CdefPreset = []
+            CdefPreset = Memory<int>.Empty,
+            TileDataOffsets = Memory<int>.Empty,
+            TileDataLengths = Memory<int>.Empty
         };
 
         Point position = new(column, row);
@@ -218,6 +222,137 @@ public class Av1CoefficientsEntropyTests
         Assert.Equal(8, Unsafe.SizeOf<Av1MacroBlockModeInfo>());
     }
 
+    /// <summary>
+    /// Verifies that segment, reference, filter, and flag updates preserve adjacent packed values.
+    /// </summary>
+    [Fact]
+    public void EncoderBlockModeInfoPackedFieldsRemainIndependent()
+    {
+        for (int segment = 0; segment < 8; segment++)
+        {
+            for (int reference = 0; reference < 8; reference++)
+            {
+                for (int vertical = 0; vertical < 4; vertical++)
+                {
+                    for (int horizontal = 0; horizontal < 4; horizontal++)
+                    {
+                        Av1EncoderBlockModeInfo modeInfo = new()
+                        {
+                            Skip = true,
+                            SkipMode = true,
+                            UseIntraBlockCopy = true,
+                            SegmentId = segment,
+                            ReferenceFrame = (Av1ReferenceFrameType)reference,
+                            VerticalInterpolationFilter = (Av1InterpolationFilter)vertical,
+                            HorizontalInterpolationFilter = (Av1InterpolationFilter)horizontal
+                        };
+
+                        Assert.Equal(segment, modeInfo.SegmentId);
+                        Assert.Equal((Av1ReferenceFrameType)reference, modeInfo.ReferenceFrame);
+                        Assert.Equal((Av1InterpolationFilter)vertical, modeInfo.VerticalInterpolationFilter);
+                        Assert.Equal((Av1InterpolationFilter)horizontal, modeInfo.HorizontalInterpolationFilter);
+                        Assert.True(modeInfo.Skip);
+                        Assert.True(modeInfo.SkipMode);
+                        Assert.True(modeInfo.UseIntraBlockCopy);
+
+                        // Overwrite every bit in each shared region after the adjacent value has been populated.
+                        modeInfo.SegmentId = segment ^ 7;
+                        Assert.Equal((Av1ReferenceFrameType)reference, modeInfo.ReferenceFrame);
+                        modeInfo.ReferenceFrame = (Av1ReferenceFrameType)(reference ^ 7);
+                        Assert.Equal(segment ^ 7, modeInfo.SegmentId);
+                        modeInfo.Skip = false;
+                        modeInfo.SkipMode = false;
+                        modeInfo.UseIntraBlockCopy = false;
+                        Assert.Equal((Av1InterpolationFilter)vertical, modeInfo.VerticalInterpolationFilter);
+                        Assert.Equal((Av1InterpolationFilter)horizontal, modeInfo.HorizontalInterpolationFilter);
+                        modeInfo.VerticalInterpolationFilter = (Av1InterpolationFilter)(vertical ^ 3);
+                        modeInfo.HorizontalInterpolationFilter = (Av1InterpolationFilter)(horizontal ^ 3);
+                        Assert.False(modeInfo.Skip);
+                        Assert.False(modeInfo.SkipMode);
+                        Assert.False(modeInfo.UseIntraBlockCopy);
+                    }
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Verifies both filter directions for matching references, mismatches, and unavailable tile neighbors.
+    /// </summary>
+    [Theory]
+    [InlineData(false, false)]
+    [InlineData(false, true)]
+    [InlineData(true, false)]
+    [InlineData(true, true)]
+    public void EncoderInterpolationContextUsesTileNeighbors(bool aboveAvailable, bool leftAvailable)
+    {
+        // Rows are the above filter and columns the left filter; index three means a nonmatching reference.
+        ReadOnlySpan<int> expectedContexts = [0, 3, 3, 0, 3, 1, 3, 1, 3, 3, 2, 2, 0, 1, 2, 3];
+        Av1MacroBlockModeInfo[] allocation = new Av1MacroBlockModeInfo[3];
+        int[] grid = new int[9];
+        grid[1] = 0;
+        grid[3] = 1;
+        grid[4] = 2;
+        Av1MacroBlockD macroBlock = CreateMacroBlock();
+        macroBlock.ModeInfoStride = 3;
+        macroBlock.IsUpAvailable = aboveAvailable;
+        macroBlock.IsLeftAvailable = leftAvailable;
+        macroBlock.SetModeInfoGrid(grid, allocation, 4);
+        Av1EncoderBlockModeInfo current = new() { ReferenceFrame = Av1ReferenceFrameType.Last };
+        for (int above = 0; above < 4; above++)
+        {
+            for (int left = 0; left < 4; left++)
+            {
+                allocation[0].Block.ReferenceFrame = above == 3 ? Av1ReferenceFrameType.Golden : Av1ReferenceFrameType.Last;
+                allocation[0].Block.VerticalInterpolationFilter = (Av1InterpolationFilter)(above % 3);
+                allocation[0].Block.HorizontalInterpolationFilter = (Av1InterpolationFilter)((above + 1) % 3);
+                allocation[1].Block.ReferenceFrame = left == 3 ? Av1ReferenceFrameType.Golden : Av1ReferenceFrameType.Last;
+                allocation[1].Block.VerticalInterpolationFilter = (Av1InterpolationFilter)(left % 3);
+                allocation[1].Block.HorizontalInterpolationFilter = (Av1InterpolationFilter)((left + 1) % 3);
+                int aboveVertical = aboveAvailable ? above : 3;
+                int leftVertical = leftAvailable ? left : 3;
+                int aboveHorizontal = aboveVertical == 3 ? 3 : (above + 1) % 3;
+                int leftHorizontal = leftVertical == 3 ? 3 : (left + 1) % 3;
+
+                Assert.Equal(expectedContexts[(aboveVertical * 4) + leftVertical], Av1SymbolContextHelper.GetSwitchableInterpolationContext(current, macroBlock, 0));
+                Assert.Equal(8 + expectedContexts[(aboveHorizontal * 4) + leftHorizontal], Av1SymbolContextHelper.GetSwitchableInterpolationContext(current, macroBlock, 1));
+            }
+        }
+    }
+
+    /// <summary>
+    /// Verifies which inter modes signal filters and distinguishes residual skip from compound skip mode.
+    /// </summary>
+    [Theory]
+    [InlineData(Av1GlobalMotionType.Identity, Av1PredictionMode.GlobalMotionVector, false)]
+    [InlineData(Av1GlobalMotionType.Translation, Av1PredictionMode.GlobalMotionVector, true)]
+    [InlineData(Av1GlobalMotionType.RotationZoom, Av1PredictionMode.GlobalMotionVector, false)]
+    [InlineData(Av1GlobalMotionType.Affine, Av1PredictionMode.GlobalMotionVector, false)]
+    [InlineData(Av1GlobalMotionType.Identity, Av1PredictionMode.NewMotionVector, true)]
+    [InlineData(Av1GlobalMotionType.Identity, Av1PredictionMode.NearestMotionVector, true)]
+    public void EncoderInterpolationSyntaxMatchesModeEligibility(int globalType, int predictionMode, bool expected)
+    {
+        ObuFrameHeader frameHeader = new() { InterpolationFilter = Av1InterpolationFilter.Switchable };
+        frameHeader.GetGlobalMotionParameters()[0].Type = (Av1GlobalMotionType)globalType;
+        Av1EncoderBlockModeInfo modeInfo = new()
+        {
+            BlockSize = Av1BlockSize.Block8x8,
+            ReferenceFrame = Av1ReferenceFrameType.Last,
+            Mode = (Av1PredictionMode)predictionMode
+        };
+
+        Assert.Equal(expected, Av1TileWriter.UsesSwitchableInterpolation(frameHeader, modeInfo));
+        modeInfo.Skip = true;
+        Assert.Equal(expected, Av1TileWriter.UsesSwitchableInterpolation(frameHeader, modeInfo));
+        modeInfo.BlockSize = Av1BlockSize.Block4x8;
+        Assert.True(Av1TileWriter.UsesSwitchableInterpolation(frameHeader, modeInfo));
+        modeInfo.SkipMode = true;
+        Assert.False(Av1TileWriter.UsesSwitchableInterpolation(frameHeader, modeInfo));
+        modeInfo.SkipMode = false;
+        frameHeader.InterpolationFilter = Av1InterpolationFilter.Regular;
+        Assert.False(Av1TileWriter.UsesSwitchableInterpolation(frameHeader, modeInfo));
+    }
+
     [Fact]
     public void EncoderSuperblockWorkspaceUsesOneExactSizeOwner()
     {
@@ -231,9 +366,9 @@ public class Av1CoefficientsEntropyTests
         {
             allocation = Assert.Single(allocator.AllocationLog);
             Assert.Empty(allocator.ReturnLog);
-            Assert.Equal(typeof(Av1EncoderBlockStruct), allocation.ElementType);
+            Assert.Equal(typeof(byte), allocation.ElementType);
             Assert.Equal(AllocationOptions.None, allocation.AllocationOptions);
-            Assert.Equal(Av1EncoderSuperblockWorkspace.StorageLength, allocation.Length);
+            Assert.Equal(Av1EncoderSuperblockWorkspace.StorageByteLength, allocation.Length);
             Assert.Equal(Av1EncoderSuperblockWorkspace.MaximumFinalBlockCount, workspace.FinalBlocks.Length);
             Assert.Equal(Av1EncoderSuperblockWorkspace.MaximumPartitionCount, workspace.PartitionTypes.Length);
             Assert.Equal(Av1EncoderBlockStruct.StorageSize, Unsafe.SizeOf<Av1EncoderBlockStruct>());
@@ -295,39 +430,34 @@ public class Av1CoefficientsEntropyTests
     }
 
     [Fact]
-    public void EncoderPaletteMapsUseOneLazyExactSizeOwner()
+    public void EncoderSuperblockWorkspaceExposesReusablePaletteMaps()
     {
         TestMemoryAllocator allocator = new();
         allocator.EnableNonThreadSafeLogging();
         Configuration configuration = Configuration.Default.Clone();
         configuration.MemoryAllocator = allocator;
 
-        TestMemoryAllocator.AllocationRequest[] allocations;
+        TestMemoryAllocator.AllocationRequest allocation;
         using (Av1EncoderSuperblockWorkspace workspace = new(configuration))
         {
-            Assert.Single(allocator.AllocationLog);
+            allocation = Assert.Single(allocator.AllocationLog);
             Assert.Empty(allocator.ReturnLog);
 
             Av1EncoderPaletteMapBuffer maps = workspace.GetPaletteMaps();
             Assert.Same(maps, workspace.GetPaletteMaps());
-            allocations = allocator.AllocationLog.ToArray();
-            Assert.Equal(2, allocations.Length);
-            Assert.Equal(typeof(byte), allocations[1].ElementType);
-            Assert.Equal(Av1EncoderPaletteMapBuffer.StorageLength, allocations[1].Length);
-            Assert.Equal(AllocationOptions.None, allocations[1].AllocationOptions);
+            Assert.Single(allocator.AllocationLog);
 
             Buffer2DRegion<byte> luma = maps.GetMap(Av1PlaneType.Y, 64, 64);
             Buffer2DRegion<byte> chroma = maps.GetMap(Av1PlaneType.Uv, 32, 32);
             luma.DangerousGetRowSpan(0)[0] = 3;
             chroma.DangerousGetRowSpan(0)[0] = 5;
-            Assert.Equal(3, luma.DangerousGetRowSpan(0)[0]);
-            Assert.Equal(5, chroma.DangerousGetRowSpan(0)[0]);
+            Av1EncoderPaletteMapBuffer retainedMaps = workspace.GetPaletteMaps();
+            Assert.Equal(3, retainedMaps.GetMap(Av1PlaneType.Y, 64, 64).DangerousGetRowSpan(0)[0]);
+            Assert.Equal(5, retainedMaps.GetMap(Av1PlaneType.Uv, 32, 32).DangerousGetRowSpan(0)[0]);
         }
 
-        Assert.Equal(2, allocator.ReturnLog.Count);
-        Assert.Equal(
-            allocations.Select(x => x.AllocationId).Order(),
-            allocator.ReturnLog.Select(x => x.AllocationId).Order());
+        TestMemoryAllocator.ReturnRequest returned = Assert.Single(allocator.ReturnLog);
+        Assert.Equal(allocation.AllocationId, returned.AllocationId);
     }
 
     [Fact]
@@ -841,7 +971,7 @@ public class Av1CoefficientsEntropyTests
             skip: false,
             modeInfoPosition: new Point(20, 4));
 
-        Assert.Equal(new[] { -1, 3, -1, -1 }, picture.CdefPreset[0]);
+        Assert.True(picture.CdefPreset.Span.SequenceEqual([-1, 3, -1, -1]));
     }
 
     [Fact]
@@ -1431,13 +1561,15 @@ public class Av1CoefficientsEntropyTests
                     TilesInfo = tiles
                 },
                 FrameHeader = frameHeader,
-                PreviousQIndex = []
+                PreviousQIndex = Memory<int>.Empty
             },
             SegmentationNeighborMap = Memory<byte>.Empty,
             ModeInfoGrid = modeInfoGrid,
             ModeInfoAllocation = modeInfoAllocation,
             ModeInfoStride = modeInfoColumnCount,
-            CdefPreset = [[-1, -1, -1, -1]]
+            CdefPreset = new int[] { -1, -1, -1, -1 },
+            TileDataOffsets = Memory<int>.Empty,
+            TileDataLengths = Memory<int>.Empty
         };
     }
 

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1EncoderFrameTests.cs

@@ -3,9 +3,11 @@
 
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Heif.Av1;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Motion;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Formats.Heif.Components;
@@ -26,6 +28,42 @@ public class Av1EncoderFrameTests
     private const int Yuv422 = (int)Av1ColorFormat.Yuv422;
     private const int Yuv444 = (int)Av1ColorFormat.Yuv444;
 
+    [Fact]
+    public void EncodeUsesMultipleTilesWhenSingleTileWidthLimitIsExceeded()
+    {
+        const int Width = Av1Constants.MaxTileWidth + 1;
+        const int SuperblockSize = 1 << (Av1Constants.MaxSuperBlockSizeLog2 - 1);
+        const int Height = SuperblockSize;
+        int superblockColumns = (Width + SuperblockSize - 1) / SuperblockSize;
+        int secondTileStart = ((superblockColumns + 1) / 2) * SuperblockSize;
+        using Image<L8> source = new(Width, Height, new L8(128));
+        source[0, 0] = new L8(1);
+        source[secondTileStart - 1, 0] = new L8(17);
+        source[secondTileStart, 0] = new L8(241);
+        source[Width - 1, Height - 1] = new L8(255);
+        using MemoryStream stream = new();
+        ObuColorConfig colorConfig = CreateColorConfig(Av1BitDepth.EightBit, Av1ColorFormat.Yuv400);
+
+        Av1FrameEncoder.Encode(
+            Configuration.Default,
+            source.Frames.RootFrame,
+            stream,
+            colorConfig,
+            qIndex: 0,
+            effort: 0);
+
+        using Av1Decoder decoder = new(Configuration.Default);
+        using Image<L8> decoded = decoder.Decode<L8>(stream.ToArray());
+
+        Assert.Equal(2, decoder.FrameHeader.TilesInfo.TileColumnCount);
+        Assert.Equal(1, decoder.FrameHeader.TilesInfo.TileRowCount);
+        Assert.Equal(source.Size, decoded.Size);
+        Assert.Equal(source[0, 0], decoded[0, 0]);
+        Assert.Equal(source[secondTileStart - 1, 0], decoded[secondTileStart - 1, 0]);
+        Assert.Equal(source[secondTileStart, 0], decoded[secondTileStart, 0]);
+        Assert.Equal(source[Width - 1, Height - 1], decoded[Width - 1, Height - 1]);
+    }
+
     [Theory]
     [InlineData(8, 8, false, EightBit, Yuv400)]
     [InlineData(8, 8, true, EightBit, Yuv400)]
@@ -153,6 +191,250 @@ public class Av1EncoderFrameTests
         }
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void EncodeSequenceFrameWritesNonReducedHeaderConsumedByProductionDecoder(bool encodeAlpha)
+    {
+        const int Width = 16;
+        const int Height = 16;
+        using Image<Rgba32> source = new(Width, Height, new Rgba32(48, 96, 192));
+        using MemoryStream stream = new();
+        ObuColorConfig colorConfig = encodeAlpha
+            ? CreateColorConfig(Av1BitDepth.EightBit)
+            : CreateColorConfig(Av1BitDepth.EightBit, Av1ColorFormat.Yuv420);
+
+        using Av1FrameEncoder.SequenceEncoder encoder = encodeAlpha
+            ? Av1FrameEncoder.CreateAlphaSequenceEncoder(
+                Configuration.Default,
+                Width,
+                Height,
+                colorConfig,
+                qIndex: 37,
+                effort: 5)
+            : Av1FrameEncoder.CreateColorSequenceEncoder(
+                Configuration.Default,
+                Width,
+                Height,
+                colorConfig,
+                qIndex: 37,
+                effort: 5);
+
+        encoder.EncodeKeyFrame(source.Frames.RootFrame, stream);
+        ObuSequenceHeader encodedHeader = encoder.SequenceHeader;
+        byte[] payload = stream.ToArray();
+        using Av1Decoder decoder = new(Configuration.Default);
+        using Image<Rgba32> decoded = decoder.Decode<Rgba32>(payload);
+        ObuSequenceHeader decodedHeader = decoder.SequenceHeader;
+
+        Assert.False(encodedHeader.IsStillPicture);
+        Assert.False(encodedHeader.IsReducedStillPictureHeader);
+        Assert.Equal(encodeAlpha, encodedHeader.ColorConfig.IsMonochrome);
+        Assert.NotNull(decodedHeader);
+        Assert.False(decodedHeader.IsStillPicture);
+        Assert.False(decodedHeader.IsReducedStillPictureHeader);
+        Assert.Equal(new Size(Width, Height), decoded.Size);
+    }
+
+    /// <summary>
+    /// Verifies retained reference reconstruction and effort-dependent filter signaling through production sequence decoding.
+    /// </summary>
+    [Theory]
+    [InlineData(5, false, false)]
+    [InlineData(7, false, false)]
+    [InlineData(8, true, false)]
+    [InlineData(9, true, true)]
+    public void SequenceEncoderUsesRetainedReconstructionForInterFrame(int effort, bool switchableFilters, bool dualFilters)
+    {
+        const int Width = 16;
+        const int Height = 16;
+        Rgba32 sourceColor = new(48, 96, 192);
+        using Image<Rgba32> source = new(Width, Height, sourceColor);
+        using MemoryStream firstSample = new();
+        using MemoryStream secondSample = new();
+        ObuColorConfig colorConfig = CreateColorConfig(Av1BitDepth.EightBit, Av1ColorFormat.Yuv420);
+        using Av1FrameEncoder.SequenceEncoder encoder = Av1FrameEncoder.CreateColorSequenceEncoder(
+            Configuration.Default,
+            Width,
+            Height,
+            colorConfig,
+            qIndex: 37,
+            effort);
+
+        encoder.EncodeKeyFrame(source.Frames.RootFrame, firstSample);
+        encoder.EncodeInterFrame(source.Frames.RootFrame, secondSample);
+
+        // Retain the exact two-sample elementary stream for independent reference-decoder acceptance.
+        string outputDirectory = TestEnvironment.CreateOutputDirectory("Heif", "Av1", nameof(this.SequenceEncoderUsesRetainedReconstructionForInterFrame));
+        using (FileStream output = File.Create(Path.Combine(outputDirectory, $"effort-{effort}.obu")))
+        {
+            firstSample.Position = 0;
+            firstSample.CopyTo(output);
+            secondSample.Position = 0;
+            secondSample.CopyTo(output);
+        }
+
+        using Av1Decoder decoder = new(Configuration.Default);
+        using ImageFrame<Rgba32> decodedFirst = decoder.DecodeSequenceFrame<Rgba32>(
+            firstSample.ToArray(),
+            null,
+            null);
+
+        using ImageFrame<Rgba32> decodedSecond = decoder.DecodeSequenceFrame<Rgba32>(
+            secondSample.ToArray(),
+            null,
+            null);
+
+        ObuFrameHeader frameHeader = decoder.FrameHeader;
+        Assert.Equal(ObuFrameType.InterFrame, frameHeader.FrameType);
+        Assert.False(frameHeader.SegmentationParameters.Enabled);
+        Assert.False(frameHeader.AllowScreenContentTools);
+        Assert.False(frameHeader.ForceIntegerMotionVector);
+        Assert.Equal(effort >= 8, frameHeader.AllowHighPrecisionMotionVector);
+        Assert.Equal(37, frameHeader.QuantizationParameters.BaseQIndex);
+        Assert.Equal(switchableFilters ? Av1InterpolationFilter.Switchable : Av1InterpolationFilter.Regular, frameHeader.InterpolationFilter);
+        Assert.Equal(dualFilters, decoder.SequenceHeader.EnableDualFilter);
+        for (int y = 0; y < Height; y++)
+        {
+            Assert.Equal(
+                decodedFirst.PixelBuffer.DangerousGetRowSpan(y),
+                decodedSecond.PixelBuffer.DangerousGetRowSpan(y));
+        }
+    }
+
+    [Fact]
+    public void SequenceEncoderWritesSelectedGlobalTranslation()
+    {
+        const int Width = 64;
+        const int Height = 64;
+        const int HorizontalOffset = 4;
+        using Image<Rgba32> first = new(Width, Height);
+        using Image<Rgba32> second = new(Width, Height);
+        for (int y = 0; y < Height; y++)
+        {
+            Span<Rgba32> firstRow = first.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(y);
+            for (int x = 0; x < Width; x++)
+            {
+                byte value = (byte)(((x * 37) + (y * 53) + ((x * y) * 11)) & byte.MaxValue);
+                firstRow[x] = new Rgba32(value, value, value);
+            }
+        }
+
+        for (int y = 0; y < Height; y++)
+        {
+            ReadOnlySpan<Rgba32> firstRow = first.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(y);
+            Span<Rgba32> secondRow = second.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(y);
+            for (int x = 0; x < Width; x++)
+            {
+                secondRow[x] = firstRow[Math.Min(x + HorizontalOffset, Width - 1)];
+            }
+        }
+
+        using MemoryStream firstSample = new();
+        using MemoryStream secondSample = new();
+        ObuColorConfig colorConfig = CreateColorConfig(Av1BitDepth.EightBit, Av1ColorFormat.Yuv420);
+        using Av1FrameEncoder.SequenceEncoder encoder = Av1FrameEncoder.CreateColorSequenceEncoder(
+            Configuration.Default,
+            Width,
+            Height,
+            colorConfig,
+            qIndex: 4,
+            effort: 6);
+
+        encoder.EncodeKeyFrame(first.Frames.RootFrame, firstSample);
+        encoder.EncodeInterFrame(second.Frames.RootFrame, secondSample);
+
+        using Av1Decoder decoder = new(Configuration.Default);
+        using ImageFrame<Rgba32> decodedFirst = decoder.DecodeSequenceFrame<Rgba32>(
+            firstSample.ToArray(),
+            null,
+            null);
+
+        using ImageFrame<Rgba32> decodedSecond = decoder.DecodeSequenceFrame<Rgba32>(
+            secondSample.ToArray(),
+            null,
+            null);
+
+        ObuFrameHeader frameHeader = decoder.FrameHeader;
+        Av1GlobalMotionParameters globalMotion = frameHeader.GetGlobalMotionParameters()[0];
+        Av1MotionVector vector = globalMotion.GetMotionVector(
+            frameHeader.AllowHighPrecisionMotionVector,
+            Av1BlockSize.Block8x8,
+            default,
+            frameHeader.ForceIntegerMotionVector);
+
+        Assert.Equal(Av1GlobalMotionType.RotationZoom, globalMotion.Type);
+        Assert.False(frameHeader.AllowScreenContentTools);
+        Assert.False(frameHeader.ForceIntegerMotionVector);
+        Assert.Equal(0, vector.Row);
+        Assert.Equal(HorizontalOffset * 8, vector.Column);
+        Assert.Equal(first.Size, decodedFirst.Size);
+        Assert.Equal(second.Size, decodedSecond.Size);
+    }
+
+    [Theory]
+    [InlineData(false, EightBit, Yuv420, 384)]
+    [InlineData(false, TwelveBit, Yuv444, 288)]
+    [InlineData(true, EightBit, Yuv400, 192)]
+    [InlineData(true, TwelveBit, Yuv400, 192)]
+    public void SequenceEncoderReusesAllocatorOwnedRowStorage(
+        bool encodeAlpha,
+        int bitDepthValue,
+        int colorFormatValue,
+        int expectedRowStorageLength)
+    {
+        const int Width = 64;
+        const int Height = 64;
+        Av1BitDepth bitDepth = (Av1BitDepth)bitDepthValue;
+        Av1ColorFormat colorFormat = (Av1ColorFormat)colorFormatValue;
+        using Image<Rgba64> source = new(
+            Width,
+            Height,
+            new Rgba64(ushort.MaxValue, 32768, 16384, 49152));
+
+        TestMemoryAllocator allocator = new();
+        allocator.EnableNonThreadSafeLogging();
+        Configuration configuration = Configuration.Default.Clone();
+        configuration.MemoryAllocator = allocator;
+        ObuColorConfig colorConfig = CreateColorConfig(bitDepth, colorFormat);
+        TestMemoryAllocator.AllocationRequest rowStorage;
+        int allocationCount;
+        using (Av1FrameEncoder.SequenceEncoder encoder = encodeAlpha
+            ? Av1FrameEncoder.CreateAlphaSequenceEncoder(
+                configuration,
+                Width,
+                Height,
+                colorConfig,
+                qIndex: 37,
+                effort: 6)
+            : Av1FrameEncoder.CreateColorSequenceEncoder(
+                configuration,
+                Width,
+                Height,
+                colorConfig,
+                qIndex: 37,
+                effort: 6))
+        {
+            rowStorage = Assert.Single(
+                allocator.AllocationLog,
+                allocation => allocation.ElementType == typeof(float));
+
+            allocationCount = allocator.AllocationLog.Count;
+            using MemoryStream output = new(256 * 1024);
+            encoder.EncodeKeyFrame(source.Frames.RootFrame, output);
+            encoder.EncodeInterFrame(source.Frames.RootFrame, output);
+
+            // Fixed sequence geometry lets libaom retain its frame-sized compressor data. The ImageSharp
+            // sequence encoder must likewise perform every sample conversion and coding pass without another rent.
+            Assert.Equal(allocationCount, allocator.AllocationLog.Count);
+        }
+
+        Assert.Equal(expectedRowStorageLength, rowStorage.Length);
+        Assert.Contains(
+            allocator.ReturnLog,
+            returned => returned.AllocationId == rowStorage.AllocationId);
+    }
+
     [Theory]
     [InlineData(TenBit)]
     [InlineData(TwelveBit)]

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1EncoderModeInfoBufferTests.cs

@@ -54,10 +54,13 @@ public class Av1EncoderModeInfoBufferTests
     public unsafe void PictureBufferPacksAllPictureStateIntoTwoAllocatorOwners(
         bool allowScreenContentTools,
         bool allowIntraBlockCopy,
-        int expectedStateStorageLength)
+        int expectedContextStorageLength)
     {
         const int Width = 16;
         const int Height = 16;
+
+        // Four CDEF presets, the preceding quantizer, and two payload bounds follow the context regions.
+        const int TileStateStorageLength = 7 * sizeof(int);
         TestMemoryAllocator allocator = new();
         allocator.EnableNonThreadSafeLogging();
         Configuration configuration = Configuration.Default.Clone();
@@ -108,7 +111,7 @@ public class Av1EncoderModeInfoBufferTests
             Assert.Equal(6_144, allocations[0].Length);
             Assert.Equal(AllocationOptions.Clean, allocations[0].AllocationOptions);
             Assert.Equal(typeof(byte), allocations[1].ElementType);
-            Assert.Equal(expectedStateStorageLength, allocations[1].Length);
+            Assert.Equal(expectedContextStorageLength + TileStateStorageLength, allocations[1].Length);
             Assert.Equal(AllocationOptions.Clean, allocations[1].AllocationOptions);
             Assert.Empty(allocator.ReturnLog);
 
@@ -122,6 +125,29 @@ public class Av1EncoderModeInfoBufferTests
             Assert.Equal(16, picture.CbDcSignLevelCoefficientNeighbors[0].Top.Length);
             Assert.Equal(32, picture.TransformFunctionContexts[0].Left.Length);
             Assert.Equal(32, picture.TransformFunctionContexts[0].Top.Length);
+            Assert.Equal(4, picture.CdefPreset.Length);
+            Assert.Equal(1, picture.Parent.PreviousQIndex.Length);
+            Assert.Equal(1, picture.TileDataOffsets.Length);
+            Assert.Equal(1, picture.TileDataLengths.Length);
+
+            // Exact offsets prove that all four typed views occupy the trailing region of the same owner,
+            // without gaps, overlapping fields, or a separate allocation hidden behind a memory manager.
+            fixed (byte* state = picture.SegmentationNeighborMap.Span)
+            {
+                fixed (int* cdef = picture.CdefPreset.Span,
+                    quantizer = picture.Parent.PreviousQIndex.Span,
+                    offsets = picture.TileDataOffsets.Span,
+                    lengths = picture.TileDataLengths.Span)
+                {
+                    Assert.Equal((nuint)0, (nuint)cdef % (nuint)sizeof(int));
+                    Assert.Equal(expectedContextStorageLength, (byte*)cdef - state);
+                    Assert.Equal(4, quantizer - cdef);
+                    Assert.Equal(1, offsets - quantizer);
+                    Assert.Equal(1, lengths - offsets);
+                    Assert.Equal(allocations[1].Length, (byte*)(lengths + 1) - state);
+                }
+            }
+
             if (allowScreenContentTools)
             {
                 Av1NeighborArrayUnit<Av1EncoderPaletteInfo> paletteContext = Assert.Single(picture.PaletteContexts);
@@ -176,6 +202,157 @@ public class Av1EncoderModeInfoBufferTests
             allocator.ReturnLog.Select(x => x.AllocationId).Order());
     }
 
+    [Fact]
+    public void InterPictureBufferExposesPackedMotionVectorStorage()
+    {
+        const int Width = 16;
+        const int Height = 16;
+        ObuColorConfig colorConfig = new()
+        {
+            IsMonochrome = false,
+            SubSamplingX = true,
+            SubSamplingY = true,
+            BitDepth = Av1BitDepth.EightBit
+        };
+
+        ObuTileGroupHeader tiles = new()
+        {
+            TileColumnCount = 1,
+            TileRowCount = 1
+        };
+
+        tiles.TileColumnStartModeInfo[1] = Width >> Av1Constants.ModeInfoSizeLog2;
+        tiles.TileRowStartModeInfo[1] = Height >> Av1Constants.ModeInfoSizeLog2;
+        ObuSequenceHeader sequenceHeader = new() { ColorConfig = colorConfig };
+        ObuFrameHeader frameHeader = new()
+        {
+            FrameType = ObuFrameType.InterFrame,
+            ModeInfoColumnCount = Width >> Av1Constants.ModeInfoSizeLog2,
+            ModeInfoRowCount = Height >> Av1Constants.ModeInfoSizeLog2,
+            TilesInfo = tiles
+        };
+
+        using Av1EncoderPictureBuffer buffer = new(
+            Configuration.Default,
+            sequenceHeader,
+            frameHeader,
+            Width,
+            Height,
+            disallow4x4AllFrames: true);
+
+        Av1PictureControlSet picture = buffer.Picture;
+        Assert.Equal(256, picture.DisplacementVectors.Length);
+        Assert.Equal(0, picture.IntraBlockCopySearch.OriginWidth);
+
+        Point position = new(2, 2);
+        Av1MotionVector vector = new(-32, 40);
+        picture.MapModeInfoBlock(position, Av1BlockSize.Block8x8);
+        picture.SetDisplacementVector(position, vector);
+        Assert.Equal(vector, picture.GetDisplacementVector(new Point(3, 3)));
+    }
+
+    [Fact]
+    public void PictureBufferResetReusesStorageAndRestoresFrameState()
+    {
+        const int Width = 16;
+        const int Height = 16;
+        const int InitialQIndex = 37;
+        const int NextQIndex = 91;
+        TestMemoryAllocator allocator = new();
+        allocator.EnableNonThreadSafeLogging();
+        Configuration configuration = Configuration.Default.Clone();
+        configuration.MemoryAllocator = allocator;
+        ObuColorConfig colorConfig = new()
+        {
+            IsMonochrome = false,
+            SubSamplingX = true,
+            SubSamplingY = true,
+            BitDepth = Av1BitDepth.EightBit
+        };
+
+        ObuTileGroupHeader initialTiles = new()
+        {
+            TileColumnCount = 1,
+            TileRowCount = 1
+        };
+
+        initialTiles.TileColumnStartModeInfo[1] = Width >> Av1Constants.ModeInfoSizeLog2;
+        initialTiles.TileRowStartModeInfo[1] = Height >> Av1Constants.ModeInfoSizeLog2;
+        ObuSequenceHeader sequenceHeader = new()
+        {
+            Use128x128Superblock = true,
+            ColorConfig = colorConfig
+        };
+
+        ObuFrameHeader initialFrameHeader = new()
+        {
+            FrameType = ObuFrameType.KeyFrame,
+            ModeInfoColumnCount = Width >> Av1Constants.ModeInfoSizeLog2,
+            ModeInfoRowCount = Height >> Av1Constants.ModeInfoSizeLog2,
+            TilesInfo = initialTiles
+        };
+
+        initialFrameHeader.QuantizationParameters.BaseQIndex = InitialQIndex;
+        using Av1EncoderPictureBuffer buffer = new(
+            configuration,
+            sequenceHeader,
+            initialFrameHeader,
+            Width,
+            Height,
+            disallow4x4AllFrames: true,
+            allocateScreenContentState: true,
+            allocateMotionVectorState: true,
+            allocateIntraBlockCopySearch: true);
+
+        Av1PictureControlSet picture = buffer.Picture;
+        int allocationCount = allocator.AllocationLog.Count;
+        picture.ModeInfoGrid.Span[0] = 7;
+        picture.ModeInfoAllocation.Span[0].Block.Mode = Av1PredictionMode.Paeth;
+        picture.SegmentationNeighborMap.Span[0] = 3;
+        picture.PartitionContexts[0].Left[0] = new Av1PartitionContext(5, 7);
+        picture.TransformFunctionContexts[0].Top[0] = 8;
+        picture.PaletteContexts[0].Left[0].PaletteSizes[0] = 2;
+        picture.DisplacementVectors.Span[0] = new Av1EncoderDisplacementVector { Row = -8, Column = 16 };
+        picture.CdefPreset.Span[0] = 2;
+        picture.Parent.PreviousQIndex.Span[0] = InitialQIndex + 1;
+        picture.TileDataOffsets.Span[0] = 11;
+        picture.TileDataLengths.Span[0] = 13;
+        ObuTileGroupHeader nextTiles = new()
+        {
+            TileColumnCount = 1,
+            TileRowCount = 1
+        };
+
+        nextTiles.TileColumnStartModeInfo[1] = Width >> Av1Constants.ModeInfoSizeLog2;
+        nextTiles.TileRowStartModeInfo[1] = Height >> Av1Constants.ModeInfoSizeLog2;
+        ObuFrameHeader nextFrameHeader = new()
+        {
+            FrameType = ObuFrameType.InterFrame,
+            ModeInfoColumnCount = Width >> Av1Constants.ModeInfoSizeLog2,
+            ModeInfoRowCount = Height >> Av1Constants.ModeInfoSizeLog2,
+            TilesInfo = nextTiles
+        };
+
+        nextFrameHeader.QuantizationParameters.BaseQIndex = NextQIndex;
+        buffer.Reset(nextFrameHeader);
+
+        Assert.Equal(allocationCount, allocator.AllocationLog.Count);
+        Assert.Empty(allocator.ReturnLog);
+        Assert.Equal(0, picture.ModeInfoGrid.Span[0]);
+        Assert.Equal(Av1PredictionMode.DC, picture.ModeInfoAllocation.Span[0].Block.Mode);
+        Assert.Equal(0, picture.SegmentationNeighborMap.Span[0]);
+        Assert.Equal(default, picture.PartitionContexts[0].Left[0]);
+        Assert.Equal(Av1Constants.MaxTransformSize, picture.TransformFunctionContexts[0].Top[0]);
+        Assert.Equal(0, picture.PaletteContexts[0].Left[0].PaletteSizes[0]);
+        Assert.Equal(default, picture.DisplacementVectors.Span[0]);
+        Assert.Equal(-1, picture.CdefPreset.Span[0]);
+        Assert.Equal(NextQIndex, picture.Parent.PreviousQIndex.Span[0]);
+        Assert.Equal(0, picture.TileDataOffsets.Span[0]);
+        Assert.Equal(0, picture.TileDataLengths.Span[0]);
+        Assert.Same(nextFrameHeader, picture.Parent.FrameHeader);
+        Assert.Same(nextTiles, picture.Parent.Common.TilesInfo);
+    }
+
     [Theory]
     [InlineData(false, 2, 3, 98)]
     [InlineData(true, 2, 2, 17)]
@@ -248,14 +425,16 @@ public class Av1EncoderModeInfoBufferTests
                     TilesInfo = tiles
                 },
                 FrameHeader = frameHeader,
-                PreviousQIndex = []
+                PreviousQIndex = Memory<int>.Empty
             },
             SegmentationNeighborMap = Memory<byte>.Empty,
             ModeInfoGrid = buffer.Grid,
             ModeInfoAllocation = buffer.Allocation,
             ModeInfoStride = buffer.ModeInfoStride,
             Disallow4x4AllFrames = buffer.Disallow4x4AllFrames,
-            CdefPreset = []
+            CdefPreset = Memory<int>.Empty,
+            TileDataOffsets = Memory<int>.Empty,
+            TileDataLengths = Memory<int>.Empty
         };
     }
 }

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1EntropyTests.cs

@@ -155,6 +155,27 @@ public class Av1EntropyTests
             encoder.GetTransformBlockSkipCost(true, TransformSize, SkipContext));
     }
 
+    [Fact]
+    public void SymbolEncoderResetRestoresNormativeDistributionState()
+    {
+        const byte TopContext = 0;
+        const byte LeftContext = 0;
+        const Av1PredictionMode LumaMode = Av1PredictionMode.DC;
+        using Av1SymbolEncoder encoder = new(Configuration.Default, 64, BaseQIndex, updateCdf: true);
+        int initialCost = encoder.GetLumaModeCost(LumaMode, TopContext, LeftContext);
+
+        for (int i = 0; i < 8; i++)
+        {
+            encoder.WriteLumaMode(LumaMode, TopContext, LeftContext);
+        }
+
+        Assert.NotEqual(initialCost, encoder.GetLumaModeCost(LumaMode, TopContext, LeftContext));
+
+        encoder.Reset();
+
+        Assert.Equal(initialCost, encoder.GetLumaModeCost(LumaMode, TopContext, LeftContext));
+    }
+
     [Fact]
     public void BlockSkipDecisionUsesAdaptedRatesForEmptyTransforms()
     {
@@ -246,7 +267,8 @@ public class Av1EntropyTests
                 macroBlock,
                 Av1BlockSize.Block8x8,
                 Mode,
-                angleDelta));
+                angleDelta,
+                isIntraFrame: true));
     }
 
     /// <summary>
@@ -698,6 +720,48 @@ public class Av1EntropyTests
         long expected)
         => Assert.Equal(expected, Av1RateDistortion.GetCost(rateMultiplier, rate, distortion));
 
+    /// <summary>
+    /// Verifies fixed curve samples, interpolation, error categories, native quantizer normalization, and skip selection.
+    /// </summary>
+    [Theory]
+    [InlineData(Av1BlockSize.Block4x4, 16L, 16, 8, Av1BitDepth.EightBit, 1, 13243, 17L)]
+    [InlineData(Av1BlockSize.Block8x8, 64L, 64, 8, Av1BitDepth.EightBit, 1, 45715, 66L)]
+    [InlineData(Av1BlockSize.Block16x16, 256L, 256, 8, Av1BitDepth.EightBit, 1, 154928, 265L)]
+    [InlineData(Av1BlockSize.Block32x32, 1024L, 1024, 8, Av1BitDepth.EightBit, 1, 410224, 1061L)]
+    [InlineData(Av1BlockSize.Block8x8, 96L, 64, 8, Av1BitDepth.EightBit, 1, 53253, 71L)]
+    [InlineData(Av1BlockSize.Block8x8, 1024L, 64, 8, Av1BitDepth.EightBit, 1, 96672, 95L)]
+    [InlineData(Av1BlockSize.Block8x8, 1056L, 64, 8, Av1BitDepth.EightBit, 1, 97253, 95L)]
+    [InlineData(Av1BlockSize.Block8x8, 64L, 64, 32, Av1BitDepth.TenBit, 1, 45715, 66L)]
+    [InlineData(Av1BlockSize.Block8x8, 64L, 64, 128, Av1BitDepth.TwelveBit, 1, 45715, 66L)]
+    [InlineData(Av1BlockSize.Block8x8, 64L, 64, 8, Av1BitDepth.EightBit, 1000000, 0, 1024L)]
+    [InlineData(Av1BlockSize.Block8x8, 0L, 64, 8, Av1BitDepth.EightBit, 1, 0, 0L)]
+    [InlineData(Av1BlockSize.Block128x128, 1L, 16384, 21387, Av1BitDepth.TwelveBit, 1, 0, 16L)]
+    public void PredictionErrorModelMatchesReferenceCurveSamples(
+        int blockSize,
+        long squaredError,
+        int sampleCount,
+        int acQuantizer,
+        int bitDepth,
+        int rateMultiplier,
+        int expectedRate,
+        long expectedDistortion)
+    {
+        // Expectations come from the published curve samples and cubic polynomial, not from an encode/decode
+        // round trip. Unit normalized error and unit quantizer hit rate column 31 in each block-size category.
+        Av1RateDistortion.ModelPredictionError(
+            (Av1BlockSize)blockSize,
+            squaredError,
+            sampleCount,
+            acQuantizer,
+            (Av1BitDepth)bitDepth,
+            rateMultiplier,
+            out int rate,
+            out long distortion);
+
+        Assert.Equal(expectedRate, rate);
+        Assert.Equal(expectedDistortion, distortion);
+    }
+
     [Theory]
     [InlineData(1, 8191, 100, 100)]
     [InlineData(1, 8192, 100, 101)]
@@ -744,6 +808,19 @@ public class Av1EntropyTests
         int expected)
         => Assert.Equal(expected, Av1RateDistortion.GetKeyFrameRateMultiplier(qIndex, (Av1BitDepth)bitDepth));
 
+    [Theory]
+    [InlineData(0, 0, 51)]
+    [InlineData(0, 1, 3)]
+    [InlineData(0, 2, 1)]
+    [InlineData(255, 0, 9_288_598)]
+    [InlineData(255, 1, 20_049_918)]
+    [InlineData(255, 2, 63_036_850)]
+    public void InterFrameRateMultiplierMatchesCurrentLibaom(
+        int qIndex,
+        int bitDepth,
+        int expected)
+        => Assert.Equal(expected, Av1RateDistortion.GetInterFrameRateMultiplier(qIndex, (Av1BitDepth)bitDepth));
+
     [Fact]
     public void SymbolWriterMatchesCurrentLibaomCarryRegression()
     {
@@ -782,7 +859,7 @@ public class Av1EntropyTests
     }
 
     [Fact]
-    public void SymbolWriterExposesExistingOutputAllocationWithoutCopy()
+    public void SymbolWriterResetReusesExistingOutputAllocation()
     {
         const int bufferLength = 257;
         TestMemoryAllocator allocator = new();
@@ -805,14 +882,46 @@ public class Av1EntropyTests
             Assert.Equal(63, encoded.Span[0]);
             Assert.Single(allocator.AllocationLog);
             Assert.Empty(allocator.ReturnLog);
+
+            int firstLength = length;
+            writer.Reset(firstLength);
+            writer.WriteBoolean(false, 16_384);
+            writer.WriteBoolean(false, 16_384);
+            writer.WriteBoolean(true, 512);
+            writer.WriteBoolean(false, 8_192);
+            encoded = writer.Exit(out length);
+
+            Assert.Equal(2, length);
+            Assert.Equal(length, encoded.Length);
+            Assert.Equal(63, encoded.Span[0]);
+            ReadOnlyMemory<byte> output = writer.GetOutput(firstLength + length);
+            Assert.True(output.Span[..firstLength].SequenceEqual(output.Span[firstLength..]));
+            Assert.Single(allocator.AllocationLog);
+            Assert.Empty(allocator.ReturnLog);
+
+            writer.Reset();
+            writer.WriteBoolean(false, 16_384);
+            writer.WriteBoolean(false, 16_384);
+            writer.WriteBoolean(true, 512);
+            writer.WriteBoolean(false, 8_192);
+            encoded = writer.Exit(out length);
+
+            Assert.Equal(2, length);
+            Assert.Equal(length, encoded.Length);
+            Assert.Equal(63, encoded.Span[0]);
+            Assert.Single(allocator.AllocationLog);
+            Assert.Empty(allocator.ReturnLog);
         }
 
         TestMemoryAllocator.ReturnRequest returned = Assert.Single(allocator.ReturnLog);
         Assert.Equal(allocation.AllocationId, returned.AllocationId);
     }
 
+    /// <summary>
+    /// Verifies bounded coefficient scratch is rented at construction and reused by costing, coding, and frame resets.
+    /// </summary>
     [Fact]
-    public void SymbolEncoderRentsCoefficientScratchOnlyForNonzeroBlocks()
+    public void SymbolEncoderReusesConstructorOwnedCoefficientScratchAcrossFrames()
     {
         TestMemoryAllocator allocator = new();
         allocator.EnableNonThreadSafeLogging();
@@ -822,54 +931,10 @@ public class Av1EntropyTests
 
         using (Av1SymbolEncoder encoder = new(configuration, 64, BaseQIndex, updateCdf: true))
         {
-            TestMemoryAllocator.AllocationRequest outputScratch = Assert.Single(allocator.AllocationLog);
-            Assert.Equal(typeof(byte), outputScratch.ElementType);
-
-            int emptyContext = encoder.WriteCoefficients(
-                Av1TransformSize.Size4x4,
-                Av1TransformType.DctDct,
-                Av1PredictionMode.DC,
-                coefficients,
-                Av1ComponentType.Luminance,
-                default,
-                0,
-                false,
-                Av1FilterIntraMode.DC,
-                usesInterTransformSet: false);
-
-            Assert.Equal(0, emptyContext);
-            Assert.Single(allocator.AllocationLog);
-
-            coefficients[0] = 1;
-            _ = encoder.GetCoefficientCost(
-                Av1TransformSize.Size4x4,
-                Av1TransformType.DctDct,
-                Av1PredictionMode.DC,
-                coefficients,
-                Av1ComponentType.Luminance,
-                default,
-                1,
-                false,
-                Av1FilterIntraMode.DC,
-                usesInterTransformSet: false);
-
             Assert.Equal(3, allocator.AllocationLog.Count);
-
-            encoder.WriteCoefficients(
-                Av1TransformSize.Size4x4,
-                Av1TransformType.DctDct,
-                Av1PredictionMode.DC,
-                coefficients,
-                Av1ComponentType.Luminance,
-                default,
-                1,
-                false,
-                Av1FilterIntraMode.DC,
-                usesInterTransformSet: false);
-
-            Assert.Equal(3, allocator.AllocationLog.Count);
-            TestMemoryAllocator.AllocationRequest levelScratch = allocator.AllocationLog[1];
-            TestMemoryAllocator.AllocationRequest contextScratch = allocator.AllocationLog[2];
+            TestMemoryAllocator.AllocationRequest levelScratch = allocator.AllocationLog[0];
+            TestMemoryAllocator.AllocationRequest contextScratch = allocator.AllocationLog[1];
+            TestMemoryAllocator.AllocationRequest outputScratch = allocator.AllocationLog[2];
             int maximumTransformDimension = Av1Constants.MaxTransformSize / 2;
             int expectedLevelLength =
                 (Av1Constants.TransformPadHorizontal + maximumTransformDimension) *
@@ -880,6 +945,59 @@ public class Av1EntropyTests
             Assert.Equal(AllocationOptions.Clean, levelScratch.AllocationOptions);
             Assert.Equal(typeof(sbyte), contextScratch.ElementType);
             Assert.Equal(maximumTransformDimension * maximumTransformDimension, contextScratch.Length);
+            Assert.Equal(typeof(byte), outputScratch.ElementType);
+            Assert.Equal(64, outputScratch.Length);
+
+            // Exercise both an empty and a coded transform on each side of a frame reset. The second pass must
+            // reuse every constructor-owned buffer even after nonzero levels and probability updates exist.
+            for (int frame = 0; frame < 2; frame++)
+            {
+                coefficients.Clear();
+                int emptyContext = encoder.WriteCoefficients(
+                    Av1TransformSize.Size4x4,
+                    Av1TransformType.DctDct,
+                    Av1PredictionMode.DC,
+                    coefficients,
+                    Av1ComponentType.Luminance,
+                    default,
+                    0,
+                    false,
+                    Av1FilterIntraMode.DC,
+                    usesInterTransformSet: false);
+
+                Assert.Equal(0, emptyContext);
+                Assert.Equal(3, allocator.AllocationLog.Count);
+
+                coefficients[0] = 1;
+                _ = encoder.GetCoefficientCost(
+                    Av1TransformSize.Size4x4,
+                    Av1TransformType.DctDct,
+                    Av1PredictionMode.DC,
+                    coefficients,
+                    Av1ComponentType.Luminance,
+                    default,
+                    1,
+                    false,
+                    Av1FilterIntraMode.DC,
+                    usesInterTransformSet: false);
+
+                encoder.WriteCoefficients(
+                    Av1TransformSize.Size4x4,
+                    Av1TransformType.DctDct,
+                    Av1PredictionMode.DC,
+                    coefficients,
+                    Av1ComponentType.Luminance,
+                    default,
+                    1,
+                    false,
+                    Av1FilterIntraMode.DC,
+                    usesInterTransformSet: false);
+
+                encoder.Exit(out _);
+                encoder.Reset();
+                Assert.Equal(3, allocator.AllocationLog.Count);
+                Assert.Empty(allocator.ReturnLog);
+            }
         }
 
         Assert.Equal(3, allocator.ReturnLog.Count);

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1InterpolationFilterEntropyTests.cs

@@ -114,6 +114,50 @@ public class Av1InterpolationFilterEntropyTests
         }
     }
 
+    /// <summary>
+    /// Verifies the encoder's context selection, read-only costing, and adaptive output against independently seeded distributions.
+    /// </summary>
+    [Theory]
+    [MemberData(nameof(GetContexts))]
+    public void EncoderUsesRequestedContextAndLiveCosts(int context)
+    {
+        ReadOnlySpan<Av1InterpolationFilter> filters =
+        [
+            Av1InterpolationFilter.Sharp,
+            Av1InterpolationFilter.Smooth,
+            Av1InterpolationFilter.Regular,
+            Av1InterpolationFilter.Sharp,
+            Av1InterpolationFilter.Regular,
+            Av1InterpolationFilter.Smooth,
+        ];
+
+        // The constructor converts forward thresholds to inverse CDF storage. Supply the published forward
+        // values directly, independently of the production context factory.
+        Av1Distribution distribution = new(
+            ForwardThresholds[context * 2],
+            ForwardThresholds[(context * 2) + 1]);
+
+        using Av1SymbolWriter expectedWriter = new(Configuration.Default, 64, updateCdf: true);
+        using Av1SymbolEncoder encoder = new(Configuration.Default, 64, qIndex: 0, updateCdf: true);
+        foreach (Av1InterpolationFilter filter in filters)
+        {
+            int expectedCost = Av1ProbabilityCost.GetSymbolCost(distribution, (int)filter);
+            Assert.Equal(expectedCost, encoder.GetSwitchableInterpolationFilterCost(filter, context));
+            Assert.Equal(expectedCost, encoder.GetSwitchableInterpolationFilterCost(filter, context));
+            expectedWriter.WriteSymbol((int)filter, distribution);
+            encoder.WriteSwitchableInterpolationFilter(filter, context);
+        }
+
+        using IMemoryOwner<byte> expected = expectedWriter.Exit();
+        using IMemoryOwner<byte> actual = encoder.Exit();
+        Assert.Equal(expected.Memory.Span, actual.Memory.Span);
+        Av1SymbolDecoder decoder = new(Configuration.Default, actual.Memory.Span, 0, updateCdf: true);
+        foreach (Av1InterpolationFilter filter in filters)
+        {
+            Assert.Equal(filter, decoder.ReadSwitchableInterpolationFilter(context));
+        }
+    }
+
     /// <summary>
     /// Verifies all sixteen combinations of reference type, direction, and contributing neighbor filter state.
     /// </summary>

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1IntraSuperblockEncoderTests.cs

@@ -8,12 +8,15 @@ using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline.Quantizers;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.Inter;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Tests.Memory;
 
 namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
 
@@ -23,6 +26,168 @@ namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
 [Trait("Format", "Avif")]
 public class Av1IntraSuperblockEncoderTests
 {
+    /// <summary>
+    /// Verifies non-regular filter selection, retained reconstruction, and allocation-free inter tile coding.
+    /// </summary>
+    [Theory]
+    [InlineData((int)Av1InterpolationFilter.Smooth, false)]
+    [InlineData((int)Av1InterpolationFilter.Sharp, false)]
+    [InlineData((int)Av1InterpolationFilter.Smooth, true)]
+    [InlineData((int)Av1InterpolationFilter.Sharp, true)]
+    public void ProductionTileSelectsNonRegularInterpolation(int filterValue, bool dualFilter)
+    {
+        const int Width = 32;
+        const int Height = 8;
+        const int TargetColumn = 8;
+        const int BlockWidth = 8;
+        const int QIndex = 37;
+        const int TileBufferLength = 4096;
+        Av1InterpolationFilter filter = (Av1InterpolationFilter)filterValue;
+        int effort = dualFilter ? 9 : 8;
+        ReadOnlySpan<byte> referencePeriod = [128, 184, 208, 184, 128, 72, 48, 72];
+
+        // These are fixed half-sample responses of the reference's eight-tap smooth and sharp kernels.
+        // The horizontal pass rounds first by three bits and then by four; edge samples are replicated.
+        // Keeping the results literal avoids using the predictor under test to manufacture its own target.
+        ReadOnlySpan<byte> targetRow = filter == Av1InterpolationFilter.Smooth
+            ? [159, 189, 190, 154, 102, 66, 66, 102, 154, 190, 190, 154, 102, 66, 66, 102,
+               154, 190, 190, 154, 102, 66, 66, 102, 154, 190, 190, 154, 102, 67, 61, 69]
+            : [153, 204, 200, 158, 98, 55, 55, 98, 158, 202, 202, 158, 98, 55, 55, 98,
+               158, 202, 202, 158, 98, 55, 55, 98, 158, 202, 202, 158, 100, 53, 59, 75];
+
+        TestMemoryAllocator allocator = new();
+        allocator.EnableNonThreadSafeLogging();
+        Configuration configuration = Configuration.Default.Clone();
+        configuration.MemoryAllocator = allocator;
+        ObuColorConfig colorConfig = new()
+        {
+            IsMonochrome = true,
+            ColorRange = true,
+            SubSamplingX = true,
+            SubSamplingY = true,
+            BitDepth = Av1BitDepth.EightBit
+        };
+
+        using Image<L8> referenceImage = new(Width, Height);
+        using Av1EncoderFrameBuffer<byte> reference = new(configuration, Width, Height, 8, Av1ColorFormat.Yuv400, 0, 0);
+        using Av1EncoderFrameBuffer<byte> source = new(configuration, Width, Height, 8, Av1ColorFormat.Yuv400, 0, 0);
+        using Av1EncoderFrameBuffer<byte> reconstruction = new(configuration, Width, Height, 8, Av1ColorFormat.Yuv400, 0, 0);
+        for (int y = 0; y < Height; y++)
+        {
+            Span<L8> pixels = referenceImage.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(y);
+            Span<byte> referenceRow = reference.Frame.CodedView.GetPlane(Av1Plane.Y).DangerousGetRowSpan(y);
+            for (int x = 0; x < Width; x++)
+            {
+                byte sample = referencePeriod[x % referencePeriod.Length];
+                referenceRow[x] = sample;
+                pixels[x] = new L8(sample);
+            }
+
+            targetRow.CopyTo(source.Frame.CodedView.GetPlane(Av1Plane.Y).DangerousGetRowSpan(y));
+        }
+
+        reference.Frame.ExtendBorders();
+        source.Frame.ExtendBorders();
+        ClearPlane(reconstruction.Luma);
+
+        // A lossless key frame gives an independent decoder exactly the reference samples used by tile search.
+        using MemoryStream firstSample = new();
+        using Av1FrameEncoder.SequenceEncoder keyEncoder = Av1FrameEncoder.CreateColorSequenceEncoder(
+            configuration,
+            Width,
+            Height,
+            colorConfig,
+            qIndex: 0,
+            effort);
+
+        keyEncoder.EncodeKeyFrame(referenceImage.Frames.RootFrame, firstSample);
+        ObuSequenceHeader sequenceHeader = keyEncoder.SequenceHeader;
+        using Av1EncoderModeInfoBuffer modeInfo = new(configuration, Width, Height, disallow4x4AllFrames: true);
+        Av1PictureControlSet template = CreatePicture(modeInfo, colorConfig, use128x128Superblock: false, QIndex);
+        ObuFrameHeader frameHeader = template.Parent.FrameHeader;
+        frameHeader.FrameType = ObuFrameType.InterFrame;
+        frameHeader.ShowFrame = true;
+        frameHeader.ErrorResilientMode = true;
+        frameHeader.RefreshFrameFlags = byte.MaxValue;
+        frameHeader.DisableFrameEndUpdateCdf = true;
+        frameHeader.ReferenceMode = ObuReferenceMode.SingleReference;
+        frameHeader.InterpolationFilter = Av1InterpolationFilter.Switchable;
+        frameHeader.AllowHighPrecisionMotionVector = true;
+        frameHeader.TransformMode = Av1TransformMode.Select;
+        frameHeader.FrameSize.FrameWidth = Width;
+        frameHeader.FrameSize.FrameHeight = Height;
+        frameHeader.FrameSize.SuperResolutionUpscaledWidth = Width;
+        frameHeader.FrameSize.RenderWidth = Width;
+        frameHeader.FrameSize.RenderHeight = Height;
+        frameHeader.TilesInfo.HasUniformTileSpacing = true;
+        Av1QuantizationLookup.UpdateFrameQuantizationState(frameHeader);
+
+        using Av1EncoderPictureBuffer picture = new(configuration, sequenceHeader, frameHeader, Width, Height, disallow4x4AllFrames: true);
+        using Av1EncoderCoefficientBuffer coefficients = new(configuration, sequenceHeader, Width, Height);
+        using Av1EncoderSuperblockWorkspace superblockWorkspace = new(configuration);
+        using Av1EncoderBlockWorkspace blockWorkspace = new(configuration);
+        using Av1SymbolEncoder symbolEncoder = new(configuration, TileBufferLength, QIndex, updateCdf: true);
+        Av1EncoderTileWorkspace tileWorkspace = new(frameHeader, superblockWorkspace);
+        int allocationCount = allocator.AllocationLog.Count;
+        Av1TileEncoder tileWriter = new(
+            symbolEncoder,
+            source.Frame,
+            reference.Frame,
+            reconstruction.Frame,
+            picture.Picture,
+            coefficients,
+            tileWorkspace,
+            blockWorkspace,
+            effort);
+
+        Assert.Equal(allocationCount, allocator.AllocationLog.Count);
+        Point targetPosition = new(TargetColumn >> Av1Constants.ModeInfoSizeLog2, 0);
+        ref Av1MacroBlockModeInfo targetMode = ref picture.Picture.GetMacroBlockModeInfo(targetPosition);
+        Assert.Equal(Av1ReferenceFrameType.Last, targetMode.Block.ReferenceFrame);
+        Assert.Equal(filter, targetMode.Block.HorizontalInterpolationFilter);
+        Assert.Equal(dualFilter ? Av1InterpolationFilter.Regular : filter, targetMode.Block.VerticalInterpolationFilter);
+        Assert.Equal(4, picture.Picture.GetDisplacementVector(targetPosition).Column);
+        Assert.Equal(0, picture.Picture.GetDisplacementVector(targetPosition).Row);
+        for (int y = 0; y < Height; y++)
+        {
+            Assert.Equal(
+                targetRow.Slice(TargetColumn, BlockWidth),
+                reconstruction.Frame.View.GetPlane(Av1Plane.Y).DangerousGetRowSpan(y).Slice(TargetColumn, BlockWidth));
+        }
+
+        using MemoryStream secondSample = new();
+        using ObuWriter obuWriter = new(configuration);
+        obuWriter.WriteFrame(secondSample, sequenceHeader, frameHeader, tileWriter);
+        using Av1Decoder decoder = new(configuration);
+        using Av1FrameBuffer<byte> decodedFirst = decoder.DecodeFrameBuffer(firstSample.ToArray(), null, null, out _);
+        using Av1FrameBuffer<byte> decodedSecond = decoder.DecodeFrameBuffer(secondSample.ToArray(), null, null, out _);
+
+        // Preserve both the production stream and every managed reconstructed luma sample for exact libaom comparison.
+        string outputDirectory = TestEnvironment.CreateOutputDirectory("Heif", "Av1", nameof(this.ProductionTileSelectsNonRegularInterpolation));
+        string outputName = $"{filter}-{dualFilter}";
+        using FileStream output = File.Create(Path.Combine(outputDirectory, outputName + ".obu"));
+        firstSample.Position = 0;
+        firstSample.CopyTo(output);
+        secondSample.Position = 0;
+        secondSample.CopyTo(output);
+        using FileStream rawOutput = File.Create(Path.Combine(outputDirectory, outputName + ".managed.yuv"));
+        for (int y = 0; y < Height; y++)
+        {
+            ReadOnlySpan<byte> expected = reference.Frame.View.GetPlane(Av1Plane.Y).DangerousGetRowSpan(y);
+            ReadOnlySpan<byte> actual = decodedFirst.DeriveBlockPointer(Av1Plane.Y, new Point(0, y), 0, 0, out _)[..Width];
+            Assert.Equal(expected, actual);
+            rawOutput.Write(actual);
+        }
+
+        for (int y = 0; y < Height; y++)
+        {
+            ReadOnlySpan<byte> expected = reconstruction.Frame.View.GetPlane(Av1Plane.Y).DangerousGetRowSpan(y);
+            ReadOnlySpan<byte> actual = decodedSecond.DeriveBlockPointer(Av1Plane.Y, new Point(0, y), 0, 0, out _)[..Width];
+            Assert.Equal(expected, actual);
+            rawOutput.Write(actual);
+        }
+    }
+
     /// <summary>
     /// Gets the normative eight-sample weights used to build independent smooth-mode fixtures.
     /// </summary>
@@ -259,7 +424,7 @@ public class Av1IntraSuperblockEncoderTests
             tilePicture.Picture,
             512);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             tileSymbolEncoder,
             source.Frame,
             tileReconstruction.Frame,
@@ -413,7 +578,7 @@ public class Av1IntraSuperblockEncoderTests
             livePicture.Picture,
             256);
 
-        Av1IntraTileWriter liveTileWriter = new(
+        Av1TileEncoder liveTileWriter = new(
             liveSymbolEncoder,
             source.Frame,
             liveReconstruction.Frame,
@@ -541,7 +706,7 @@ public class Av1IntraSuperblockEncoderTests
             tilePicture.Picture,
             256);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             tileSymbolEncoder,
             source.Frame,
             tileReconstruction.Frame,
@@ -749,7 +914,7 @@ public class Av1IntraSuperblockEncoderTests
             32,
             224,
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -770,7 +935,7 @@ public class Av1IntraSuperblockEncoderTests
             512,
             3584,
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -791,7 +956,7 @@ public class Av1IntraSuperblockEncoderTests
             48,
             208,
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -816,7 +981,7 @@ public class Av1IntraSuperblockEncoderTests
             64,
             192,
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -837,7 +1002,7 @@ public class Av1IntraSuperblockEncoderTests
             1024,
             3072,
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -946,7 +1111,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             256);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -1162,7 +1327,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             512);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -1279,7 +1444,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             512);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -1442,7 +1607,7 @@ public class Av1IntraSuperblockEncoderTests
             pilotPicture.Picture,
             TileBufferLength);
 
-        Av1IntraTileWriter pilotWriter = createWriter(
+        Av1TileEncoder pilotWriter = createWriter(
             pilotSymbolEncoder,
             pilotSource.Frame,
             pilotReconstruction.Frame,
@@ -1549,7 +1714,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             TileBufferLength);
 
-        Av1IntraTileWriter tileWriter = createWriter(
+        Av1TileEncoder tileWriter = createWriter(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -1792,7 +1957,7 @@ public class Av1IntraSuperblockEncoderTests
             pilotPicture.Picture,
             TileBufferLength);
 
-        Av1IntraTileWriter pilotWriter = createWriter(
+        Av1TileEncoder pilotWriter = createWriter(
             pilotSymbolEncoder,
             pilotSource.Frame,
             pilotReconstruction.Frame,
@@ -1954,7 +2119,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             TileBufferLength);
 
-        Av1IntraTileWriter tileWriter = createWriter(
+        Av1TileEncoder tileWriter = createWriter(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -2158,7 +2323,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             2048);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -2183,7 +2348,7 @@ public class Av1IntraSuperblockEncoderTests
             8,
             static value => (byte)value,
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -2198,7 +2363,7 @@ public class Av1IntraSuperblockEncoderTests
             12,
             static value => (ushort)(value << 4),
             static (writer, source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
-                new Av1IntraTileWriter(
+                new Av1TileEncoder(
                     writer,
                     source,
                     reconstruction,
@@ -2303,7 +2468,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             TileBufferLength);
 
-        Av1IntraTileWriter tileWriter = createTileWriter(
+        Av1TileEncoder tileWriter = createTileWriter(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -2436,7 +2601,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             4096);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -2550,7 +2715,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             4096);
 
-        Av1IntraTileWriter tileWriter = new(
+        Av1TileEncoder tileWriter = new(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -2631,8 +2796,8 @@ public class Av1IntraSuperblockEncoderTests
 
         frameHeader.TilesInfo.HasUniformTileSpacing = true;
         using MemoryStream stream = new();
-        new ObuWriter().WriteAll(
-            Configuration.Default,
+        using ObuWriter obuWriter = new(Configuration.Default);
+        obuWriter.WriteSequenceFrame(
             stream,
             sequenceHeader,
             frameHeader,
@@ -2689,14 +2854,16 @@ public class Av1IntraSuperblockEncoderTests
                     FrameSize = new ObuFrameSize()
                 },
                 FrameHeader = frameHeader,
-                PreviousQIndex = [qIndex]
+                PreviousQIndex = new int[] { qIndex }
             },
             SegmentationNeighborMap = new byte[modeInfo.ModeInfoColumnCount * modeInfo.ModeInfoRowCount],
             ModeInfoGrid = modeInfo.Grid,
             ModeInfoAllocation = modeInfo.Allocation,
             ModeInfoStride = modeInfo.ModeInfoStride,
             Disallow4x4AllFrames = modeInfo.Disallow4x4AllFrames,
-            CdefPreset = [[-1, -1, -1, -1]]
+            CdefPreset = new int[] { -1, -1, -1, -1 },
+            TileDataOffsets = Memory<int>.Empty,
+            TileDataLengths = Memory<int>.Empty
         };
     }
 
@@ -2813,7 +2980,7 @@ public class Av1IntraSuperblockEncoderTests
             picture.Picture,
             TileBufferLength);
 
-        Av1IntraTileWriter tileWriter = createTileWriter(
+        Av1TileEncoder tileWriter = createTileWriter(
             symbolEncoder,
             source.Frame,
             reconstruction.Frame,
@@ -3044,7 +3211,7 @@ public class Av1IntraSuperblockEncoderTests
             updateCdf: !frameHeader.DisableCdfUpdate);
     }
 
-    private delegate Av1IntraTileWriter TileWriterFactory<TSample>(
+    private delegate Av1TileEncoder TileWriterFactory<TSample>(
         Av1SymbolEncoder writer,
         Av1EncoderFrame<TSample> source,
         Av1EncoderFrame<TSample> reconstruction,
@@ -3143,7 +3310,8 @@ public class Av1IntraSuperblockEncoderTests
                 macroBlock,
                 Av1BlockSize.Block8x8,
                 Av1PredictionMode.DC,
-                0);
+                0,
+                isIntraFrame: true);
 
             modeInfo.Block = new Av1EncoderBlockModeInfo
             {

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1MotionVectorEntropyTests.cs

@@ -112,6 +112,37 @@ public class Av1MotionVectorEntropyTests
         Assert.True(decoder.ReadDrl(1));
     }
 
+    [Theory]
+    [InlineData((int)Av1MotionVectorPrecision.Integer, 16, -24)]
+    [InlineData((int)Av1MotionVectorPrecision.QuarterSample, 6, -10)]
+    [InlineData((int)Av1MotionVectorPrecision.EighthSample, 11, -17)]
+    public void WriteMotionVectorRoundTripsRequestedPrecision(
+        int precisionValue,
+        int rowDelta,
+        int columnDelta)
+    {
+        Av1MotionVectorPrecision precision = (Av1MotionVectorPrecision)precisionValue;
+        Av1MotionVector reference = new(27, -11);
+        Av1MotionVector value = reference + new Av1MotionVector(rowDelta, columnDelta);
+        Av1MotionVectorContext writerContext = new();
+        Av1Distribution trailingDistribution = Av1DefaultDistributions.Drl[1];
+        using Av1SymbolWriter writer = new(Configuration.Default, 32, updateCdf: true);
+
+        writerContext.Write(writer, value, reference, precision);
+        writer.WriteSymbol(true, trailingDistribution);
+
+        using IMemoryOwner<byte> encoded = writer.Exit();
+        Av1FrameEntropyContext decoderContext = new(0);
+        Av1SymbolDecoder decoder = new(
+            Configuration.Default,
+            encoded.Memory.Span,
+            decoderContext,
+            updateCdf: true);
+
+        Assert.Equal(value, decoder.ReadMotionVector(reference, precision));
+        Assert.True(decoder.ReadDrl(1));
+    }
+
     /// <summary>
     /// Verifies that normal and displacement motion vectors never share adaptive distribution state.
     /// </summary>

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1ResidualBuilderTests.cs

@@ -15,6 +15,128 @@ namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
 [Trait("Format", "Avif")]
 public class Av1ResidualBuilderTests
 {
+    /// <summary>
+    /// Verifies precision limits, independent strides, candidate order, and exact final-row bounds.
+    /// </summary>
+    [Theory]
+    [InlineData(255, false)]
+    [InlineData(255, true)]
+    public void ByteSearchMetricsMatchKnownMoments(int maximum, bool negative)
+    {
+        const int SourceStride = 13;
+        const int PredictionStride = 17;
+        const int SourceOffset = 1;
+        const int PredictionOffset = 3;
+        byte[] sourceBuffer = new byte[SourceOffset + (7 * SourceStride) + 8];
+        byte[] predictionBuffer = new byte[PredictionOffset + (7 * PredictionStride) + 11];
+        sourceBuffer.AsSpan().Fill((byte)maximum);
+        predictionBuffer.AsSpan().Fill((byte)maximum);
+        Span<byte> source = sourceBuffer.AsSpan(SourceOffset);
+        Span<byte> prediction = predictionBuffer.AsSpan(PredictionOffset);
+        int[] sums = [-1, 0, 0, 0, 0, -1];
+
+        // The last source row contains exactly eight samples, and the four prediction windows require
+        // exactly eleven. Distinct nonzero padding catches accidental participation of neighboring rows.
+        for (int row = 0; row < 8; row++)
+        {
+            source.Slice(row * SourceStride, 8).Fill((byte)(negative ? 0 : maximum));
+            prediction.Slice(row * PredictionStride, 11).Fill((byte)(negative ? maximum : 0));
+        }
+
+        Assert.Equal(64 * maximum, Av1ResidualBuilder.SumAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride));
+        Av1ResidualBuilder.GetMoments8x8(source, SourceStride, prediction, PredictionStride, out int sum, out int squaredSum);
+        Assert.Equal((negative ? -64 : 64) * maximum, sum);
+        Assert.Equal(64 * maximum * maximum, squaredSum);
+        Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride, sums.AsSpan(1, 4));
+        Assert.Equal(new[] { -1, 64 * maximum, 64 * maximum, 64 * maximum, 64 * maximum, -1 }, sums);
+
+        // Source (2*y + x) and prediction (y + 2*x) give residual (y - x) * step, exercising both signs
+        // and distinct rows. Across the 8x8 square its signed sum is zero, absolute sum 168, and squared sum 672.
+        // Offsetting prediction by 1, 2, or 3 columns gives absolute sums 198, 276, and 386 respectively.
+        int step = maximum / 32;
+        for (int row = 0; row < 8; row++)
+        {
+            for (int column = 0; column < 8; column++)
+            {
+                source[(row * SourceStride) + column] = (byte)(((2 * row) + column) * step);
+            }
+
+            for (int column = 0; column < 11; column++)
+            {
+                prediction[(row * PredictionStride) + column] = (byte)((row + (2 * column)) * step);
+            }
+        }
+
+        Assert.Equal(168 * step, Av1ResidualBuilder.SumAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride));
+        Av1ResidualBuilder.GetMoments8x8(source, SourceStride, prediction, PredictionStride, out sum, out squaredSum);
+        Assert.Equal(0, sum);
+        Assert.Equal(672 * step * step, squaredSum);
+        Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride, sums.AsSpan(1, 4));
+        Assert.Equal(new[] { -1, 168 * step, 198 * step, 276 * step, 386 * step, -1 }, sums);
+    }
+
+    /// <summary>
+    /// Verifies precision limits, independent strides, candidate order, and exact final-row bounds.
+    /// </summary>
+    [Theory]
+    [InlineData(1023, false)]
+    [InlineData(1023, true)]
+    [InlineData(4095, false)]
+    [InlineData(4095, true)]
+    public void HighBitDepthSearchMetricsMatchKnownMoments(int maximum, bool negative)
+    {
+        const int SourceStride = 13;
+        const int PredictionStride = 17;
+        const int SourceOffset = 1;
+        const int PredictionOffset = 3;
+        ushort[] sourceBuffer = new ushort[SourceOffset + (7 * SourceStride) + 8];
+        ushort[] predictionBuffer = new ushort[PredictionOffset + (7 * PredictionStride) + 11];
+        sourceBuffer.AsSpan().Fill((ushort)maximum);
+        predictionBuffer.AsSpan().Fill((ushort)maximum);
+        Span<ushort> source = sourceBuffer.AsSpan(SourceOffset);
+        Span<ushort> prediction = predictionBuffer.AsSpan(PredictionOffset);
+        int[] sums = [-1, 0, 0, 0, 0, -1];
+
+        // The last source row contains exactly eight samples, and the four prediction windows require
+        // exactly eleven. Distinct nonzero padding catches accidental participation of neighboring rows.
+        for (int row = 0; row < 8; row++)
+        {
+            source.Slice(row * SourceStride, 8).Fill((ushort)(negative ? 0 : maximum));
+            prediction.Slice(row * PredictionStride, 11).Fill((ushort)(negative ? maximum : 0));
+        }
+
+        Assert.Equal(64 * maximum, Av1ResidualBuilder.SumAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride));
+        Av1ResidualBuilder.GetMoments8x8(source, SourceStride, prediction, PredictionStride, out int sum, out int squaredSum);
+        Assert.Equal((negative ? -64 : 64) * maximum, sum);
+        Assert.Equal(64 * maximum * maximum, squaredSum);
+        Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride, sums.AsSpan(1, 4));
+        Assert.Equal(new[] { -1, 64 * maximum, 64 * maximum, 64 * maximum, 64 * maximum, -1 }, sums);
+
+        // Source (2*y + x) and prediction (y + 2*x) give residual (y - x) * step, exercising both signs
+        // and distinct rows. Across the 8x8 square its signed sum is zero, absolute sum 168, and squared sum 672.
+        // Offsetting prediction by 1, 2, or 3 columns gives absolute sums 198, 276, and 386 respectively.
+        int step = maximum / 32;
+        for (int row = 0; row < 8; row++)
+        {
+            for (int column = 0; column < 8; column++)
+            {
+                source[(row * SourceStride) + column] = (ushort)(((2 * row) + column) * step);
+            }
+
+            for (int column = 0; column < 11; column++)
+            {
+                prediction[(row * PredictionStride) + column] = (ushort)((row + (2 * column)) * step);
+            }
+        }
+
+        Assert.Equal(168 * step, Av1ResidualBuilder.SumAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride));
+        Av1ResidualBuilder.GetMoments8x8(source, SourceStride, prediction, PredictionStride, out sum, out squaredSum);
+        Assert.Equal(0, sum);
+        Assert.Equal(672 * step * step, squaredSum);
+        Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(source, SourceStride, prediction, PredictionStride, sums.AsSpan(1, 4));
+        Assert.Equal(new[] { -1, 168 * step, 198 * step, 276 * step, 386 * step, -1 }, sums);
+    }
+
     private const HwIntrinsics ResidualConfigurations =
         HwIntrinsics.AllowAll | HwIntrinsics.DisableAVX512F | HwIntrinsics.DisableAVX | HwIntrinsics.DisableHWIntrinsic;
 
@@ -133,6 +255,14 @@ public class Av1ResidualBuilderTests
             width,
             height);
 
+        int[] candidateSums = new int[4];
+        _ = Av1ResidualBuilder.SumAbsoluteDifferences8x8(source, width, prediction, width);
+        _ = Av1ResidualBuilder.SumAbsoluteDifferences8x8(highBitDepthSource, width, highBitDepthPrediction, width);
+        Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(source, width, prediction, width, candidateSums);
+        Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(highBitDepthSource, width, highBitDepthPrediction, width, candidateSums);
+        Av1ResidualBuilder.GetMoments8x8(source, width, prediction, width, out _, out _);
+        Av1ResidualBuilder.GetMoments8x8(highBitDepthSource, width, highBitDepthPrediction, width, out _, out _);
+
         long sum = 0;
         long before = GC.GetAllocatedBytesForCurrentThread();
         for (int iteration = 0; iteration < 32; iteration++)
@@ -147,6 +277,16 @@ public class Av1ResidualBuilderTests
                 width,
                 width,
                 height);
+
+            sum += Av1ResidualBuilder.SumAbsoluteDifferences8x8(source, width, prediction, width);
+            sum += Av1ResidualBuilder.SumAbsoluteDifferences8x8(highBitDepthSource, width, highBitDepthPrediction, width);
+            Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(source, width, prediction, width, candidateSums);
+            sum += candidateSums[0];
+            Av1ResidualBuilder.SumFourAbsoluteDifferences8x8(highBitDepthSource, width, highBitDepthPrediction, width, candidateSums);
+            sum += candidateSums[3];
+            Av1ResidualBuilder.GetMoments8x8(source, width, prediction, width, out int byteSum, out int byteSquares);
+            Av1ResidualBuilder.GetMoments8x8(highBitDepthSource, width, highBitDepthPrediction, width, out int wordSum, out int wordSquares);
+            sum += byteSum + byteSquares + wordSum + wordSquares;
         }
 
         long allocated = GC.GetAllocatedBytesForCurrentThread() - before;

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1SingleReferenceEntropyTests.cs

@@ -14,6 +14,11 @@ namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
 [Trait("Format", "Avif")]
 public class Av1SingleReferenceEntropyTests
 {
+    /// <summary>
+    /// The quantizer index used to initialize an otherwise unrelated tile entropy encoder.
+    /// </summary>
+    private const int BaseQIndex = 128;
+
     /// <summary>
     /// Verifies all eighteen normative single-reference distributions against the reference decoder's forward Q15 defaults.
     /// </summary>
@@ -121,6 +126,41 @@ public class Av1SingleReferenceEntropyTests
         }
     }
 
+    /// <summary>
+    /// Verifies that the production writer emits every single-reference branch consumed by the decoder.
+    /// </summary>
+    /// <param name="referenceFrameValue">The encoded reference-frame label.</param>
+    [Theory]
+    [InlineData((int)Av1ReferenceFrameType.Last)]
+    [InlineData((int)Av1ReferenceFrameType.Last2)]
+    [InlineData((int)Av1ReferenceFrameType.Last3)]
+    [InlineData((int)Av1ReferenceFrameType.Golden)]
+    [InlineData((int)Av1ReferenceFrameType.Backward)]
+    [InlineData((int)Av1ReferenceFrameType.Alternate2)]
+    [InlineData((int)Av1ReferenceFrameType.Alternate)]
+    public void SingleReferenceWriterRoundTripsEveryReference(int referenceFrameValue)
+    {
+        Av1ReferenceFrameType referenceFrame = (Av1ReferenceFrameType)referenceFrameValue;
+        InlineArray8<byte> referenceCountStorage = default;
+        Span<byte> referenceCounts = referenceCountStorage;
+        referenceCounts[(int)Av1ReferenceFrameType.Last] = 5;
+        referenceCounts[(int)Av1ReferenceFrameType.Last2] = 1;
+        referenceCounts[(int)Av1ReferenceFrameType.Last3] = 2;
+        referenceCounts[(int)Av1ReferenceFrameType.Golden] = 2;
+        referenceCounts[(int)Av1ReferenceFrameType.Backward] = 3;
+        referenceCounts[(int)Av1ReferenceFrameType.Alternate2] = 3;
+        referenceCounts[(int)Av1ReferenceFrameType.Alternate] = 6;
+
+        using Av1SymbolEncoder encoder = new(Configuration.Default, 8, BaseQIndex, updateCdf: true);
+        encoder.WriteSingleReference(referenceFrame, referenceCounts);
+
+        using IMemoryOwner<byte> encoded = encoder.Exit();
+        Av1SymbolDecoder decoder = new(Configuration.Default, encoded.Memory.Span, BaseQIndex, updateCdf: true);
+        Av1ReferenceFrameType decodedReference = ReadSingleReference(ref decoder, referenceCounts);
+
+        Assert.Equal(referenceFrame, decodedReference);
+    }
+
     /// <summary>
     /// Verifies one-pass neighbor collection, compound-neighbor votes, clearing, and intra-neighbor exclusion.
     /// </summary>
@@ -314,6 +354,43 @@ public class Av1SingleReferenceEntropyTests
             _ => decoder.ReadSingleReferenceIsAlternate2(context),
         };
 
+    /// <summary>
+    /// Reads one complete single-reference branch through the production semantic entry points.
+    /// </summary>
+    private static Av1ReferenceFrameType ReadSingleReference(
+        ref Av1SymbolDecoder decoder,
+        scoped ReadOnlySpan<byte> referenceCounts)
+    {
+        int context = Av1SymbolContextHelper.GetSingleReferenceBackwardContext(referenceCounts);
+        if (decoder.ReadSingleReferenceIsBackward(context))
+        {
+            context = Av1SymbolContextHelper.GetSingleReferenceAlternateContext(referenceCounts);
+            if (decoder.ReadSingleReferenceIsAlternate(context))
+            {
+                return Av1ReferenceFrameType.Alternate;
+            }
+
+            context = Av1SymbolContextHelper.GetSingleReferenceAlternate2Context(referenceCounts);
+            return decoder.ReadSingleReferenceIsAlternate2(context)
+                ? Av1ReferenceFrameType.Alternate2
+                : Av1ReferenceFrameType.Backward;
+        }
+
+        context = Av1SymbolContextHelper.GetSingleReferenceLast3OrGoldenContext(referenceCounts);
+        if (decoder.ReadSingleReferenceIsLast3OrGolden(context))
+        {
+            context = Av1SymbolContextHelper.GetSingleReferenceGoldenContext(referenceCounts);
+            return decoder.ReadSingleReferenceIsGolden(context)
+                ? Av1ReferenceFrameType.Golden
+                : Av1ReferenceFrameType.Last3;
+        }
+
+        context = Av1SymbolContextHelper.GetSingleReferenceLast2Context(referenceCounts);
+        return decoder.ReadSingleReferenceIsLast2(context)
+            ? Av1ReferenceFrameType.Last2
+            : Av1ReferenceFrameType.Last;
+    }
+
     /// <summary>
     /// Creates decoded block-mode state with the requested primary and secondary reference labels.
     /// </summary>

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1TransformBlockEncoderTests.cs

@@ -614,8 +614,8 @@ public class Av1TransformBlockEncoderTests
 
             Av1EncoderModeDecisionWorkspace<ushort> modeWorkspace = workspace.GetModeDecisionWorkspace<ushort>();
             Av1EncoderPaletteWorkspace<ushort> paletteWorkspace = modeWorkspace.Palette;
-            Av1EncoderIntraBlockCopyWorkspace<ushort> intraBlockCopyWorkspace =
-                workspace.GetIntraBlockCopyWorkspace<ushort>();
+            Av1EncoderInterPredictionWorkspace<ushort> intraBlockCopyWorkspace =
+                workspace.GetInterPredictionWorkspace<ushort>();
 
             Assert.Equal(
                 (2 * Av1Constants.MaxTransformSize) + 1,

tests/ImageSharp.Tests/Formats/Heif/Av1/ObuFrameHeaderTests.cs

@@ -129,8 +129,8 @@ public class ObuFrameHeaderTests
         MemoryStream encoded = new();
 
         // Act 2
-        ObuWriter obuWriter = new();
-        obuWriter.WriteAll(Configuration.Default, encoded, obuReader.SequenceHeader, obuReader.FrameHeader, tileStub);
+        using ObuWriter obuWriter = new(Configuration.Default);
+        obuWriter.WriteSequenceFrame(encoded, obuReader.SequenceHeader, obuReader.FrameHeader, tileStub);
 
         // Assert
         byte[] encodedArray = encoded.ToArray();
@@ -157,8 +157,8 @@ public class ObuFrameHeaderTests
         MemoryStream encoded = new();
 
         // Act 2
-        ObuWriter obuWriter = new();
-        obuWriter.WriteAll(Configuration.Default, encoded, obuReader1.SequenceHeader, obuReader1.FrameHeader, tileStub);
+        using ObuWriter obuWriter = new(Configuration.Default);
+        obuWriter.WriteSequenceFrame(encoded, obuReader1.SequenceHeader, obuReader1.FrameHeader, tileStub);
 
         // Assign 2
         Span<byte> encodedBuffer = encoded.ToArray();
@@ -456,8 +456,8 @@ public class ObuFrameHeaderTests
         tileStub.ReadTile(emptyTile, 0);
 
         using MemoryStream stream = new();
-        ObuWriter writer = new();
-        writer.WriteAll(Configuration.Default, stream, sequenceHeader, frameHeader, tileStub);
+        using ObuWriter writer = new(Configuration.Default);
+        writer.WriteSequenceFrame(stream, sequenceHeader, frameHeader, tileStub);
         byte[] bitStream = stream.ToArray();
 
         int sequenceObuOffset = DefaultTemporalDelimiterBitStream.Length;
@@ -639,10 +639,9 @@ public class ObuFrameHeaderTests
     {
         // Arrange
         using MemoryStream stream = new(2);
-        ObuWriter obuWriter = new();
 
         // Act
-        obuWriter.WriteAll(Configuration.Default, stream, null, null, null);
+        ObuWriter.WriteTemporalDelimiter(stream);
         byte[] actual = stream.GetBuffer();
 
         // Assert
@@ -655,10 +654,9 @@ public class ObuFrameHeaderTests
         // Arrange
         using MemoryStream stream = new(10);
         ObuSequenceHeader input = GetDefaultSequenceHeader();
-        ObuWriter obuWriter = new();
 
         // Act
-        obuWriter.WriteAll(Configuration.Default, stream, input, null, null);
+        ObuWriter.WriteSequenceHeader(Configuration.Default, stream, input);
         byte[] buffer = stream.GetBuffer();
 
         // Assert
@@ -667,6 +665,68 @@ public class ObuFrameHeaderTests
         Assert.Equal(DefaultSequenceHeaderBitStream, actual);
     }
 
+    [Fact]
+    public void WriteNonReducedSequenceHeaderPreservesTimingAndDecoderModel()
+    {
+        ObuSequenceHeader input = GetDefaultSequenceHeader();
+        input.IsStillPicture = false;
+        input.IsReducedStillPictureHeader = false;
+        input.TimingInfoPresentFlag = true;
+        input.TimingInfo = new ObuTimingInfo
+        {
+            NumUnitsInDisplayTick = 1001,
+            TimeScale = 60000,
+            EqualPictureInterval = true,
+            NumTicksPerPicture = 2
+        };
+
+        input.DecoderModelInfoPresentFlag = true;
+        input.DecoderModelInfo = new ObuDecoderModelInfo
+        {
+            BufferDelayLength = 12,
+            NumUnitsInDecodingTick = 1000,
+            BufferRemovalTimeLength = 10,
+            FramePresentationTimeLength = 9
+        };
+
+        input.InitialDisplayDelayPresentFlag = true;
+        ObuOperatingPoint inputOperatingPoint = input.OperatingPoint[0];
+        inputOperatingPoint.SequenceLevelIndex = Av1Constants.SequenceTierMinimumLevelIndex;
+        inputOperatingPoint.SequenceTier = 1;
+        inputOperatingPoint.IsDecoderModelInfoPresent = true;
+        inputOperatingPoint.DecoderBufferDelay = 137;
+        inputOperatingPoint.EncoderBufferDelay = 211;
+        inputOperatingPoint.LowDelayMode = true;
+        inputOperatingPoint.IsInitialDisplayDelayPresent = true;
+        inputOperatingPoint.InitialDisplayDelay = 4;
+
+        using MemoryStream stream = new();
+        ObuWriter.WriteSequenceHeader(Configuration.Default, stream, input);
+        byte[] bitStream = stream.ToArray();
+        Av1BitStreamReader reader = new(bitStream);
+        ObuReader obuReader = new();
+        obuReader.ReadAll(ref reader, bitStream.Length, () => new Av1TileDecoderStub());
+
+        ObuSequenceHeader output = obuReader.SequenceHeader;
+        ObuTimingInfo outputTiming = output.GetTimingInfo();
+        ObuDecoderModelInfo outputDecoderModel = output.GetDecoderModelInfo();
+        ObuOperatingPoint outputOperatingPoint = output.OperatingPoint[0];
+        Assert.False(output.IsStillPicture);
+        Assert.False(output.IsReducedStillPictureHeader);
+        Assert.Equal(input.TimingInfo.NumUnitsInDisplayTick, outputTiming.NumUnitsInDisplayTick);
+        Assert.Equal(input.TimingInfo.TimeScale, outputTiming.TimeScale);
+        Assert.Equal(input.TimingInfo.NumTicksPerPicture, outputTiming.NumTicksPerPicture);
+        Assert.Equal(input.DecoderModelInfo.BufferDelayLength, outputDecoderModel.BufferDelayLength);
+        Assert.Equal(input.DecoderModelInfo.NumUnitsInDecodingTick, outputDecoderModel.NumUnitsInDecodingTick);
+        Assert.Equal(input.DecoderModelInfo.BufferRemovalTimeLength, outputDecoderModel.BufferRemovalTimeLength);
+        Assert.Equal(input.DecoderModelInfo.FramePresentationTimeLength, outputDecoderModel.FramePresentationTimeLength);
+        Assert.Equal(inputOperatingPoint.SequenceTier, outputOperatingPoint.SequenceTier);
+        Assert.Equal(inputOperatingPoint.DecoderBufferDelay, outputOperatingPoint.DecoderBufferDelay);
+        Assert.Equal(inputOperatingPoint.EncoderBufferDelay, outputOperatingPoint.EncoderBufferDelay);
+        Assert.Equal(inputOperatingPoint.LowDelayMode, outputOperatingPoint.LowDelayMode);
+        Assert.Equal(inputOperatingPoint.InitialDisplayDelay, outputOperatingPoint.InitialDisplayDelay);
+    }
+
     /// <summary>
     /// Verifies that the combined frame OBU declares exactly the payload bytes emitted by the writer.
     /// </summary>
@@ -680,10 +740,10 @@ public class ObuFrameHeaderTests
         Av1TileDecoderStub tileStub = new();
         byte[] tileData = [0x80];
         tileStub.ReadTile(tileData, 0);
-        ObuWriter obuWriter = new();
+        using ObuWriter obuWriter = new(Configuration.Default);
 
         // Act
-        obuWriter.WriteAll(Configuration.Default, stream, sequenceInput, frameInput, tileStub);
+        obuWriter.WriteSequenceFrame(stream, sequenceInput, frameInput, tileStub);
         byte[] bitStream = stream.ToArray();
 
         // Assert
@@ -706,10 +766,10 @@ public class ObuFrameHeaderTests
     }
 
     /// <summary>
-    /// Verifies that the OBU writer streams an encoded tile from its owning buffer without renting a second payload-sized buffer.
+    /// Verifies that the OBU writer reuses header scratch and streams encoded tiles without a payload-sized copy.
     /// </summary>
     [Fact]
-    public void WriteFrameStreamsTilePayloadWithoutRentingPayloadCopy()
+    public void WriterReusesHeaderScratchAndStreamsTilePayloadWithoutCopy()
     {
         const int TilePayloadLength = 64 * 1024;
 
@@ -725,8 +785,13 @@ public class ObuFrameHeaderTests
         tileStub.ReadTile(tileData, 0);
 
         using MemoryStream stream = new();
-        ObuWriter writer = new();
-        writer.WriteAll(configuration, stream, sequenceHeader, frameHeader, tileStub);
+        using (ObuWriter writer = new(configuration))
+        {
+            writer.WriteSequenceFrame(stream, sequenceHeader, frameHeader, tileStub);
+
+            // A second complete write must reuse the same bounded header owner retained by the writer.
+            writer.WriteSequenceFrame(stream, sequenceHeader, frameHeader, tileStub);
+        }
 
         TestMemoryAllocator.AllocationRequest headerScratch = Assert.Single(allocator.AllocationLog);
         Assert.Equal(typeof(byte), headerScratch.ElementType);
@@ -760,8 +825,8 @@ public class ObuFrameHeaderTests
         sourceTiles.ReadTile([0x80], 1);
 
         using MemoryStream stream = new();
-        ObuWriter writer = new();
-        writer.WriteAll(Configuration.Default, stream, sequenceHeader, frameHeader, sourceTiles);
+        using ObuWriter writer = new(Configuration.Default);
+        writer.WriteSequenceFrame(stream, sequenceHeader, frameHeader, sourceTiles);
         byte[] bitStream = stream.ToArray();
         Assert.Equal([0x00, 0x80, 0x80], bitStream[^3..]);
 
@@ -936,8 +1001,8 @@ public class ObuFrameHeaderTests
         tileStub.ReadTile([0x80], 1);
 
         using MemoryStream stream = new();
-        ObuWriter writer = new();
-        writer.WriteAll(Configuration.Default, stream, sequenceHeader, frameHeader, tileStub);
+        using ObuWriter writer = new(Configuration.Default);
+        writer.WriteSequenceFrame(stream, sequenceHeader, frameHeader, tileStub);
         return stream.ToArray();
     }
 

tests/ImageSharp.Tests/Formats/Heif/HeifEncoderTests.cs

@@ -4,14 +4,13 @@
 using System.Buffers;
 using System.Buffers.Binary;
 using System.Text;
-using SixLabors.ImageSharp.ColorProfiles;
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Heif;
 using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
-using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.Metadata.Profiles.Cicp;
 using SixLabors.ImageSharp.Metadata.Profiles.Exif;
 using SixLabors.ImageSharp.Metadata.Profiles.Icc;
@@ -171,7 +170,7 @@ public class HeifEncoderTests
     }
 
     [Fact]
-    public void LegacyJpegRejectsLosslessEncoding()
+    public void LegacyJpegIgnoresLosslessOption()
     {
         using Image<Rgba32> image = new(1, 1);
         using MemoryStream stream = new();
@@ -181,14 +180,19 @@ public class HeifEncoderTests
             Lossless = true
         };
 
-        Assert.Throws<NotSupportedException>(() => image.Save(stream, encoder));
-        Assert.Equal(0, stream.Length);
+        image.Save(stream, encoder);
+        stream.Position = 0;
+        using Image<Rgba32> decoded = Image.Load<Rgba32>(stream);
+        Assert.Equal(image.Size, decoded.Size);
+        Assert.Equal(
+            HeifCompressionMethod.LegacyJpeg,
+            decoded.Metadata.GetHeifMetadata().CompressionMethod);
     }
 
     [Theory]
     [InlineData(HeifBitDepth.Bit10)]
     [InlineData(HeifBitDepth.Bit12)]
-    public void LegacyJpegRejectsHighBitDepth(HeifBitDepth bitDepth)
+    public void LegacyJpegNormalizesHighBitDepthToEightBit(HeifBitDepth bitDepth)
     {
         using Image<Rgba32> image = new(1, 1);
         using MemoryStream stream = new();
@@ -198,8 +202,310 @@ public class HeifEncoderTests
             BitDepth = bitDepth
         };
 
-        Assert.Throws<NotSupportedException>(() => image.Save(stream, encoder));
-        Assert.Equal(0, stream.Length);
+        image.Save(stream, encoder);
+        stream.Position = 0;
+        using Image<Rgba32> decoded = Image.Load<Rgba32>(stream);
+        Assert.Equal(HeifBitDepth.Bit8, decoded.Metadata.GetHeifMetadata().BitDepth);
+    }
+
+    [Fact]
+    public void LegacyJpegEncodesRootFrameFromImageSequence()
+    {
+        using Image<Rgba32> image = new(1, 1);
+        image.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(0)[0] = new Rgba32(255, 255, 255);
+        image.Frames.AddFrame(image.Frames.RootFrame);
+        image.Frames[1].PixelBuffer.DangerousGetRowSpan(0)[0] = new Rgba32(0, 0, 0);
+        using MemoryStream stream = new();
+        HeifEncoder encoder = new()
+        {
+            CompressionMethod = HeifCompressionMethod.LegacyJpeg,
+            Quality = 100
+        };
+
+        image.Save(stream, encoder);
+        stream.Position = 0;
+        using Image<Rgba32> decoded = Image.Load<Rgba32>(stream);
+        Assert.Single(decoded.Frames);
+        Assert.Equal(new Rgba32(255, 255, 255), decoded.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(0)[0]);
+    }
+
+    [Fact]
+    public void Av1ImageSequencePreservesSeparateRootFrame()
+    {
+        const int width = 8;
+        const int height = 8;
+        using Image<Rgb24> image = new(width, height);
+        for (int row = 0; row < height; row++)
+        {
+            image.Frames.RootFrame.PixelBuffer.DangerousGetRowSpan(row).Fill(new Rgb24(255, 255, 255));
+        }
+
+        image.Frames.AddFrame(image.Frames.RootFrame);
+        for (int row = 0; row < height; row++)
+        {
+            image.Frames[1].PixelBuffer.DangerousGetRowSpan(row).Fill(new Rgb24(0, 0, 0));
+        }
+
+        image.Frames[1].Metadata.GetHeifMetadata().FrameDelay = new Rational(1, 20);
+        using MemoryStream stream = new();
+        HeifEncoder encoder = new()
+        {
+            CompressionMethod = HeifCompressionMethod.Av1,
+            AnimateRootFrame = false,
+            Lossless = true,
+            Effort = 0
+        };
+
+        image.Save(stream, encoder);
+        byte[] file = stream.ToArray();
+        Span<byte> fileType = GetTopLevelBox(file, Heif4CharCode.Ftyp);
+        Assert.Equal(Heif4CharCode.Miaf, (Heif4CharCode)BinaryPrimitives.ReadUInt32BigEndian(fileType[^sizeof(uint)..]));
+        Assert.Equal(1, BinaryPrimitives.ReadUInt16BigEndian(GetMetadataChild(file, Heif4CharCode.Pitm)[12..]));
+
+        stream.Position = 0;
+        using Image<Rgb24> decoded = Image.Load<Rgb24>(stream);
+        Assert.Equal(2, decoded.Frames.Count);
+        Assert.False(decoded.Metadata.GetHeifMetadata().AnimateRootFrame);
+        Assert.Empty(ImageComparer.Exact.CompareImages(image, decoded));
+        Assert.Equal(
+            image.Frames[1].Metadata.GetHeifMetadata().FrameDelay,
+            decoded.Frames[1].Metadata.GetHeifMetadata().FrameDelay);
+    }
+
+    [Fact]
+    public void GridDecoderAcceptsSmallerRightAndBottomColorAndAlphaCells()
+    {
+        const int tileWidth = 64;
+        const int tileHeight = 64;
+        const int outputWidth = 96;
+        const int outputHeight = 96;
+        Size[] tileSizes =
+        [
+            new(tileWidth, tileHeight),
+            new(outputWidth - tileWidth, tileHeight),
+            new(tileWidth, outputHeight - tileHeight),
+            new(outputWidth - tileWidth, outputHeight - tileHeight)
+        ];
+
+        Rgba32[] tileColors =
+        [
+            new(32, 32, 32),
+            new(64, 64, 64),
+            new(96, 96, 96),
+            new(128, 128, 128)
+        ];
+
+        ObuColorConfig colorConfig = new()
+        {
+            IsColorDescriptionPresent = true,
+            ColorPrimaries = ObuColorPrimaries.Bt709,
+            TransferCharacteristics = ObuTransferCharacteristics.Srgb,
+            MatrixCoefficients = ObuMatrixCoefficients.Bt709,
+            ColorRange = true,
+            IsMonochrome = true,
+            SubSamplingX = true,
+            SubSamplingY = true,
+            BitDepth = Av1BitDepth.EightBit
+        };
+
+        List<HeifItem> items = [];
+        Dictionary<uint, byte[]> payloads = [];
+        HeifItem gridItem = new(Heif4CharCode.Grid, 1);
+        gridItem.SetExtent(new Size(outputWidth, outputHeight));
+        items.Add(gridItem);
+        HeifItemLink gridLink = new(Heif4CharCode.Dimg, gridItem.Id);
+        for (int tileIndex = 0; tileIndex < tileSizes.Length; tileIndex++)
+        {
+            Size tileSize = tileSizes[tileIndex];
+            using Image<Rgba32> tile = new(tileSize.Width, tileSize.Height, tileColors[tileIndex]);
+            using MemoryStream payload = new();
+            ObuSequenceHeader header = Av1FrameEncoder.Encode(
+                Configuration.Default,
+                tile.Frames.RootFrame,
+                payload,
+                colorConfig,
+                qIndex: 0,
+                effort: 0);
+
+            uint itemId = (uint)tileIndex + 2;
+            HeifItem tileItem = new(Heif4CharCode.Av01, itemId)
+            {
+                Av1CodecConfiguration = new Av1CodecConfiguration(header)
+            };
+
+            tileItem.SetExtent(tileSize);
+            items.Add(tileItem);
+            gridLink.DestinationIds.Add(itemId);
+            payloads.Add(itemId, payload.ToArray());
+        }
+
+        List<HeifItemLink> links = [gridLink];
+        GridHeifItemDecoder<Rgba32> decoder = new(items, links, ReadItem);
+        Span<byte> descriptor = [0, 0, 1, 1, 0, outputWidth, 0, outputHeight];
+        using Image<Rgba32> result = decoder.DecodeItemData(
+            new DecoderOptions { Configuration = Configuration.Default },
+            gridItem,
+            descriptor,
+            null,
+            TestContext.Current.CancellationToken);
+
+        for (int y = 0; y < outputHeight; y++)
+        {
+            for (int x = 0; x < outputWidth; x++)
+            {
+                int tileIndex = (y < tileHeight ? 0 : 2) + (x < tileWidth ? 0 : 1);
+                Assert.Equal(tileColors[tileIndex], result[x, y]);
+            }
+        }
+
+        Rgba32 opaqueColor = new(7, 11, 13);
+        using Image<Rgba32> alphaResult = new(outputWidth, outputHeight, opaqueColor);
+        decoder.DecodeAlphaItemData(
+            new DecoderOptions { Configuration = Configuration.Default },
+            gridItem,
+            descriptor,
+            alphaResult.Frames.RootFrame,
+            alphaResult.Size,
+            new Rectangle(Point.Empty, alphaResult.Size),
+            false,
+            TestContext.Current.CancellationToken);
+
+        for (int y = 0; y < outputHeight; y++)
+        {
+            for (int x = 0; x < outputWidth; x++)
+            {
+                int tileIndex = (y < tileHeight ? 0 : 2) + (x < tileWidth ? 0 : 1);
+                Rgba32 expected = opaqueColor;
+                expected.A = tileColors[tileIndex].R;
+                Assert.Equal(expected, alphaResult[x, y]);
+            }
+        }
+
+        IMemoryOwner<byte> ReadItem(HeifItem item)
+        {
+            byte[] payload = payloads[item.Id];
+            IMemoryOwner<byte> owner = Configuration.Default.MemoryAllocator.Allocate<byte>(payload.Length);
+            payload.CopyTo(owner.Memory.Span);
+            return owner;
+        }
+    }
+
+    [Fact]
+    public void GridDecoderRejectsCellSmallerThanMiafMinimum()
+    {
+        ObuColorConfig colorConfig = new()
+        {
+            IsColorDescriptionPresent = true,
+            ColorPrimaries = ObuColorPrimaries.Bt709,
+            TransferCharacteristics = ObuTransferCharacteristics.Srgb,
+            MatrixCoefficients = ObuMatrixCoefficients.Bt709,
+            ColorRange = true,
+            IsMonochrome = true,
+            SubSamplingX = true,
+            SubSamplingY = true,
+            BitDepth = Av1BitDepth.EightBit
+        };
+
+        InvalidImageContentException exception = Assert.Throws<InvalidImageContentException>(
+            () =>
+            {
+                using Image<Rgba32> decoded = DecodeSingleCellGrid(63, 64, colorConfig);
+            });
+
+        Assert.Contains("grid cells must be at least 64 samples", exception.Message, StringComparison.Ordinal);
+    }
+
+    /// <summary>
+    /// Verifies grid dimensions preserve chroma alignment for AV1's 4:2:2 and 4:2:0 layouts.
+    /// </summary>
+    [Theory]
+    [InlineData(65, 64, true, false)]
+    [InlineData(65, 64, true, true)]
+    [InlineData(64, 65, true, true)]
+    public void GridDecoderRejectsOddSubsampledDimension(
+        int width,
+        int height,
+        bool subsamplingX,
+        bool subsamplingY)
+    {
+        ObuColorConfig colorConfig = new()
+        {
+            IsColorDescriptionPresent = true,
+            ColorPrimaries = ObuColorPrimaries.Bt709,
+            TransferCharacteristics = ObuTransferCharacteristics.Srgb,
+            MatrixCoefficients = ObuMatrixCoefficients.Bt709,
+            ColorRange = true,
+            BitDepth = Av1BitDepth.EightBit,
+            SubSamplingX = subsamplingX,
+            SubSamplingY = subsamplingY
+        };
+
+        InvalidImageContentException exception = Assert.Throws<InvalidImageContentException>(
+            () =>
+            {
+                using Image<Rgba32> decoded = DecodeSingleCellGrid(width, height, colorConfig);
+            });
+
+        Assert.Contains("must be even", exception.Message, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void Av1OversizedStillImageWritesAndDecodesGrid()
+    {
+        const int width = 65537;
+        using Image<Rgb24> image = new(width, 1);
+        image[0, 0] = new Rgb24(1, 2, 3);
+        image[32768, 0] = new Rgb24(11, 13, 17);
+        image[32769, 0] = new Rgb24(19, 23, 29);
+        image[width - 1, 0] = new Rgb24(31, 37, 41);
+
+        // Identity-matrix 4:4:4 makes the lossless AV1 cells preserve the packed RGB channels exactly.
+        image.Metadata.CicpProfile = new CicpProfile(1, 13, 0, true);
+        using MemoryStream stream = new();
+        HeifEncoder encoder = new()
+        {
+            CompressionMethod = HeifCompressionMethod.Av1,
+            Lossless = true,
+            Effort = 0
+        };
+
+        image.Save(stream, encoder);
+        byte[] file = stream.ToArray();
+        Assert.Equal(
+            [0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1],
+            GetItemPayload(file, 1).ToArray());
+
+        using Av1Decoder firstCellDecoder = new(Configuration.Default);
+        using Image<Rgb24> firstCell = firstCellDecoder.Decode<Rgb24>(GetItemPayload(file, 2));
+        using Av1Decoder secondCellDecoder = new(Configuration.Default);
+        using Image<Rgb24> secondCell = secondCellDecoder.Decode<Rgb24>(GetItemPayload(file, 3));
+
+        // AVIF requires the first grid cell to be at least 64 samples on both axes. The derived grid trims
+        // the replicated right and bottom edges back to the presentation encoded in its descriptor.
+        Assert.Equal(new Size(32769, 64), firstCell.Size);
+        Assert.Equal(new Size(32769, 64), secondCell.Size);
+        Assert.Equal(image[32768, 0], firstCell[32768, 0]);
+        Assert.Equal(image[32769, 0], secondCell[0, 0]);
+        Assert.Equal(image[width - 1, 0], secondCell[32767, 0]);
+        Assert.Equal(secondCell[32767, 0], secondCell[32768, 0]);
+        Assert.Equal(secondCell[0, 0], secondCell[0, 63]);
+        Assert.Equal(1U, GetItemInfoFlags(file, 2));
+        Assert.Equal(1U, GetItemInfoFlags(file, 3));
+
+        ReadOnlySpan<byte> references = GetMetadataChild(file, Heif4CharCode.Iref);
+        const int FirstReferenceOffset = 12;
+        Assert.Equal(
+            Heif4CharCode.Dimg,
+            (Heif4CharCode)BinaryPrimitives.ReadUInt32BigEndian(references[(FirstReferenceOffset + 4)..]));
+
+        Assert.Equal(1, BinaryPrimitives.ReadUInt16BigEndian(references[(FirstReferenceOffset + 8)..]));
+        Assert.Equal(2, BinaryPrimitives.ReadUInt16BigEndian(references[(FirstReferenceOffset + 10)..]));
+        Assert.Equal(2, BinaryPrimitives.ReadUInt16BigEndian(references[(FirstReferenceOffset + 12)..]));
+        Assert.Equal(3, BinaryPrimitives.ReadUInt16BigEndian(references[(FirstReferenceOffset + 14)..]));
+
+        stream.Position = 0;
+        using Image<Rgb24> decoded = Image.Load<Rgb24>(stream);
+        Assert.Empty(ImageComparer.Exact.CompareImages(image, decoded));
     }
 
     [Fact]
@@ -335,9 +641,13 @@ public class HeifEncoderTests
         => (ushort)(((sample * (long)ushort.MaxValue) + (maximum / 2)) / maximum);
 
     [Theory]
-    [InlineData((ushort)0)]
-    [InlineData((ushort)3)]
-    public void Av1LosslessImageSequencePreservesFramesTimingAndAlpha(ushort repeatCount)
+    [InlineData((ushort)0, null, (ushort)0)]
+    [InlineData((ushort)3, null, (ushort)3)]
+    [InlineData((ushort)3, (ushort)7, (ushort)7)]
+    public void Av1LosslessImageSequencePreservesFramesTimingAndAlpha(
+        ushort metadataRepeatCount,
+        ushort? encoderRepeatCount,
+        ushort expectedRepeatCount)
     {
         const int width = 8;
         const int height = 8;
@@ -376,24 +686,36 @@ public class HeifEncoderTests
         image.Metadata.ExifProfile = exifProfile;
         byte[] xmpData = Encoding.UTF8.GetBytes("<xmp>ImageSharp AV1 sequence</xmp>");
         image.Metadata.XmpProfile = new XmpProfile(xmpData);
-        image.Metadata.GetHeifMetadata().RepeatCount = repeatCount;
+        image.Metadata.GetHeifMetadata().RepeatCount = metadataRepeatCount;
         using MemoryStream stream = new();
         HeifEncoder encoder = new()
         {
             CompressionMethod = HeifCompressionMethod.Av1,
             Lossless = true,
-            Effort = 0
+            Effort = 0,
+            RepeatCount = encoderRepeatCount
         };
 
         image.Save(stream, encoder);
         byte[] file = stream.ToArray();
         Assert.Equal((uint)Heif4CharCode.Avis, BinaryPrimitives.ReadUInt32BigEndian(file.AsSpan(8)));
+        Assert.Equal(1, BinaryPrimitives.ReadUInt16BigEndian(GetMetadataChild(file, Heif4CharCode.Pitm)[12..]));
+
+        using (Av1Decoder sampleDecoder = new(Configuration.Default))
+        using (Image<Rgba32> decodedSample = sampleDecoder.Decode<Rgba32>(GetItemPayload(file, 1)))
+        {
+            ObuSequenceHeader sampleHeader = sampleDecoder.SequenceHeader;
+            Assert.NotNull(sampleHeader);
+            Assert.False(sampleHeader.IsStillPicture);
+            Assert.False(sampleHeader.IsReducedStillPictureHeader);
+        }
 
         stream.Position = 0;
         DecoderOptions preserveOptions = new() { ColorProfileHandling = ColorProfileHandling.Preserve };
         using Image<Rgba32> decoded = Image.Load<Rgba32>(preserveOptions, stream);
         Assert.Equal(frameCount, decoded.Frames.Count);
-        Assert.Equal(repeatCount, decoded.Metadata.GetHeifMetadata().RepeatCount);
+        Assert.Equal(expectedRepeatCount, decoded.Metadata.GetHeifMetadata().RepeatCount);
+        Assert.True(decoded.Metadata.GetHeifMetadata().AnimateRootFrame);
         Assert.Empty(ImageComparer.Exact.CompareImages(image, decoded));
         Assert.Equal(
             IccTestDataProfiles.ProfileRandomArray,
@@ -483,7 +805,7 @@ public class HeifEncoderTests
 
         image.Save(stream, encoder);
         byte[] file = stream.ToArray();
-        ReadOnlySpan<byte> fileType = file.AsSpan(0, 28);
+        ReadOnlySpan<byte> fileType = GetTopLevelBox(file, Heif4CharCode.Ftyp);
         Assert.Equal(28, BinaryPrimitives.ReadInt32BigEndian(fileType));
         Assert.Equal(Heif4CharCode.Ftyp, (Heif4CharCode)BinaryPrimitives.ReadUInt32BigEndian(fileType[4..]));
         Assert.Equal(Heif4CharCode.Avif, (Heif4CharCode)BinaryPrimitives.ReadUInt32BigEndian(fileType[8..]));
@@ -997,6 +1319,49 @@ public class HeifEncoderTests
             ipmaPayload[8..].ToArray());
     }
 
+    [Fact]
+    public void ItemPropertiesReuseAnEarlierIdenticalCellPropertySet()
+    {
+        ObuSequenceHeader sequenceHeader = new()
+        {
+            SequenceProfile = ObuSequenceProfile.Main,
+            OperatingPoint = [new ObuOperatingPoint { SequenceLevelIndex = 31 }],
+            ColorConfig = new ObuColorConfig
+            {
+                BitDepth = Av1BitDepth.EightBit,
+                SubSamplingX = false,
+                SubSamplingY = false
+            }
+        };
+
+        HeifItem firstCell = new(Heif4CharCode.Av01, 1)
+        {
+            ChannelBitDepths = [8, 8, 8],
+            Av1CodecConfiguration = new Av1CodecConfiguration(sequenceHeader)
+        };
+
+        firstCell.SetExtent(new Size(64, 64));
+        HeifItem secondCell = new(Heif4CharCode.Av01, 2)
+        {
+            PropertySource = firstCell
+        };
+
+        secondCell.SetExtent(firstCell.Extent);
+        List<HeifItem> items = [firstCell, secondCell];
+        int expectedLength = HeifEncoderCore.GetItemPropertiesBoxLength(items);
+        using IMemoryOwner<byte> owner = Configuration.Default.MemoryAllocator.Allocate<byte>(expectedLength);
+        Span<byte> propertyBox = owner.Memory.Span[..expectedLength];
+        int length = HeifEncoderCore.WriteItemPropertiesBox(propertyBox, 0, items);
+
+        Assert.Equal(92, length);
+        const int IpcoOffset = 8;
+        int ipmaOffset = IpcoOffset + BinaryPrimitives.ReadInt32BigEndian(propertyBox[IpcoOffset..]);
+        ReadOnlySpan<byte> ipmaPayload = propertyBox[(ipmaOffset + 8)..];
+        Assert.Equal(
+            [0, 0, 0, 0, 0, 0, 0, 2, 0, 1, 3, 1, 2, 0x83, 0, 2, 3, 1, 2, 0x83],
+            ipmaPayload.ToArray());
+    }
+
     [Fact]
     public void Av1ItemPropertiesWriteIccBeforeCicpAndExcludeMetadataItemsFromAssociations()
     {
@@ -1188,6 +1553,87 @@ public class HeifEncoderTests
         throw new InvalidImageContentException($"The encoded file has no payload for item {itemId}.");
     }
 
+    private static uint GetItemInfoFlags(Span<byte> file, ushort itemId)
+    {
+        ReadOnlySpan<byte> itemInformation = GetMetadataChild(file, Heif4CharCode.Iinf);
+        int entryOffset = 14;
+        while (entryOffset < itemInformation.Length)
+        {
+            int entrySize = BinaryPrimitives.ReadInt32BigEndian(itemInformation[entryOffset..]);
+            ReadOnlySpan<byte> entry = itemInformation.Slice(entryOffset, entrySize);
+            ushort currentItemId = BinaryPrimitives.ReadUInt16BigEndian(entry[12..]);
+            if (currentItemId == itemId)
+            {
+                return (uint)((entry[9] << 16) | (entry[10] << 8) | entry[11]);
+            }
+
+            entryOffset += entrySize;
+        }
+
+        throw new InvalidImageContentException($"The encoded file has no item-information entry for item {itemId}.");
+    }
+
+    private static Image<Rgba32> DecodeSingleCellGrid(int width, int height, ObuColorConfig colorConfig)
+    {
+        using Image<Rgba32> tile = new(width, height, new Rgba32(127, 127, 127));
+        using MemoryStream payloadStream = new();
+        ObuSequenceHeader header = Av1FrameEncoder.Encode(
+            Configuration.Default,
+            tile.Frames.RootFrame,
+            payloadStream,
+            colorConfig,
+            qIndex: 0,
+            effort: 0);
+
+        byte[] payload = payloadStream.ToArray();
+        HeifItem gridItem = new(Heif4CharCode.Grid, 1);
+        gridItem.SetExtent(new Size(width, height));
+        HeifItem tileItem = new(Heif4CharCode.Av01, 2)
+        {
+            Av1CodecConfiguration = new Av1CodecConfiguration(header)
+        };
+
+        tileItem.SetExtent(new Size(width, height));
+        HeifItemLink gridLink = new(Heif4CharCode.Dimg, gridItem.Id);
+        gridLink.DestinationIds.Add(tileItem.Id);
+        GridHeifItemDecoder<Rgba32> decoder = new([gridItem, tileItem], [gridLink], ReadItem);
+        byte[] descriptor = new byte[8];
+        BinaryPrimitives.WriteUInt16BigEndian(descriptor.AsSpan(4), (ushort)width);
+        BinaryPrimitives.WriteUInt16BigEndian(descriptor.AsSpan(6), (ushort)height);
+        return decoder.DecodeItemData(
+            new DecoderOptions { Configuration = Configuration.Default },
+            gridItem,
+            descriptor,
+            null,
+            TestContext.Current.CancellationToken);
+
+        IMemoryOwner<byte> ReadItem(HeifItem item)
+        {
+            Assert.Equal(tileItem.Id, item.Id);
+            IMemoryOwner<byte> owner = Configuration.Default.MemoryAllocator.Allocate<byte>(payload.Length);
+            payload.CopyTo(owner.Memory.Span);
+            return owner;
+        }
+    }
+
+    private static Span<byte> GetTopLevelBox(Span<byte> file, Heif4CharCode requestedType)
+    {
+        int offset = 0;
+        while (offset < file.Length)
+        {
+            int boxSize = BinaryPrimitives.ReadInt32BigEndian(file[offset..]);
+            Heif4CharCode boxType = (Heif4CharCode)BinaryPrimitives.ReadUInt32BigEndian(file[(offset + sizeof(uint))..]);
+            if (boxType == requestedType)
+            {
+                return file.Slice(offset, boxSize);
+            }
+
+            offset += boxSize;
+        }
+
+        throw new InvalidImageContentException($"The encoded file has no {requestedType} top-level box.");
+    }
+
     private static Span<byte> GetMetadataChild(Span<byte> file, Heif4CharCode childType)
     {
         int offset = 0;