Add live AV1 chroma-from-luma search

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

@@ -1114,6 +1114,34 @@ internal class Av1SymbolEncoder : IDisposable
         return Av1ProbabilityCost.GetSymbolCost(this.uvMode[cflAllowed][(int)lumaMode], (int)chromaMode);
     }
 
+    /// <summary>
+    /// Gets the current fixed-point cost of joint chroma-from-luma alpha syntax.
+    /// </summary>
+    /// <param name="chromaFromLumaIndex">The packed U/V alpha-magnitude indices.</param>
+    /// <param name="joinedSign">The joint U/V sign symbol.</param>
+    /// <returns>The rate cost in 1/512-bit units.</returns>
+    public int GetChromaFromLumaCost(int chromaFromLumaIndex, int joinedSign)
+    {
+        int cost = Av1ProbabilityCost.GetSymbolCost(this.chromaFromLumaSign, joinedSign);
+        int signU = Av1ChromaFromLumaMath.SignU(joinedSign);
+        if (signU != Av1ChromaFromLumaMath.SignZero)
+        {
+            int contextU = Av1ChromaFromLumaMath.ContextU(joinedSign);
+            int indexU = Av1ChromaFromLumaMath.IndexU(chromaFromLumaIndex);
+            cost += Av1ProbabilityCost.GetSymbolCost(this.chromaFromLumaAlpha[contextU], indexU);
+        }
+
+        int signV = Av1ChromaFromLumaMath.SignV(joinedSign);
+        if (signV != Av1ChromaFromLumaMath.SignZero)
+        {
+            int contextV = Av1ChromaFromLumaMath.ContextV(joinedSign);
+            int indexV = Av1ChromaFromLumaMath.IndexV(chromaFromLumaIndex);
+            cost += Av1ProbabilityCost.GetSymbolCost(this.chromaFromLumaAlpha[contextV], indexV);
+        }
+
+        return cost;
+    }
+
     /// <summary>
     /// Writes a chroma intra prediction mode conditioned on the luma mode and chroma-from-luma availability.
     /// </summary>

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

@@ -4,6 +4,7 @@
 using SixLabors.ImageSharp.Formats.Heif.Av1.Entropy;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -52,7 +53,9 @@ internal static partial class Av1IntraSuperblockEncoder
             Span<int> retainedRedCoefficients,
             ref Av1EncoderTransformBlockState retainedBlueState,
             ref Av1EncoderTransformBlockState retainedRedState,
-            out int selectedAngleDelta)
+            out int selectedAngleDelta,
+            out byte selectedChromaFromLumaIndex,
+            out sbyte selectedChromaFromLumaSigns)
         {
             const Av1BlockSize BlockSize = Av1BlockSize.Block8x8;
             const int MaximumSampleCount = 8 * 8;
@@ -154,6 +157,8 @@ internal static partial class Av1IntraSuperblockEncoder
             long bestCost = long.MaxValue;
             Av1ChromaPredictionMode bestMode = Av1ChromaPredictionMode.DC;
             selectedAngleDelta = 0;
+            selectedChromaFromLumaIndex = 0;
+            selectedChromaFromLumaSigns = 0;
             int baseModeCount = ChromaModeSearchOrder.Length;
             int deltaCount = AngleDeltaSearchOrder.Length;
             int directionalModeCount = (int)Av1ChromaPredictionMode.Directional67Degrees - (int)Av1ChromaPredictionMode.Vertical + 1;
@@ -232,9 +237,253 @@ internal static partial class Av1IntraSuperblockEncoder
                 }
             }
 
+            bool chromaFromLumaAllowed = BlockSize.AllowsChromaFromLuma(
+                this.picture.Parent.FrameHeader.LosslessArray[modeInfo.Block.SegmentId],
+                colorConfig.SubSamplingX,
+                colorConfig.SubSamplingY);
+
+            if (chromaFromLumaAllowed)
+            {
+                Span<short> lumaQ3 = stackalloc short[Av1ChromaFromLumaContext.BufferLine * 8];
+                TOperator.PrepareChromaFromLuma(
+                    this.reconstruction.GetPlane(Av1Plane.Y),
+                    lumaOrigin,
+                    lumaQ3,
+                    transformSize,
+                    colorConfig.SubSamplingX,
+                    colorConfig.SubSamplingY);
+
+                // Every alpha candidate uses the same constant DC predictor, so compute each plane once and
+                // refill the candidate block from its sample instead of rebuilding the identical edge average.
+                TOperator.PrepareChromaFromLumaDc(
+                    candidateBlueReconstruction[..sampleCount],
+                    blueAbove,
+                    blueLeft,
+                    hasLeft,
+                    hasAbove,
+                    transformSize,
+                    this.bitDepth);
+
+                TSample blueDc = candidateBlueReconstruction[0];
+                TOperator.PrepareChromaFromLumaDc(
+                    candidateRedReconstruction[..sampleCount],
+                    redAbove,
+                    redLeft,
+                    hasLeft,
+                    hasAbove,
+                    transformSize,
+                    this.bitDepth);
+
+                TSample redDc = candidateRedReconstruction[0];
+                Span<int> blueRates = stackalloc int[Av1ChromaFromLumaMath.AlphaCandidateCount];
+                Span<int> redRates = stackalloc int[Av1ChromaFromLumaMath.AlphaCandidateCount];
+                Span<long> blueDistortions = stackalloc long[Av1ChromaFromLumaMath.AlphaCandidateCount];
+                Span<long> redDistortions = stackalloc long[Av1ChromaFromLumaMath.AlphaCandidateCount];
+
+                // Each plane has only 33 signed alpha values. Caching those complete transform results reduces
+                // the joint search from 1089 transform pairs to 66 transforms plus inexpensive rate combinations.
+                for (int alphaCandidateIndex = 0; alphaCandidateIndex < Av1ChromaFromLumaMath.AlphaCandidateCount; alphaCandidateIndex++)
+                {
+                    int alphaQ3 = Av1ChromaFromLumaMath.CandidateIndexToAlpha(alphaCandidateIndex);
+                    Av1EncoderTransformBlockState candidateBlueState = default;
+                    blueDistortions[alphaCandidateIndex] = this.GetChromaFromLumaPlaneCost(
+                        writer,
+                        lumaMode,
+                        Av1Plane.U,
+                        chromaOrigin,
+                        transformSize,
+                        blueSource,
+                        blueDc,
+                        blueContext,
+                        lumaQ3,
+                        alphaQ3,
+                        candidateBlueReconstruction[..sampleCount],
+                        candidateBlueCoefficients[..sampleCount],
+                        ref candidateBlueState,
+                        out blueRates[alphaCandidateIndex]);
+
+                    Av1EncoderTransformBlockState candidateRedState = default;
+                    redDistortions[alphaCandidateIndex] = this.GetChromaFromLumaPlaneCost(
+                        writer,
+                        lumaMode,
+                        Av1Plane.V,
+                        chromaOrigin,
+                        transformSize,
+                        redSource,
+                        redDc,
+                        redContext,
+                        lumaQ3,
+                        alphaQ3,
+                        candidateRedReconstruction[..sampleCount],
+                        candidateRedCoefficients[..sampleCount],
+                        ref candidateRedState,
+                        out redRates[alphaCandidateIndex]);
+                }
+
+                int chromaFromLumaModeRate = Av1TileWriter.GetChromaModeCost(
+                    writer,
+                    this.picture.Parent.FrameHeader,
+                    colorConfig,
+                    modeInfo,
+                    BlockSize,
+                    lumaMode,
+                    Av1ChromaPredictionMode.ChromaFromLuma,
+                    0);
+
+                bool chromaFromLumaSelected = false;
+                int selectedBlueCandidateIndex = 0;
+                int selectedRedCandidateIndex = 0;
+                for (int blueCandidateIndex = 0; blueCandidateIndex < Av1ChromaFromLumaMath.AlphaCandidateCount; blueCandidateIndex++)
+                {
+                    int alphaU = Av1ChromaFromLumaMath.CandidateIndexToAlpha(blueCandidateIndex);
+                    int signU = Av1ChromaFromLumaMath.AlphaToSign(alphaU);
+                    int indexU = Av1ChromaFromLumaMath.AlphaToMagnitudeIndex(alphaU);
+                    for (int redCandidateIndex = 0; redCandidateIndex < Av1ChromaFromLumaMath.AlphaCandidateCount; redCandidateIndex++)
+                    {
+                        int alphaV = Av1ChromaFromLumaMath.CandidateIndexToAlpha(redCandidateIndex);
+                        int signV = Av1ChromaFromLumaMath.AlphaToSign(alphaV);
+                        if (signU == Av1ChromaFromLumaMath.SignZero && signV == Av1ChromaFromLumaMath.SignZero)
+                        {
+                            continue;
+                        }
+
+                        int indexV = Av1ChromaFromLumaMath.AlphaToMagnitudeIndex(alphaV);
+                        int jointSign = Av1ChromaFromLumaMath.JointSign(signU, signV);
+                        int packedIndex = Av1ChromaFromLumaMath.PackIndices(indexU, indexV);
+                        int rate = chromaFromLumaModeRate
+                            + blueRates[blueCandidateIndex]
+                            + redRates[redCandidateIndex]
+                            + writer.GetChromaFromLumaCost(packedIndex, jointSign);
+
+                        long distortion = blueDistortions[blueCandidateIndex] + redDistortions[redCandidateIndex];
+                        long candidateCost = Av1RateDistortion.GetCost(this.rateMultiplier, rate, distortion);
+                        bool winsSearchOrderTie = candidateCost == bestCost
+                            && !chromaFromLumaSelected
+                            && bestMode != Av1ChromaPredictionMode.DC;
+
+                        // CfL follows DC and precedes every other chroma mode in the reference search order.
+                        if (candidateCost < bestCost || winsSearchOrderTie)
+                        {
+                            bestCost = candidateCost;
+                            bestMode = Av1ChromaPredictionMode.ChromaFromLuma;
+                            selectedAngleDelta = 0;
+                            selectedBlueCandidateIndex = blueCandidateIndex;
+                            selectedRedCandidateIndex = redCandidateIndex;
+                            selectedChromaFromLumaIndex = (byte)packedIndex;
+                            selectedChromaFromLumaSigns = (sbyte)jointSign;
+                            chromaFromLumaSelected = true;
+                        }
+                    }
+                }
+
+                if (chromaFromLumaSelected)
+                {
+                    Av1EncoderTransformBlockState candidateBlueState = default;
+                    _ = this.GetChromaFromLumaPlaneCost(
+                        writer,
+                        lumaMode,
+                        Av1Plane.U,
+                        chromaOrigin,
+                        transformSize,
+                        blueSource,
+                        blueDc,
+                        blueContext,
+                        lumaQ3,
+                        Av1ChromaFromLumaMath.CandidateIndexToAlpha(selectedBlueCandidateIndex),
+                        candidateBlueReconstruction[..sampleCount],
+                        candidateBlueCoefficients[..sampleCount],
+                        ref candidateBlueState,
+                        out _);
+
+                    CopyCandidate(
+                        candidateBlueReconstruction,
+                        candidateBlueCoefficients,
+                        blueReconstruction,
+                        chromaOrigin,
+                        retainedBlueCoefficients,
+                        transformSize,
+                        candidateBlueState,
+                        ref retainedBlueState);
+
+                    Av1EncoderTransformBlockState candidateRedState = default;
+                    _ = this.GetChromaFromLumaPlaneCost(
+                        writer,
+                        lumaMode,
+                        Av1Plane.V,
+                        chromaOrigin,
+                        transformSize,
+                        redSource,
+                        redDc,
+                        redContext,
+                        lumaQ3,
+                        Av1ChromaFromLumaMath.CandidateIndexToAlpha(selectedRedCandidateIndex),
+                        candidateRedReconstruction[..sampleCount],
+                        candidateRedCoefficients[..sampleCount],
+                        ref candidateRedState,
+                        out _);
+
+                    CopyCandidate(
+                        candidateRedReconstruction,
+                        candidateRedCoefficients,
+                        redReconstruction,
+                        chromaOrigin,
+                        retainedRedCoefficients,
+                        transformSize,
+                        candidateRedState,
+                        ref retainedRedState);
+                }
+            }
+
             return bestMode;
         }
 
+        private long GetChromaFromLumaPlaneCost(
+            Av1SymbolEncoder writer,
+            Av1PredictionMode lumaMode,
+            Av1Plane plane,
+            Point chromaOrigin,
+            Av1TransformSize transformSize,
+            Buffer2DRegion<TSample> source,
+            TSample dc,
+            Av1TransformBlockContext context,
+            ReadOnlySpan<short> lumaQ3,
+            int alphaQ3,
+            Span<TSample> reconstruction,
+            Span<int> coefficients,
+            ref Av1EncoderTransformBlockState state,
+            out int rate)
+        {
+            long distortion = TOperator.EncodeChromaFromLumaCandidate(
+                this.blockWorkspace,
+                source,
+                chromaOrigin,
+                reconstruction,
+                dc,
+                lumaQ3,
+                alphaQ3,
+                coefficients,
+                transformSize,
+                plane,
+                this.quantization.QIndex[0],
+                this.quantization.DeltaQDc[(int)plane],
+                this.quantization.DeltaQAc[(int)plane],
+                this.bitDepth,
+                ref state);
+
+            rate = writer.GetCoefficientCost(
+                transformSize,
+                Av1TransformType.DctDct,
+                lumaMode,
+                coefficients,
+                Av1ComponentType.Chroma,
+                context,
+                state.EndOfBlock,
+                this.picture.Parent.FrameHeader.UseReducedTransformSet,
+                Av1FilterIntraMode.AllFilterIntraModes);
+
+            return distortion;
+        }
+
         private long GetChromaCandidateCost(
             Av1SymbolEncoder writer,
             Av1MacroBlockModeInfo modeInfo,

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

@@ -237,9 +237,13 @@ internal static partial class Av1IntraSuperblockEncoder
                 redCoefficients[this.codedAreaChroma..],
                 ref blueState,
                 ref redState,
-                out int chromaAngleDelta);
+                out int chromaAngleDelta,
+                out byte chromaFromLumaIndex,
+                out sbyte chromaFromLumaSigns);
 
             block.PredictionUnit.AngleDelta[(int)Av1PlaneType.Uv] = (sbyte)chromaAngleDelta;
+            block.PredictionUnit.ChromaFromLumaIndex = chromaFromLumaIndex;
+            block.PredictionUnit.ChromaFromLumaSigns = chromaFromLumaSigns;
 
             // A block-level skip suppresses every coefficient symbol, so all coded planes must be empty.
             modeInfo.Block.Skip = skipTransform && blueState.EndOfBlock == 0 && redState.EndOfBlock == 0;

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

@@ -3,6 +3,7 @@
 
 using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -36,6 +37,42 @@ internal static partial class Av1IntraSuperblockEncoder
         /// <returns>The converted sample.</returns>
         public static abstract TSample CreateSample(int value);
 
+        /// <summary>
+        /// Builds the zero-mean Q3 luma surface shared by chroma-from-luma candidates.
+        /// </summary>
+        /// <param name="reconstruction">The coded reconstructed luma plane.</param>
+        /// <param name="blockOrigin">The luma block origin in plane samples.</param>
+        /// <param name="lumaQ3">The fixed-stride Q3 predictor workspace.</param>
+        /// <param name="transformSize">The chroma transform dimensions.</param>
+        /// <param name="subsamplingX">Whether luma is subsampled horizontally for chroma.</param>
+        /// <param name="subsamplingY">Whether luma is subsampled vertically for chroma.</param>
+        public static abstract void PrepareChromaFromLuma(
+            Buffer2DRegion<TSample> reconstruction,
+            Point blockOrigin,
+            Span<short> lumaQ3,
+            Av1TransformSize transformSize,
+            bool subsamplingX,
+            bool subsamplingY);
+
+        /// <summary>
+        /// Computes the DC predictor shared by every chroma-from-luma alpha candidate.
+        /// </summary>
+        /// <param name="reconstruction">The contiguous candidate reconstruction.</param>
+        /// <param name="above">The top reference samples.</param>
+        /// <param name="left">The left reference samples.</param>
+        /// <param name="hasLeft">Whether the left reference is available.</param>
+        /// <param name="hasAbove">Whether the top reference is available.</param>
+        /// <param name="transformSize">The chroma transform dimensions.</param>
+        /// <param name="bitDepth">The coded sample bit depth.</param>
+        public static abstract void PrepareChromaFromLumaDc(
+            Span<TSample> reconstruction,
+            ReadOnlySpan<TSample> above,
+            ReadOnlySpan<TSample> left,
+            bool hasLeft,
+            bool hasAbove,
+            Av1TransformSize transformSize,
+            Av1BitDepth bitDepth);
+
         /// <summary>
         /// Encodes and reconstructs one DC intra transform block.
         /// </summary>
@@ -116,6 +153,42 @@ internal static partial class Av1IntraSuperblockEncoder
             int acDeltaQ,
             Av1BitDepth bitDepth,
             ref Av1EncoderTransformBlockState state);
+
+        /// <summary>
+        /// Encodes one chroma-from-luma candidate into contiguous decision scratch.
+        /// </summary>
+        /// <param name="workspace">The reusable block workspace.</param>
+        /// <param name="source">The coded source plane.</param>
+        /// <param name="blockOrigin">The transform-block origin in plane samples.</param>
+        /// <param name="reconstruction">The contiguous candidate reconstruction.</param>
+        /// <param name="dc">The cached DC predictor sample shared by every alpha.</param>
+        /// <param name="lumaQ3">The zero-mean reconstructed-luma predictor surface.</param>
+        /// <param name="alphaQ3">The signed chroma-from-luma multiplier.</param>
+        /// <param name="quantizedCoefficients">The candidate entropy-coding coefficients.</param>
+        /// <param name="transformSize">The transform dimensions.</param>
+        /// <param name="plane">The component plane containing the block.</param>
+        /// <param name="qIndex">The effective segment quantizer index.</param>
+        /// <param name="dcDeltaQ">The plane DC quantizer adjustment.</param>
+        /// <param name="acDeltaQ">The plane AC quantizer adjustment.</param>
+        /// <param name="bitDepth">The coded sample bit depth.</param>
+        /// <param name="state">The candidate transform state.</param>
+        /// <returns>The normalized pixel-domain distortion in AV1 transform units.</returns>
+        public static abstract long EncodeChromaFromLumaCandidate(
+            Av1EncoderBlockWorkspace workspace,
+            Buffer2DRegion<TSample> source,
+            Point blockOrigin,
+            Span<TSample> reconstruction,
+            TSample dc,
+            ReadOnlySpan<short> lumaQ3,
+            int alphaQ3,
+            Span<int> quantizedCoefficients,
+            Av1TransformSize transformSize,
+            Av1Plane plane,
+            int qIndex,
+            int dcDeltaQ,
+            int acDeltaQ,
+            Av1BitDepth bitDepth,
+            ref Av1EncoderTransformBlockState state);
     }
 
     /// <summary>
@@ -130,6 +203,44 @@ internal static partial class Av1IntraSuperblockEncoder
         /// <inheritdoc/>
         public static byte CreateSample(int value) => (byte)value;
 
+        /// <inheritdoc/>
+        public static void PrepareChromaFromLuma(
+            Buffer2DRegion<byte> reconstruction,
+            Point blockOrigin,
+            Span<short> lumaQ3,
+            Av1TransformSize transformSize,
+            bool subsamplingX,
+            bool subsamplingY)
+            => Av1ChromaFromLumaContext.PrepareBlock(
+                Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, blockOrigin),
+                reconstruction.Stride,
+                lumaQ3,
+                transformSize,
+                subsamplingX,
+                subsamplingY);
+
+        /// <inheritdoc/>
+        public static void PrepareChromaFromLumaDc(
+            Span<byte> reconstruction,
+            ReadOnlySpan<byte> above,
+            ReadOnlySpan<byte> left,
+            bool hasLeft,
+            bool hasAbove,
+            Av1TransformSize transformSize,
+            Av1BitDepth bitDepth)
+        {
+            int width = transformSize.GetWidth();
+            Av1DcIntraPredictor.Predict(
+                hasLeft,
+                hasAbove,
+                reconstruction,
+                width,
+                above,
+                left,
+                width,
+                transformSize.GetHeight());
+        }
+
         /// <inheritdoc/>
         public static void Encode(
             Av1EncoderBlockWorkspace workspace,
@@ -206,6 +317,39 @@ internal static partial class Av1IntraSuperblockEncoder
                 acDeltaQ,
                 plane,
                 ref state);
+
+        /// <inheritdoc/>
+        public static long EncodeChromaFromLumaCandidate(
+            Av1EncoderBlockWorkspace workspace,
+            Buffer2DRegion<byte> source,
+            Point blockOrigin,
+            Span<byte> reconstruction,
+            byte dc,
+            ReadOnlySpan<short> lumaQ3,
+            int alphaQ3,
+            Span<int> quantizedCoefficients,
+            Av1TransformSize transformSize,
+            Av1Plane plane,
+            int qIndex,
+            int dcDeltaQ,
+            int acDeltaQ,
+            Av1BitDepth bitDepth,
+            ref Av1EncoderTransformBlockState state)
+            => Av1TransformBlockEncoder.EncodeChromaFromLumaLossyCandidate(
+                workspace,
+                source,
+                blockOrigin,
+                reconstruction,
+                dc,
+                lumaQ3,
+                alphaQ3,
+                quantizedCoefficients,
+                transformSize,
+                qIndex,
+                dcDeltaQ,
+                acDeltaQ,
+                plane,
+                ref state);
     }
 
     /// <summary>
@@ -220,6 +364,45 @@ internal static partial class Av1IntraSuperblockEncoder
         /// <inheritdoc/>
         public static ushort CreateSample(int value) => (ushort)value;
 
+        /// <inheritdoc/>
+        public static void PrepareChromaFromLuma(
+            Buffer2DRegion<ushort> reconstruction,
+            Point blockOrigin,
+            Span<short> lumaQ3,
+            Av1TransformSize transformSize,
+            bool subsamplingX,
+            bool subsamplingY)
+            => Av1ChromaFromLumaContext.PrepareBlock(
+                MemoryMarshal.Cast<ushort, short>(Av1TransformBlockEncoder.GetPlaneSpan(reconstruction, blockOrigin)),
+                reconstruction.Stride,
+                lumaQ3,
+                transformSize,
+                subsamplingX,
+                subsamplingY);
+
+        /// <inheritdoc/>
+        public static void PrepareChromaFromLumaDc(
+            Span<ushort> reconstruction,
+            ReadOnlySpan<ushort> above,
+            ReadOnlySpan<ushort> left,
+            bool hasLeft,
+            bool hasAbove,
+            Av1TransformSize transformSize,
+            Av1BitDepth bitDepth)
+        {
+            int width = transformSize.GetWidth();
+            Av1DcIntraPredictor.Predict(
+                hasLeft,
+                hasAbove,
+                MemoryMarshal.Cast<ushort, short>(reconstruction),
+                width,
+                MemoryMarshal.Cast<ushort, short>(above),
+                MemoryMarshal.Cast<ushort, short>(left),
+                width,
+                transformSize.GetHeight(),
+                bitDepth.GetBitCount());
+        }
+
         /// <inheritdoc/>
         public static void Encode(
             Av1EncoderBlockWorkspace workspace,
@@ -298,5 +481,39 @@ internal static partial class Av1IntraSuperblockEncoder
                 plane,
                 bitDepth,
                 ref state);
+
+        /// <inheritdoc/>
+        public static long EncodeChromaFromLumaCandidate(
+            Av1EncoderBlockWorkspace workspace,
+            Buffer2DRegion<ushort> source,
+            Point blockOrigin,
+            Span<ushort> reconstruction,
+            ushort dc,
+            ReadOnlySpan<short> lumaQ3,
+            int alphaQ3,
+            Span<int> quantizedCoefficients,
+            Av1TransformSize transformSize,
+            Av1Plane plane,
+            int qIndex,
+            int dcDeltaQ,
+            int acDeltaQ,
+            Av1BitDepth bitDepth,
+            ref Av1EncoderTransformBlockState state)
+            => Av1TransformBlockEncoder.EncodeChromaFromLumaLossyCandidate(
+                workspace,
+                source,
+                blockOrigin,
+                reconstruction,
+                dc,
+                lumaQ3,
+                alphaQ3,
+                quantizedCoefficients,
+                transformSize,
+                qIndex,
+                dcDeltaQ,
+                acDeltaQ,
+                plane,
+                bitDepth,
+                ref state);
     }
 }

src/ImageSharp/Formats/Heif/Av1/Pipeline/Av1TransformBlockEncoder.cs

@@ -4,6 +4,7 @@
 using System.Runtime.InteropServices;
 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.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -157,6 +158,96 @@ internal static class Av1TransformBlockEncoder
         return Av1ResidualBuilder.SumSquares(workspace.Residual[..transformSize.GetSize2d()]) << 4;
     }
 
+    /// <summary>
+    /// Encodes one eight-bit chroma-from-luma candidate into contiguous decision scratch.
+    /// </summary>
+    /// <param name="workspace">The reusable residual, coefficient, and transform storage.</param>
+    /// <param name="source">The coded source plane.</param>
+    /// <param name="blockOrigin">The block origin in plane samples.</param>
+    /// <param name="reconstruction">The contiguous candidate reconstruction.</param>
+    /// <param name="dc">The cached DC predictor sample shared by every alpha.</param>
+    /// <param name="lumaQ3">The zero-mean reconstructed-luma predictor surface.</param>
+    /// <param name="alphaQ3">The signed chroma-from-luma multiplier.</param>
+    /// <param name="quantizedCoefficients">The candidate entropy-coding coefficients.</param>
+    /// <param name="transformSize">The selected chroma transform dimensions.</param>
+    /// <param name="qIndex">The segment quantizer index.</param>
+    /// <param name="dcDeltaQ">The plane DC quantizer adjustment.</param>
+    /// <param name="acDeltaQ">The plane AC quantizer adjustment.</param>
+    /// <param name="plane">The component plane containing the block.</param>
+    /// <param name="state">The candidate transform type and end-of-block syntax.</param>
+    /// <returns>The normalized pixel-domain distortion in AV1 transform units.</returns>
+    public static long EncodeChromaFromLumaLossyCandidate(
+        Av1EncoderBlockWorkspace workspace,
+        Buffer2DRegion<byte> source,
+        Point blockOrigin,
+        Span<byte> reconstruction,
+        byte dc,
+        ReadOnlySpan<short> lumaQ3,
+        int alphaQ3,
+        Span<int> quantizedCoefficients,
+        Av1TransformSize transformSize,
+        int qIndex,
+        int dcDeltaQ,
+        int acDeltaQ,
+        Av1Plane plane,
+        ref Av1EncoderTransformBlockState state)
+    {
+        int width = transformSize.GetWidth();
+        int height = transformSize.GetHeight();
+        ReadOnlySpan<byte> sourceSamples = GetPlaneSpan(source, blockOrigin);
+        reconstruction[..transformSize.GetSize2d()].Fill(dc);
+
+        // CfL adds its scaled reconstructed-luma AC contribution to the cached DC predictor before residual coding.
+        Av1ChromaFromLumaPredictor.Predict(lumaQ3, reconstruction, width, alphaQ3, width, height);
+        Av1ResidualBuilder.Subtract(
+            sourceSamples,
+            source.Stride,
+            reconstruction,
+            width,
+            workspace.Residual,
+            width,
+            width,
+            height);
+
+        EncodeLossy(
+            workspace,
+            quantizedCoefficients,
+            transformSize,
+            Av1TransformType.DctDct,
+            qIndex,
+            dcDeltaQ,
+            acDeltaQ,
+            Av1BitDepth.EightBit,
+            ref state);
+
+        if (state.EndOfBlock > 0)
+        {
+            Av1InverseTransformer.Reconstruct8Bit(
+                workspace.DequantizedCoefficients,
+                reconstruction,
+                width,
+                transformSize,
+                Av1TransformType.DctDct,
+                (int)plane,
+                state.EndOfBlock,
+                false,
+                workspace.TransformWorkspace);
+        }
+
+        // Final distortion is measured against the samples a decoder reconstructs, not the unquantized predictor.
+        Av1ResidualBuilder.Subtract(
+            sourceSamples,
+            source.Stride,
+            reconstruction,
+            width,
+            workspace.Residual,
+            width,
+            width,
+            height);
+
+        return Av1ResidualBuilder.SumSquares(workspace.Residual[..transformSize.GetSize2d()]) << 4;
+    }
+
     /// <summary>
     /// Encodes and reconstructs one high-bit-depth lossy DC intra block in contiguous encoder planes.
     /// </summary>
@@ -311,6 +402,112 @@ internal static class Av1TransformBlockEncoder
         return normalizedDistortion << 4;
     }
 
+    /// <summary>
+    /// Encodes one high-bit-depth chroma-from-luma candidate into contiguous decision scratch.
+    /// </summary>
+    /// <param name="workspace">The reusable residual, coefficient, and transform storage.</param>
+    /// <param name="source">The coded source plane.</param>
+    /// <param name="blockOrigin">The block origin in plane samples.</param>
+    /// <param name="reconstruction">The contiguous candidate reconstruction.</param>
+    /// <param name="dc">The cached DC predictor sample shared by every alpha.</param>
+    /// <param name="lumaQ3">The zero-mean reconstructed-luma predictor surface.</param>
+    /// <param name="alphaQ3">The signed chroma-from-luma multiplier.</param>
+    /// <param name="quantizedCoefficients">The candidate entropy-coding coefficients.</param>
+    /// <param name="transformSize">The selected chroma transform dimensions.</param>
+    /// <param name="qIndex">The segment quantizer index.</param>
+    /// <param name="dcDeltaQ">The plane DC quantizer adjustment.</param>
+    /// <param name="acDeltaQ">The plane AC quantizer adjustment.</param>
+    /// <param name="plane">The component plane containing the block.</param>
+    /// <param name="bitDepth">The coded sample bit depth.</param>
+    /// <param name="state">The candidate transform type and end-of-block syntax.</param>
+    /// <returns>The normalized pixel-domain distortion in AV1 transform units.</returns>
+    public static long EncodeChromaFromLumaLossyCandidate(
+        Av1EncoderBlockWorkspace workspace,
+        Buffer2DRegion<ushort> source,
+        Point blockOrigin,
+        Span<ushort> reconstruction,
+        ushort dc,
+        ReadOnlySpan<short> lumaQ3,
+        int alphaQ3,
+        Span<int> quantizedCoefficients,
+        Av1TransformSize transformSize,
+        int qIndex,
+        int dcDeltaQ,
+        int acDeltaQ,
+        Av1Plane plane,
+        Av1BitDepth bitDepth,
+        ref Av1EncoderTransformBlockState state)
+    {
+        int width = transformSize.GetWidth();
+        int height = transformSize.GetHeight();
+        ReadOnlySpan<ushort> sourceSamples = GetPlaneSpan(source, blockOrigin);
+        Span<short> signedReconstruction = MemoryMarshal.Cast<ushort, short>(reconstruction);
+        reconstruction[..transformSize.GetSize2d()].Fill(dc);
+
+        Av1ChromaFromLumaPredictor.Predict(
+            lumaQ3,
+            signedReconstruction,
+            width,
+            alphaQ3,
+            bitDepth.GetBitCount(),
+            width,
+            height);
+
+        Av1ResidualBuilder.Subtract(
+            sourceSamples,
+            source.Stride,
+            reconstruction,
+            width,
+            workspace.Residual,
+            width,
+            width,
+            height);
+
+        EncodeLossy(
+            workspace,
+            quantizedCoefficients,
+            transformSize,
+            Av1TransformType.DctDct,
+            qIndex,
+            dcDeltaQ,
+            acDeltaQ,
+            bitDepth,
+            ref state);
+
+        if (state.EndOfBlock > 0)
+        {
+            Av1InverseTransformer.ReconstructHighBitDepth(
+                workspace.DequantizedCoefficients,
+                signedReconstruction,
+                width,
+                transformSize,
+                Av1TransformType.DctDct,
+                (int)plane,
+                state.EndOfBlock,
+                false,
+                bitDepth,
+                workspace.TransformWorkspace);
+        }
+
+        Av1ResidualBuilder.Subtract(
+            sourceSamples,
+            source.Stride,
+            reconstruction,
+            width,
+            workspace.Residual,
+            width,
+            width,
+            height);
+
+        long distortion = Av1ResidualBuilder.SumSquares(workspace.Residual[..transformSize.GetSize2d()]);
+        int shift = (bitDepth.GetBitCount() - 8) * 2;
+        long normalizedDistortion = shift == 0
+            ? distortion
+            : (distortion + (1L << (shift - 1))) >> shift;
+
+        return normalizedDistortion << 4;
+    }
+
     /// <summary>
     /// Encodes and reconstructs one eight-bit lossy intra block.
     /// </summary>
@@ -591,7 +788,7 @@ internal static class Av1TransformBlockEncoder
         state.TransformType = transformType;
     }
 
-    private static Span<TSample> GetPlaneSpan<TSample>(Buffer2DRegion<TSample> plane, Point blockOrigin)
+    public static Span<TSample> GetPlaneSpan<TSample>(Buffer2DRegion<TSample> plane, Point blockOrigin)
         where TSample : unmanaged
     {
         int offset =

src/ImageSharp/Formats/Heif/Av1/Prediction/ChromaFromLuma/Av1ChromaFromLumaContext.Operations.cs

@@ -18,6 +18,52 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
 /// </content>
 internal partial class Av1ChromaFromLumaContext
 {
+    /// <summary>
+    /// Subsamples one reconstructed eight-bit luma block and removes its rounded Q3 mean.
+    /// </summary>
+    /// <param name="input">The reconstructed luma samples.</param>
+    /// <param name="inputStride">The distance, in samples, between input rows.</param>
+    /// <param name="output">The fixed-stride Q3 predictor workspace.</param>
+    /// <param name="transformSize">The chroma transform dimensions.</param>
+    /// <param name="subsamplingX">Whether two horizontal luma samples map to each chroma sample.</param>
+    /// <param name="subsamplingY">Whether two vertical luma samples map to each chroma sample.</param>
+    public static void PrepareBlock(
+        ReadOnlySpan<byte> input,
+        int inputStride,
+        Span<short> output,
+        Av1TransformSize transformSize,
+        bool subsamplingX,
+        bool subsamplingY)
+    {
+        int lumaWidth = transformSize.GetWidth() << (subsamplingX ? 1 : 0);
+        int lumaHeight = transformSize.GetHeight() << (subsamplingY ? 1 : 0);
+        StoreSamples(input, inputStride, 0, lumaWidth, lumaHeight, output, subsamplingX, subsamplingY);
+        SubtractAverage(output, transformSize);
+    }
+
+    /// <summary>
+    /// Subsamples one reconstructed high-bit-depth luma block and removes its rounded Q3 mean.
+    /// </summary>
+    /// <param name="input">The reconstructed luma samples.</param>
+    /// <param name="inputStride">The distance, in samples, between input rows.</param>
+    /// <param name="output">The fixed-stride Q3 predictor workspace.</param>
+    /// <param name="transformSize">The chroma transform dimensions.</param>
+    /// <param name="subsamplingX">Whether two horizontal luma samples map to each chroma sample.</param>
+    /// <param name="subsamplingY">Whether two vertical luma samples map to each chroma sample.</param>
+    public static void PrepareBlock(
+        ReadOnlySpan<short> input,
+        int inputStride,
+        Span<short> output,
+        Av1TransformSize transformSize,
+        bool subsamplingX,
+        bool subsamplingY)
+    {
+        int lumaWidth = transformSize.GetWidth() << (subsamplingX ? 1 : 0);
+        int lumaHeight = transformSize.GetHeight() << (subsamplingY ? 1 : 0);
+        StoreSamples(input, inputStride, 0, lumaWidth, lumaHeight, output, subsamplingX, subsamplingY);
+        SubtractAverage(output, transformSize);
+    }
+
     /// <summary>
     /// Stores 8-bit reconstructed luma samples in the Q3 predictor surface.
     /// </summary>
@@ -26,12 +72,23 @@ internal partial class Av1ChromaFromLumaContext
     /// <param name="outputOffset">The first destination sample in the fixed-stride predictor buffer.</param>
     /// <param name="width">The luma width in samples.</param>
     /// <param name="height">The luma height in samples.</param>
-    private void StoreSamples(ReadOnlySpan<byte> input, int inputStride, int outputOffset, int width, int height)
+    /// <param name="output">The fixed-stride Q3 predictor workspace.</param>
+    /// <param name="subsamplingX">Whether horizontal luma pairs are subsampled.</param>
+    /// <param name="subsamplingY">Whether vertical luma pairs are subsampled.</param>
+    private static void StoreSamples(
+        ReadOnlySpan<byte> input,
+        int inputStride,
+        int outputOffset,
+        int width,
+        int height,
+        Span<short> output,
+        bool subsamplingX,
+        bool subsamplingY)
     {
         ref byte inputBase = ref MemoryMarshal.GetReference(input);
-        ref short outputBase = ref MemoryMarshal.GetReference(this.Q3Buffer);
+        ref short outputBase = ref MemoryMarshal.GetReference(output);
 
-        if (!this.subX)
+        if (!subsamplingX)
         {
             // One luma sample maps directly to one chroma sample, so multiplying by eight converts it to Q3.
             for (int row = 0; row < height; row++)
@@ -94,13 +151,13 @@ internal partial class Av1ChromaFromLumaContext
 
         Vector256<sbyte> ones256 = Vector256.Create((sbyte)1);
         Vector128<sbyte> ones128 = Vector128.Create((sbyte)1);
-        int rowStep = this.subY ? 2 : 1;
+        int rowStep = subsamplingY ? 2 : 1;
-        int outputShift = this.subY ? 1 : 2;
+        int outputShift = subsamplingY ? 1 : 2;
         for (int row = 0; row < height; row += rowStep)
         {
             ref byte inputRow = ref Unsafe.Add(ref inputBase, row * inputStride);
-            ref byte nextInputRow = ref Unsafe.Add(ref inputRow, this.subY ? inputStride : 0);
+            ref byte nextInputRow = ref Unsafe.Add(ref inputRow, subsamplingY ? inputStride : 0);
-            ref short outputRow = ref Unsafe.Add(ref outputBase, outputOffset + ((row >> (this.subY ? 1 : 0)) * BufferLine));
+            ref short outputRow = ref Unsafe.Add(ref outputBase, outputOffset + ((row >> (subsamplingY ? 1 : 0)) * BufferLine));
             int column = 0;
 
             if (Avx2.IsSupported)
@@ -109,7 +166,7 @@ internal partial class Av1ChromaFromLumaContext
                 for (; vectorCount > 0; vectorCount--, column += Vector256<byte>.Count)
                 {
                     Vector256<short> sum = Avx2.MultiplyAddAdjacent(Vector256.LoadUnsafe(ref inputRow, (nuint)column), ones256);
-                    if (this.subY)
+                    if (subsamplingY)
                     {
                         sum += Avx2.MultiplyAddAdjacent(Vector256.LoadUnsafe(ref nextInputRow, (nuint)column), ones256);
                     }
@@ -124,7 +181,7 @@ internal partial class Av1ChromaFromLumaContext
                 for (; vectorCount > 0; vectorCount--, column += Vector128<byte>.Count)
                 {
                     Vector128<short> sum = PairSum(Vector128.LoadUnsafe(ref inputRow, (nuint)column), ones128);
-                    if (this.subY)
+                    if (subsamplingY)
                     {
                         sum += PairSum(Vector128.LoadUnsafe(ref nextInputRow, (nuint)column), ones128);
                     }
@@ -139,7 +196,7 @@ internal partial class Av1ChromaFromLumaContext
                         ? Unsafe.ReadUnaligned<uint>(ref Unsafe.Add(ref inputRow, column))
                         : Unsafe.ReadUnaligned<ulong>(ref Unsafe.Add(ref inputRow, column));
                     Vector128<short> sum = PairSum(Vector128.CreateScalarUnsafe(packed).AsByte(), ones128);
-                    if (this.subY)
+                    if (subsamplingY)
                     {
                         packed = remaining == 4
                             ? Unsafe.ReadUnaligned<uint>(ref Unsafe.Add(ref nextInputRow, column))
@@ -164,7 +221,7 @@ internal partial class Av1ChromaFromLumaContext
             for (; column < width; column += 2)
             {
                 int sum = Unsafe.Add(ref inputRow, column) + Unsafe.Add(ref inputRow, column + 1);
-                if (this.subY)
+                if (subsamplingY)
                 {
                     sum += Unsafe.Add(ref nextInputRow, column) + Unsafe.Add(ref nextInputRow, column + 1);
                 }
@@ -182,12 +239,23 @@ internal partial class Av1ChromaFromLumaContext
     /// <param name="outputOffset">The first destination sample in the fixed-stride predictor buffer.</param>
     /// <param name="width">The luma width in samples.</param>
     /// <param name="height">The luma height in samples.</param>
-    private void StoreSamples(ReadOnlySpan<short> input, int inputStride, int outputOffset, int width, int height)
+    /// <param name="output">The fixed-stride Q3 predictor workspace.</param>
+    /// <param name="subsamplingX">Whether horizontal luma pairs are subsampled.</param>
+    /// <param name="subsamplingY">Whether vertical luma pairs are subsampled.</param>
+    private static void StoreSamples(
+        ReadOnlySpan<short> input,
+        int inputStride,
+        int outputOffset,
+        int width,
+        int height,
+        Span<short> output,
+        bool subsamplingX,
+        bool subsamplingY)
     {
         ref short inputBase = ref MemoryMarshal.GetReference(input);
-        ref short outputBase = ref MemoryMarshal.GetReference(this.Q3Buffer);
+        ref short outputBase = ref MemoryMarshal.GetReference(output);
 
-        if (!this.subX)
+        if (!subsamplingX)
         {
             for (int row = 0; row < height; row++)
             {
@@ -232,13 +300,13 @@ internal partial class Av1ChromaFromLumaContext
 
         Vector256<short> ones256 = Vector256.Create((short)1);
         Vector128<short> ones128 = Vector128.Create((short)1);
-        int rowStep = this.subY ? 2 : 1;
+        int rowStep = subsamplingY ? 2 : 1;
-        int outputShift = this.subY ? 1 : 2;
+        int outputShift = subsamplingY ? 1 : 2;
         for (int row = 0; row < height; row += rowStep)
         {
             ref short inputRow = ref Unsafe.Add(ref inputBase, row * inputStride);
-            ref short nextInputRow = ref Unsafe.Add(ref inputRow, this.subY ? inputStride : 0);
+            ref short nextInputRow = ref Unsafe.Add(ref inputRow, subsamplingY ? inputStride : 0);
-            ref short outputRow = ref Unsafe.Add(ref outputBase, outputOffset + ((row >> (this.subY ? 1 : 0)) * BufferLine));
+            ref short outputRow = ref Unsafe.Add(ref outputBase, outputOffset + ((row >> (subsamplingY ? 1 : 0)) * BufferLine));
             int column = 0;
 
             if (Vector256.IsHardwareAccelerated)
@@ -247,7 +315,7 @@ internal partial class Av1ChromaFromLumaContext
                 for (; vectorCount > 0; vectorCount--, column += Vector256<short>.Count)
                 {
                     Vector256<int> sum = Vector256_.MultiplyAddAdjacent(Vector256.LoadUnsafe(ref inputRow, (nuint)column), ones256);
-                    if (this.subY)
+                    if (subsamplingY)
                     {
                         sum += Vector256_.MultiplyAddAdjacent(Vector256.LoadUnsafe(ref nextInputRow, (nuint)column), ones256);
                     }
@@ -262,7 +330,7 @@ internal partial class Av1ChromaFromLumaContext
                 for (; vectorCount > 0; vectorCount--, column += Vector128<short>.Count)
                 {
                     Vector128<int> sum = Vector128_.MultiplyAddAdjacent(Vector128.LoadUnsafe(ref inputRow, (nuint)column), ones128);
-                    if (this.subY)
+                    if (subsamplingY)
                     {
                         sum += Vector128_.MultiplyAddAdjacent(Vector128.LoadUnsafe(ref nextInputRow, (nuint)column), ones128);
                     }
@@ -275,7 +343,7 @@ internal partial class Av1ChromaFromLumaContext
                     Vector128<short> samples = Vector128.CreateScalarUnsafe(
                         Unsafe.ReadUnaligned<ulong>(ref Unsafe.As<short, byte>(ref Unsafe.Add(ref inputRow, column)))).AsInt16();
                     Vector128<int> sum = Vector128_.MultiplyAddAdjacent(samples, ones128);
-                    if (this.subY)
+                    if (subsamplingY)
                     {
                         samples = Vector128.CreateScalarUnsafe(
                             Unsafe.ReadUnaligned<ulong>(ref Unsafe.As<short, byte>(ref Unsafe.Add(ref nextInputRow, column)))).AsInt16();
@@ -291,7 +359,7 @@ internal partial class Av1ChromaFromLumaContext
             for (; column < width; column += 2)
             {
                 int sum = Unsafe.Add(ref inputRow, column) + Unsafe.Add(ref inputRow, column + 1);
-                if (this.subY)
+                if (subsamplingY)
                 {
                     sum += Unsafe.Add(ref nextInputRow, column) + Unsafe.Add(ref nextInputRow, column + 1);
                 }
@@ -304,8 +372,9 @@ internal partial class Av1ChromaFromLumaContext
     /// <summary>
     /// Subtracts the rounded Q3 average from each predictor sample, leaving the AC contribution used by CfL.
     /// </summary>
+    /// <param name="buffer">The fixed-stride Q3 predictor workspace.</param>
     /// <param name="transformSize">The populated predictor dimensions.</param>
-    private void SubtractAverage(Av1TransformSize transformSize)
+    private static void SubtractAverage(Span<short> buffer, Av1TransformSize transformSize)
     {
         int width = transformSize.GetWidth();
         int height = transformSize.GetHeight();
@@ -313,7 +382,7 @@ internal partial class Av1ChromaFromLumaContext
         // Transform dimensions are powers of two, so division by the sample count is an exact right shift. Half
         // the sample count is accumulated first to implement the normative nearest-integer rounding.
         int sumQ3 = (width * height) >> 1;
-        ref short bufferBase = ref MemoryMarshal.GetReference(this.Q3Buffer);
+        ref short bufferBase = ref MemoryMarshal.GetReference(buffer);
 
         if (Vector256.IsHardwareAccelerated && width >= Vector256<short>.Count)
         {

src/ImageSharp/Formats/Heif/Av1/Prediction/ChromaFromLuma/Av1ChromaFromLumaContext.cs

@@ -16,7 +16,7 @@ internal sealed partial class Av1ChromaFromLumaContext
     /// <summary>
     /// The fixed row stride and maximum dimension, in chroma samples, of the luma predictor buffer.
     /// </summary>
-    private const int BufferLine = 32;
+    public const int BufferLine = 32;
 
     /// <summary>
     /// The number of samples in the fixed-stride chroma-from-luma workspace.
@@ -145,11 +145,27 @@ internal sealed partial class Av1ChromaFromLumaContext
         // here keeps sample conversion out of the row kernels and lets the JIT specialize both storage layouts.
         if (typeof(T) == typeof(byte))
         {
-            this.StoreSamples(MemoryMarshal.Cast<T, byte>(input), inputStride, outputOffset, width, height);
+            StoreSamples(
+                MemoryMarshal.Cast<T, byte>(input),
+                inputStride,
+                outputOffset,
+                width,
+                height,
+                this.Q3Buffer,
+                this.subX,
+                this.subY);
         }
         else
         {
-            this.StoreSamples(MemoryMarshal.Cast<T, short>(input), inputStride, outputOffset, width, height);
+            StoreSamples(
+                MemoryMarshal.Cast<T, short>(input),
+                inputStride,
+                outputOffset,
+                width,
+                height,
+                this.Q3Buffer,
+                this.subX,
+                this.subY);
         }
     }
 
@@ -161,7 +177,7 @@ internal sealed partial class Av1ChromaFromLumaContext
     {
         Guard.IsFalse(this.AreParametersComputed, nameof(this.AreParametersComputed), "Do not call cfl_compute_parameters multiple time on the same values.");
         this.Pad(transformSize.GetWidth(), transformSize.GetHeight());
-        this.SubtractAverage(transformSize);
+        SubtractAverage(this.Q3Buffer, transformSize);
         this.AreParametersComputed = true;
     }
 

src/ImageSharp/Formats/Heif/Av1/Prediction/ChromaFromLuma/Av1ChromaFromLumaMath.cs

@@ -18,6 +18,21 @@ internal static class Av1ChromaFromLumaMath
     /// </summary>
     private const int AlphabetSizeLog2 = 4;
 
+    /// <summary>
+    /// The number of nonzero alpha magnitudes represented by each plane's alphabet.
+    /// </summary>
+    public const int AlphaMagnitudeCount = 1 << AlphabetSizeLog2;
+
+    /// <summary>
+    /// The number of signed alpha candidates including zero.
+    /// </summary>
+    public const int AlphaCandidateCount = (AlphaMagnitudeCount * 2) + 1;
+
+    /// <summary>
+    /// The candidate index representing a zero alpha.
+    /// </summary>
+    public const int AlphaZeroIndex = AlphaMagnitudeCount;
+
     /// <summary>
     /// The alpha sign value representing a zero multiplier.
     /// </summary>
@@ -74,4 +89,42 @@ internal static class Av1ChromaFromLumaMath
     /// <param name="jointSign">The coded joint U/V sign symbol.</param>
     /// <returns>The V-plane alpha entropy context.</returns>
     public static int ContextV(int jointSign) => (SignV(jointSign) * Signs) + SignU(jointSign) - Signs;
+
+    /// <summary>
+    /// Converts a signed-candidate index to its alpha value in Q3 units.
+    /// </summary>
+    /// <param name="candidateIndex">The candidate index in negative-to-positive order.</param>
+    /// <returns>The signed Q3 alpha value.</returns>
+    public static int CandidateIndexToAlpha(int candidateIndex) => candidateIndex - AlphaZeroIndex;
+
+    /// <summary>
+    /// Converts a signed Q3 alpha value to its coded sign state.
+    /// </summary>
+    /// <param name="alphaQ3">The signed alpha value.</param>
+    /// <returns>The zero, negative, or positive sign state.</returns>
+    public static int AlphaToSign(int alphaQ3)
+        => alphaQ3 == 0 ? SignZero : alphaQ3 < 0 ? SignNegative : SignPositive;
+
+    /// <summary>
+    /// Converts a nonzero signed Q3 alpha value to its coded magnitude index.
+    /// </summary>
+    /// <param name="alphaQ3">The signed alpha value.</param>
+    /// <returns>The zero-based magnitude index, or zero for a zero alpha.</returns>
+    public static int AlphaToMagnitudeIndex(int alphaQ3) => alphaQ3 == 0 ? 0 : Math.Abs(alphaQ3) - 1;
+
+    /// <summary>
+    /// Combines the U and V sign states into the coded joint symbol.
+    /// </summary>
+    /// <param name="signU">The U-plane sign state.</param>
+    /// <param name="signV">The V-plane sign state.</param>
+    /// <returns>The joint sign symbol.</returns>
+    public static int JointSign(int signU, int signV) => (signU * Signs) + signV - 1;
+
+    /// <summary>
+    /// Packs the U and V alpha-magnitude indices into the coded byte.
+    /// </summary>
+    /// <param name="indexU">The U-plane magnitude index.</param>
+    /// <param name="indexV">The V-plane magnitude index.</param>
+    /// <returns>The packed magnitude indices.</returns>
+    public static int PackIndices(int indexU, int indexV) => (indexU << AlphabetSizeLog2) + indexV;
 }

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

@@ -7,6 +7,7 @@ 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.ChromaFromLuma;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -141,6 +142,37 @@ public class Av1EntropyTests
             encoder.GetTransformBlockSkipCost(true, TransformSize, SkipContext));
     }
 
+    [Theory]
+    [InlineData(-16, 16)]
+    [InlineData(0, 8)]
+    [InlineData(-4, 0)]
+    public void ChromaFromLumaCostMatchesCurrentDistributions(int alphaU, int alphaV)
+    {
+        int signU = Av1ChromaFromLumaMath.AlphaToSign(alphaU);
+        int signV = Av1ChromaFromLumaMath.AlphaToSign(alphaV);
+        int jointSign = Av1ChromaFromLumaMath.JointSign(signU, signV);
+        int indexU = Av1ChromaFromLumaMath.AlphaToMagnitudeIndex(alphaU);
+        int indexV = Av1ChromaFromLumaMath.AlphaToMagnitudeIndex(alphaV);
+        int packedIndex = Av1ChromaFromLumaMath.PackIndices(indexU, indexV);
+        int expected = Av1ProbabilityCost.GetSymbolCost(Av1DefaultDistributions.ChromaFromLumaSign, jointSign);
+        if (signU != Av1ChromaFromLumaMath.SignZero)
+        {
+            expected += Av1ProbabilityCost.GetSymbolCost(
+                Av1DefaultDistributions.ChromaFromLumaAlpha[Av1ChromaFromLumaMath.ContextU(jointSign)],
+                indexU);
+        }
+
+        if (signV != Av1ChromaFromLumaMath.SignZero)
+        {
+            expected += Av1ProbabilityCost.GetSymbolCost(
+                Av1DefaultDistributions.ChromaFromLumaAlpha[Av1ChromaFromLumaMath.ContextV(jointSign)],
+                indexV);
+        }
+
+        using Av1SymbolEncoder encoder = new(Configuration.Default, 64, BaseQIndex, updateCdf: false);
+        Assert.Equal(expected, encoder.GetChromaFromLumaCost(packedIndex, jointSign));
+    }
+
     /// <summary>
     /// Verifies that live luma rate accounting includes the selected signed directional adjustment.
     /// </summary>

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

@@ -2,11 +2,13 @@
 // Licensed under the Six Labors Split License.
 
 using System.Buffers;
+using System.Numerics;
 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.Prediction;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Prediction.ChromaFromLuma;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Transform;
 using SixLabors.ImageSharp.Memory;
@@ -297,12 +299,12 @@ public class Av1IntraSuperblockEncoderTests
             1,
             1);
 
-        FillPlane(source.Frame.CodedView.GetPlane(Av1Plane.Y), 128);
+        FillPlane(source.Frame.CodedView.GetPlane(Av1Plane.Y), (byte)128);
         ClearPlane(reconstruction.Luma);
         if (!isMonochrome)
         {
-            FillPlane(source.Frame.CodedView.GetPlane(Av1Plane.U), 128);
+            FillPlane(source.Frame.CodedView.GetPlane(Av1Plane.U), (byte)128);
-            FillPlane(source.Frame.CodedView.GetPlane(Av1Plane.V), 128);
+            FillPlane(source.Frame.CodedView.GetPlane(Av1Plane.V), (byte)128);
             ClearPlane(Assert.IsType<Buffer2D<byte>>(reconstruction.ChromaBlue));
             ClearPlane(Assert.IsType<Buffer2D<byte>>(reconstruction.ChromaRed));
         }
@@ -921,6 +923,277 @@ public class Av1IntraSuperblockEncoderTests
         Assert.NotEqual(0, tileWriter.GetTileData(0).Length);
     }
 
+    [Theory]
+    [InlineData((int)Av1ColorFormat.Yuv420)]
+    [InlineData((int)Av1ColorFormat.Yuv422)]
+    [InlineData((int)Av1ColorFormat.Yuv444)]
+    public void ProductionTileSelectsChromaFromReconstructedLuma(int colorFormatValue)
+        => VerifyProductionTileSelectsChromaFromReconstructedLuma<byte>(
+            colorFormatValue,
+            8,
+            static (source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
+                new(
+                    Configuration.Default,
+                    source,
+                    reconstruction,
+                    picture,
+                    coefficients,
+                    superblockWorkspace,
+                    blockWorkspace,
+                    initialSize: 512));
+
+    [Theory]
+    [InlineData((int)Av1ColorFormat.Yuv420, 10)]
+    [InlineData((int)Av1ColorFormat.Yuv420, 12)]
+    [InlineData((int)Av1ColorFormat.Yuv422, 10)]
+    [InlineData((int)Av1ColorFormat.Yuv422, 12)]
+    [InlineData((int)Av1ColorFormat.Yuv444, 10)]
+    [InlineData((int)Av1ColorFormat.Yuv444, 12)]
+    public void ProductionTileSelectsChromaFromReconstructedLumaHighBitDepth(
+        int colorFormatValue,
+        int bitDepth)
+        => VerifyProductionTileSelectsChromaFromReconstructedLuma<ushort>(
+            colorFormatValue,
+            bitDepth,
+            static (source, reconstruction, picture, coefficients, superblockWorkspace, blockWorkspace) =>
+                new(
+                    Configuration.Default,
+                    source,
+                    reconstruction,
+                    picture,
+                    coefficients,
+                    superblockWorkspace,
+                    blockWorkspace,
+                    initialSize: 512));
+
+    private static void VerifyProductionTileSelectsChromaFromReconstructedLuma<TSample>(
+        int colorFormatValue,
+        int bitDepth,
+        TileWriterFactory<TSample> createWriter)
+        where TSample : unmanaged, IBinaryInteger<TSample>
+    {
+        const int Width = 16;
+        const int Height = 16;
+        const int QIndex = 1;
+        const int AlphaU = 16;
+        const int AlphaV = -16;
+        Av1ColorFormat colorFormat = (Av1ColorFormat)colorFormatValue;
+        bool subsamplingX = colorFormat is Av1ColorFormat.Yuv420 or Av1ColorFormat.Yuv422;
+        bool subsamplingY = colorFormat == Av1ColorFormat.Yuv420;
+        int chromaSubsamplingX = subsamplingX ? 1 : 0;
+        int chromaSubsamplingY = subsamplingY ? 1 : 0;
+        int sampleScale = 1 << (bitDepth - 8);
+        int midpoint = 1 << (bitDepth - 1);
+        int maxSample = (1 << bitDepth) - 1;
+        Av1TransformSize transformSize = Av1BlockSize.Block8x8.GetMaxUvTransformSize(
+            subsamplingX,
+            subsamplingY);
+
+        ObuColorConfig colorConfig = new()
+        {
+            IsMonochrome = false,
+            SubSamplingX = subsamplingX,
+            SubSamplingY = subsamplingY,
+            BitDepth = (Av1BitDepth)((bitDepth - 8) / 2)
+        };
+
+        using Av1EncoderFrameBuffer<TSample> pilotSource = new(
+            Configuration.Default,
+            Width,
+            Height,
+            bitDepth,
+            colorFormat,
+            chromaSubsamplingX,
+            chromaSubsamplingY);
+
+        using Av1EncoderFrameBuffer<TSample> pilotReconstruction = new(
+            Configuration.Default,
+            Width,
+            Height,
+            bitDepth,
+            colorFormat,
+            chromaSubsamplingX,
+            chromaSubsamplingY);
+
+        Buffer2DRegion<TSample> pilotLuma = pilotSource.Frame.CodedView.GetPlane(Av1Plane.Y);
+        for (int y = 0; y < pilotLuma.Height; y++)
+        {
+            Span<TSample> row = pilotLuma.DangerousGetRowSpan(y);
+            for (int x = 0; x < row.Length; x++)
+            {
+                row[x] = TSample.CreateChecked(
+                    (96 + (((x * 29) + (y * 47) + (((x ^ y) & 1) * 53)) & 63)) * sampleScale);
+            }
+        }
+
+        FillPlane(pilotSource.Frame.CodedView.GetPlane(Av1Plane.U), TSample.CreateChecked(midpoint));
+        FillPlane(pilotSource.Frame.CodedView.GetPlane(Av1Plane.V), TSample.CreateChecked(midpoint));
+        ClearPlane(pilotReconstruction.Luma);
+        ClearPlane(Assert.IsType<Buffer2D<TSample>>(pilotReconstruction.ChromaBlue));
+        ClearPlane(Assert.IsType<Buffer2D<TSample>>(pilotReconstruction.ChromaRed));
+        using Av1EncoderModeInfoBuffer pilotModeInfo = new(Configuration.Default, Width, Height, disallow4x4AllFrames: true);
+        Av1PictureControlSet pilotTemplate = CreatePicture(pilotModeInfo, colorConfig, use128x128Superblock: false, QIndex);
+        using Av1EncoderPictureBuffer pilotPicture = new(
+            Configuration.Default,
+            pilotTemplate.Sequence.SequenceHeader,
+            pilotTemplate.Parent.FrameHeader,
+            Width,
+            Height);
+
+        using Av1EncoderCoefficientBuffer pilotCoefficients = new(
+            Configuration.Default,
+            pilotTemplate.Sequence.SequenceHeader,
+            Width,
+            Height);
+
+        using Av1EncoderSuperblockWorkspace pilotSuperblockWorkspace = new(Configuration.Default);
+        using Av1EncoderBlockWorkspace pilotBlockWorkspace = new(Configuration.Default);
+        using Av1IntraTileWriter pilotWriter = createWriter(
+            pilotSource.Frame,
+            pilotReconstruction.Frame,
+            pilotPicture.Picture,
+            pilotCoefficients,
+            pilotSuperblockWorkspace,
+            pilotBlockWorkspace);
+
+        Buffer2DRegion<TSample> reconstructedLuma = pilotReconstruction.Frame.CodedView.GetPlane(Av1Plane.Y);
+        int chromaWidth = transformSize.GetWidth();
+        int chromaHeight = transformSize.GetHeight();
+        int sampleCount = transformSize.GetSize2d();
+        int lumaScaleShift = 3 - chromaSubsamplingX - chromaSubsamplingY;
+        Span<short> lumaQ3 = stackalloc short[64];
+        int sumQ3 = sampleCount >> 1;
+        for (int row = 0; row < chromaHeight; row++)
+        {
+            for (int column = 0; column < chromaWidth; column++)
+            {
+                int lumaSum = 0;
+                int lumaX = 8 + (column << chromaSubsamplingX);
+                int lumaY = 8 + (row << chromaSubsamplingY);
+                for (int offsetY = 0; offsetY <= chromaSubsamplingY; offsetY++)
+                {
+                    ReadOnlySpan<TSample> lumaRow = reconstructedLuma.DangerousGetRowSpan(lumaY + offsetY);
+                    for (int offsetX = 0; offsetX <= chromaSubsamplingX; offsetX++)
+                    {
+                        lumaSum += int.CreateChecked(lumaRow[lumaX + offsetX]);
+                    }
+                }
+
+                short sampleQ3 = (short)(lumaSum << lumaScaleShift);
+                lumaQ3[(row * chromaWidth) + column] = sampleQ3;
+                sumQ3 += sampleQ3;
+            }
+        }
+
+        int averageQ3 = sumQ3 >> (transformSize.GetBlockWidthLog2() + transformSize.GetBlockHeightLog2());
+        using Av1EncoderFrameBuffer<TSample> source = new(
+            Configuration.Default,
+            Width,
+            Height,
+            bitDepth,
+            colorFormat,
+            chromaSubsamplingX,
+            chromaSubsamplingY);
+
+        using Av1EncoderFrameBuffer<TSample> reconstruction = new(
+            Configuration.Default,
+            Width,
+            Height,
+            bitDepth,
+            colorFormat,
+            chromaSubsamplingX,
+            chromaSubsamplingY);
+
+        for (int y = 0; y < pilotLuma.Height; y++)
+        {
+            pilotLuma.DangerousGetRowSpan(y).CopyTo(source.Frame.CodedView.GetPlane(Av1Plane.Y).DangerousGetRowSpan(y));
+        }
+
+        Buffer2DRegion<TSample> blue = source.Frame.CodedView.GetPlane(Av1Plane.U);
+        Buffer2DRegion<TSample> red = source.Frame.CodedView.GetPlane(Av1Plane.V);
+        FillPlane(blue, TSample.CreateChecked(midpoint));
+        FillPlane(red, TSample.CreateChecked(midpoint));
+        for (int row = 0; row < chromaHeight; row++)
+        {
+            Span<TSample> blueRow = blue.DangerousGetRowSpan(chromaHeight + row);
+            Span<TSample> redRow = red.DangerousGetRowSpan(chromaHeight + row);
+            for (int column = 0; column < chromaWidth; column++)
+            {
+                int acQ3 = lumaQ3[(row * chromaWidth) + column] - averageQ3;
+                int blueProduct = AlphaU * acQ3;
+                int redProduct = AlphaV * acQ3;
+                int blueAdjustment = (blueProduct + 32 + (blueProduct >> 31)) >> 6;
+                int redAdjustment = (redProduct + 32 + (redProduct >> 31)) >> 6;
+                blueRow[chromaWidth + column] = TSample.CreateChecked(Math.Clamp(midpoint + blueAdjustment, 0, maxSample));
+                redRow[chromaWidth + column] = TSample.CreateChecked(Math.Clamp(midpoint + redAdjustment, 0, maxSample));
+            }
+        }
+
+        ClearPlane(reconstruction.Luma);
+        ClearPlane(Assert.IsType<Buffer2D<TSample>>(reconstruction.ChromaBlue));
+        ClearPlane(Assert.IsType<Buffer2D<TSample>>(reconstruction.ChromaRed));
+        using Av1EncoderModeInfoBuffer modeInfo = new(Configuration.Default, Width, Height, disallow4x4AllFrames: true);
+        Av1PictureControlSet pictureTemplate = CreatePicture(modeInfo, colorConfig, use128x128Superblock: false, QIndex);
+        using Av1EncoderPictureBuffer picture = new(
+            Configuration.Default,
+            pictureTemplate.Sequence.SequenceHeader,
+            pictureTemplate.Parent.FrameHeader,
+            Width,
+            Height);
+
+        using Av1EncoderCoefficientBuffer coefficients = new(
+            Configuration.Default,
+            pictureTemplate.Sequence.SequenceHeader,
+            Width,
+            Height);
+
+        using Av1EncoderSuperblockWorkspace superblockWorkspace = new(Configuration.Default);
+        using Av1EncoderBlockWorkspace blockWorkspace = new(Configuration.Default);
+        using Av1IntraTileWriter tileWriter = createWriter(
+            source.Frame,
+            reconstruction.Frame,
+            picture.Picture,
+            coefficients,
+            superblockWorkspace,
+            blockWorkspace);
+
+        Buffer2DRegion<TSample> actualLuma = reconstruction.Frame.CodedView.GetPlane(Av1Plane.Y);
+        for (int y = 0; y < reconstructedLuma.Height; y++)
+        {
+            Assert.Equal(reconstructedLuma.DangerousGetRowSpan(y), actualLuma.DangerousGetRowSpan(y));
+        }
+
+        ref Av1MacroBlockModeInfo targetBlock = ref picture.Picture.GetMacroBlockModeInfo(new Point(2, 2));
+        Assert.Equal(Av1ChromaPredictionMode.ChromaFromLuma, targetBlock.Block.UvMode);
+        Assert.Equal(
+            Av1ChromaFromLumaMath.JointSign(
+                Av1ChromaFromLumaMath.SignPositive,
+                Av1ChromaFromLumaMath.SignNegative),
+            superblockWorkspace.FinalBlocks[3].PredictionUnit.ChromaFromLumaSigns);
+
+        Assert.Equal(
+            Av1ChromaFromLumaMath.PackIndices(
+                Av1ChromaFromLumaMath.AlphaToMagnitudeIndex(AlphaU),
+                Av1ChromaFromLumaMath.AlphaToMagnitudeIndex(AlphaV)),
+            superblockWorkspace.FinalBlocks[3].PredictionUnit.ChromaFromLumaIndex);
+
+        int targetTransformIndex = (3 * sampleCount) /
+            Av1EncoderCoefficientBuffer.TransformBlockUnitCoefficientCount;
+
+        Av1EncoderTransformBlockState blueState =
+            coefficients.GetTransformBlockSpan(0, Av1Plane.U)[targetTransformIndex];
+
+        Av1EncoderTransformBlockState redState =
+            coefficients.GetTransformBlockSpan(0, Av1Plane.V)[targetTransformIndex];
+
+        Assert.Equal((ushort)0, blueState.EndOfBlock);
+        Assert.Equal((ushort)0, redState.EndOfBlock);
+        Assert.Equal(Av1TransformType.DctDct, blueState.TransformType);
+        Assert.Equal(Av1TransformType.DctDct, redState.TransformType);
+        Assert.NotEqual(0, pilotWriter.GetTileData(0).Length);
+        Assert.NotEqual(0, tileWriter.GetTileData(0).Length);
+    }
+
     [Fact]
     public void ProductionDirectionalModesConsumeAvailableExtendedEdges()
     {
@@ -1313,7 +1586,8 @@ public class Av1IntraSuperblockEncoderTests
         }
     }
 
-    private static void FillPlane(Buffer2DRegion<byte> plane, byte value)
+    private static void FillPlane<TSample>(Buffer2DRegion<TSample> plane, TSample value)
+        where TSample : unmanaged
     {
         for (int y = 0; y < plane.Height; y++)
         {
@@ -1321,6 +1595,15 @@ public class Av1IntraSuperblockEncoderTests
         }
     }
 
+    private delegate Av1IntraTileWriter TileWriterFactory<TSample>(
+        Av1EncoderFrame<TSample> source,
+        Av1EncoderFrame<TSample> reconstruction,
+        Av1PictureControlSet picture,
+        Av1EncoderCoefficientBuffer coefficients,
+        Av1EncoderSuperblockWorkspace superblockWorkspace,
+        Av1EncoderBlockWorkspace blockWorkspace)
+        where TSample : unmanaged;
+
     private static void ClearPlane<TSample>(Buffer2D<TSample> plane)
         where TSample : unmanaged
     {