Isolate AV1 tile entropy contexts

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1Distribution.cs

@@ -267,6 +267,22 @@ internal class Av1Distribution
         this.speed = speed;
     }
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1Distribution"/> class with the same probability and adaptation state as another distribution.
+    /// </summary>
+    /// <param name="source">The distribution state to copy.</param>
+    private Av1Distribution(Av1Distribution source)
+    {
+        this.probabilities = new uint[source.probabilities.Length];
+        source.probabilities.CopyTo(this.probabilities, 0);
+
+        // The adaptation rate depends on both the alphabet size and prior update count, so copying only the
+        // thresholds would make the cloned frame context diverge after its next symbol.
+        this.speed = source.speed;
+        this.updateCount = source.updateCount;
+        this.NumberOfSymbols = source.NumberOfSymbols;
+    }
+
     /// <summary>
     /// Gets the number of symbols represented by the distribution.
     /// </summary>
@@ -279,6 +295,60 @@ internal class Av1Distribution
     /// <returns>The Q15 inverse cumulative threshold.</returns>
     public uint this[int index] => this.probabilities[index];
 
+    /// <summary>
+    /// Creates an independently adaptable copy of a distribution.
+    /// </summary>
+    /// <returns>A distribution initialized with the same probabilities and update count.</returns>
+    public Av1Distribution CreateCopy() => new(this);
+
+    /// <summary>
+    /// Creates independently adaptable copies of a distribution array.
+    /// </summary>
+    /// <param name="source">The distributions to copy.</param>
+    /// <returns>An array with the same shape and distribution state.</returns>
+    public static Av1Distribution[] CreateCopy(Av1Distribution[] source)
+    {
+        Av1Distribution[] result = new Av1Distribution[source.Length];
+        for (int i = 0; i < source.Length; i++)
+        {
+            result[i] = source[i].CreateCopy();
+        }
+
+        return result;
+    }
+
+    /// <summary>
+    /// Creates independently adaptable copies of a two-dimensional jagged distribution array.
+    /// </summary>
+    /// <param name="source">The distributions to copy.</param>
+    /// <returns>An array with the same shape and distribution state.</returns>
+    public static Av1Distribution[][] CreateCopy(Av1Distribution[][] source)
+    {
+        Av1Distribution[][] result = new Av1Distribution[source.Length][];
+        for (int i = 0; i < source.Length; i++)
+        {
+            result[i] = CreateCopy(source[i]);
+        }
+
+        return result;
+    }
+
+    /// <summary>
+    /// Creates independently adaptable copies of a three-dimensional jagged distribution array.
+    /// </summary>
+    /// <param name="source">The distributions to copy.</param>
+    /// <returns>An array with the same shape and distribution state.</returns>
+    public static Av1Distribution[][][] CreateCopy(Av1Distribution[][][] source)
+    {
+        Av1Distribution[][][] result = new Av1Distribution[source.Length][][];
+        for (int i = 0; i < source.Length; i++)
+        {
+            result[i] = CreateCopy(source[i]);
+        }
+
+        return result;
+    }
+
     /// <summary>
     /// Adapts the cumulative thresholds after coding one symbol.
     /// </summary>

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1SymbolDecoder.cs

@@ -21,67 +21,67 @@ internal ref struct Av1SymbolDecoder
     /// <summary>
     /// The tile-adaptive intra-block-copy distribution.
     /// </summary>
-    private readonly Av1Distribution tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy;
+    private readonly Av1Distribution tileIntraBlockCopy;
 
     /// <summary>
     /// The tile-adaptive partition-type distributions.
     /// </summary>
-    private readonly Av1Distribution[] tilePartitionTypes = Av1DefaultDistributions.PartitionTypes;
+    private readonly Av1Distribution[] tilePartitionTypes;
 
     /// <summary>
     /// The tile-adaptive key-frame luma-mode distributions.
     /// </summary>
-    private readonly Av1Distribution[][] keyFrameYMode = Av1DefaultDistributions.KeyFrameYMode;
+    private readonly Av1Distribution[][] keyFrameYMode;
 
     /// <summary>
     /// The tile-adaptive chroma intra-mode distributions.
     /// </summary>
-    private readonly Av1Distribution[][] uvMode = Av1DefaultDistributions.UvMode;
+    private readonly Av1Distribution[][] uvMode;
 
     /// <summary>
     /// The tile-adaptive transform-skip distributions.
     /// </summary>
-    private readonly Av1Distribution[] skip = Av1DefaultDistributions.Skip;
+    private readonly Av1Distribution[] skip;
 
     /// <summary>
     /// The tile-adaptive skip-mode distributions.
     /// </summary>
-    private readonly Av1Distribution[] skipMode = Av1DefaultDistributions.SkipMode;
+    private readonly Av1Distribution[] skipMode;
 
     /// <summary>
     /// The tile-adaptive absolute loop-filter delta distribution.
     /// </summary>
-    private readonly Av1Distribution deltaLoopFilterAbsolute = Av1DefaultDistributions.DeltaLoopFilterAbsolute;
+    private readonly Av1Distribution deltaLoopFilterAbsolute;
 
     /// <summary>
     /// The tile-adaptive absolute quantizer delta distribution.
     /// </summary>
-    private readonly Av1Distribution deltaQuantizerAbsolute = Av1DefaultDistributions.DeltaQuantizerAbsolute;
+    private readonly Av1Distribution deltaQuantizerAbsolute;
 
     /// <summary>
     /// The tile-adaptive spatial segment-identifier distributions.
     /// </summary>
-    private readonly Av1Distribution[] segmentId = Av1DefaultDistributions.SegmentId;
+    private readonly Av1Distribution[] segmentId;
 
     /// <summary>
     /// The tile-adaptive directional angle-delta distributions.
     /// </summary>
-    private readonly Av1Distribution[] angleDelta = Av1DefaultDistributions.AngleDelta;
+    private readonly Av1Distribution[] angleDelta;
 
     /// <summary>
     /// The tile-adaptive filter-intra mode distribution.
     /// </summary>
-    private readonly Av1Distribution filterIntraMode = Av1DefaultDistributions.FilterIntraMode;
+    private readonly Av1Distribution filterIntraMode;
 
     /// <summary>
     /// The tile-adaptive filter-intra enable distributions.
     /// </summary>
-    private readonly Av1Distribution[] filterIntra = Av1DefaultDistributions.FilterIntra;
+    private readonly Av1Distribution[] filterIntra;
 
     /// <summary>
     /// The tile-adaptive transform-size distributions.
     /// </summary>
-    private readonly Av1Distribution[][] transformSize = Av1DefaultDistributions.TransformSize;
+    private readonly Av1Distribution[][] transformSize;
 
     /// <summary>
     /// The tile-adaptive end-of-block token distributions selected for the frame base quantizer.
@@ -121,17 +121,17 @@ internal ref struct Av1SymbolDecoder
     /// <summary>
     /// The tile-adaptive joint chroma-from-luma sign distribution.
     /// </summary>
-    private readonly Av1Distribution chromaFromLumaSign = Av1DefaultDistributions.ChromaFromLumaSign;
+    private readonly Av1Distribution chromaFromLumaSign;
 
     /// <summary>
     /// The tile-adaptive chroma-from-luma alpha-magnitude distributions.
     /// </summary>
-    private readonly Av1Distribution[] chromaFromLumaAlpha = Av1DefaultDistributions.ChromaFromLumaAlpha;
+    private readonly Av1Distribution[] chromaFromLumaAlpha;
 
     /// <summary>
     /// The tile-adaptive intra transform-type distributions.
     /// </summary>
-    private readonly Av1Distribution[][][] intraExtendedTransform = Av1DefaultDistributions.IntraExtendedTransform;
+    private readonly Av1Distribution[][][] intraExtendedTransform;
 
     /// <summary>
     /// The configuration providing temporary coefficient-context memory.
@@ -154,18 +154,37 @@ internal ref struct Av1SymbolDecoder
     /// <param name="configuration">The configuration providing temporary memory.</param>
     /// <param name="tileData">The entropy-coded tile payload.</param>
     /// <param name="qIndex">The frame base quantizer index.</param>
-    public Av1SymbolDecoder(Configuration configuration, Span<byte> tileData, int qIndex)
+    /// <param name="updateCdf">A value indicating whether decoded symbols adapt their tile distributions.</param>
+    public Av1SymbolDecoder(Configuration configuration, Span<byte> tileData, int qIndex, bool updateCdf = true)
     {
+        // Every tile starts from its own frame-context copy. Sharing these objects would let one image's adaptive
+        // updates change the initial probabilities used to decode the next tile or image.
+        this.tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy.CreateCopy();
+        this.tilePartitionTypes = Av1Distribution.CreateCopy(Av1DefaultDistributions.PartitionTypes);
+        this.keyFrameYMode = Av1Distribution.CreateCopy(Av1DefaultDistributions.KeyFrameYMode);
+        this.uvMode = Av1Distribution.CreateCopy(Av1DefaultDistributions.UvMode);
+        this.skip = Av1Distribution.CreateCopy(Av1DefaultDistributions.Skip);
+        this.skipMode = Av1Distribution.CreateCopy(Av1DefaultDistributions.SkipMode);
+        this.deltaLoopFilterAbsolute = Av1DefaultDistributions.DeltaLoopFilterAbsolute.CreateCopy();
+        this.deltaQuantizerAbsolute = Av1DefaultDistributions.DeltaQuantizerAbsolute.CreateCopy();
+        this.segmentId = Av1Distribution.CreateCopy(Av1DefaultDistributions.SegmentId);
+        this.angleDelta = Av1Distribution.CreateCopy(Av1DefaultDistributions.AngleDelta);
+        this.filterIntraMode = Av1DefaultDistributions.FilterIntraMode.CreateCopy();
+        this.filterIntra = Av1Distribution.CreateCopy(Av1DefaultDistributions.FilterIntra);
+        this.transformSize = Av1Distribution.CreateCopy(Av1DefaultDistributions.TransformSize);
+        this.chromaFromLumaSign = Av1DefaultDistributions.ChromaFromLumaSign.CreateCopy();
+        this.chromaFromLumaAlpha = Av1Distribution.CreateCopy(Av1DefaultDistributions.ChromaFromLumaAlpha);
+        this.intraExtendedTransform = Av1Distribution.CreateCopy(Av1DefaultDistributions.IntraExtendedTransform);
         this.configuration = configuration;
-        this.reader = new Av1SymbolReader(tileData);
+        this.reader = new Av1SymbolReader(tileData, updateCdf);
         this.baseQIndex = qIndex;
-        this.endOfBlockFlag = Av1DefaultDistributions.GetEndOfBlockFlag(qIndex);
-        this.coefficientsBase = Av1DefaultDistributions.GetCoefficientsBase(qIndex);
-        this.baseEndOfBlock = Av1DefaultDistributions.GetBaseEndOfBlock(qIndex);
-        this.dcSign = Av1DefaultDistributions.GetDcSign(qIndex);
-        this.coefficientsBaseRange = Av1DefaultDistributions.GetCoefficientsBaseRange(qIndex);
-        this.transformBlockSkip = Av1DefaultDistributions.GetTransformBlockSkip(qIndex);
-        this.endOfBlockExtra = Av1DefaultDistributions.GetEndOfBlockExtra(qIndex);
+        this.endOfBlockFlag = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetEndOfBlockFlag(qIndex));
+        this.coefficientsBase = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetCoefficientsBase(qIndex));
+        this.baseEndOfBlock = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetBaseEndOfBlock(qIndex));
+        this.dcSign = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetDcSign(qIndex));
+        this.coefficientsBaseRange = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetCoefficientsBaseRange(qIndex));
+        this.transformBlockSkip = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetTransformBlockSkip(qIndex));
+        this.endOfBlockExtra = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetEndOfBlockExtra(qIndex));
     }
 
     /// <summary>

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

@@ -18,22 +18,22 @@ internal class Av1SymbolEncoder : IDisposable
     /// <summary>
     /// The tile-adaptive intra-block-copy distribution.
     /// </summary>
-    private readonly Av1Distribution tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy;
+    private readonly Av1Distribution tileIntraBlockCopy;
 
     /// <summary>
     /// The tile-adaptive partition-type distributions.
     /// </summary>
-    private readonly Av1Distribution[] tilePartitionTypes = Av1DefaultDistributions.PartitionTypes;
+    private readonly Av1Distribution[] tilePartitionTypes;
 
     /// <summary>
     /// The tile-adaptive key-frame luma-mode distributions.
     /// </summary>
-    private readonly Av1Distribution[][] keyFrameYMode = Av1DefaultDistributions.KeyFrameYMode;
+    private readonly Av1Distribution[][] keyFrameYMode;
 
     /// <summary>
     /// The tile-adaptive chroma intra-mode distributions.
     /// </summary>
-    private readonly Av1Distribution[][] uvMode = Av1DefaultDistributions.UvMode;
+    private readonly Av1Distribution[][] uvMode;
 
     /// <summary>
     /// The tile-adaptive transform-block skip distributions selected for the frame base quantizer.
@@ -63,17 +63,17 @@ internal class Av1SymbolEncoder : IDisposable
     /// <summary>
     /// The tile-adaptive filter-intra enable distributions.
     /// </summary>
-    private readonly Av1Distribution[] filterIntra = Av1DefaultDistributions.FilterIntra;
+    private readonly Av1Distribution[] filterIntra;
 
     /// <summary>
     /// The tile-adaptive filter-intra mode distribution.
     /// </summary>
-    private readonly Av1Distribution filterIntraMode = Av1DefaultDistributions.FilterIntraMode;
+    private readonly Av1Distribution filterIntraMode;
 
     /// <summary>
     /// The tile-adaptive absolute quantizer delta distribution.
     /// </summary>
-    private readonly Av1Distribution deltaQuantizerAbsolute = Av1DefaultDistributions.DeltaQuantizerAbsolute;
+    private readonly Av1Distribution deltaQuantizerAbsolute;
 
     /// <summary>
     /// The tile-adaptive DC sign distributions selected for the frame base quantizer.
@@ -88,37 +88,37 @@ internal class Av1SymbolEncoder : IDisposable
     /// <summary>
     /// The tile-adaptive intra transform-type distributions.
     /// </summary>
-    private readonly Av1Distribution[][][] intraExtendedTransform = Av1DefaultDistributions.IntraExtendedTransform;
+    private readonly Av1Distribution[][][] intraExtendedTransform;
 
     /// <summary>
     /// The tile-adaptive spatial segment-identifier distributions.
     /// </summary>
-    private readonly Av1Distribution[] segmentId = Av1DefaultDistributions.SegmentId;
+    private readonly Av1Distribution[] segmentId;
 
     /// <summary>
     /// The tile-adaptive directional angle-delta distributions.
     /// </summary>
-    private readonly Av1Distribution[] angleDelta = Av1DefaultDistributions.AngleDelta;
+    private readonly Av1Distribution[] angleDelta;
 
     /// <summary>
     /// The tile-adaptive transform-skip distributions.
     /// </summary>
-    private readonly Av1Distribution[] skip = Av1DefaultDistributions.Skip;
+    private readonly Av1Distribution[] skip;
 
     /// <summary>
     /// The tile-adaptive skip-mode distributions.
     /// </summary>
-    private readonly Av1Distribution[] skipMode = Av1DefaultDistributions.SkipMode;
+    private readonly Av1Distribution[] skipMode;
 
     /// <summary>
     /// The tile-adaptive joint chroma-from-luma sign distribution.
     /// </summary>
-    private readonly Av1Distribution chromaFromLumaSign = Av1DefaultDistributions.ChromaFromLumaSign;
+    private readonly Av1Distribution chromaFromLumaSign;
 
     /// <summary>
     /// The tile-adaptive chroma-from-luma alpha-magnitude distributions.
     /// </summary>
-    private readonly Av1Distribution[] chromaFromLumaAlpha = Av1DefaultDistributions.ChromaFromLumaAlpha;
+    private readonly Av1Distribution[] chromaFromLumaAlpha;
 
     /// <summary>
     /// Indicates whether the range writer has been disposed.
@@ -146,17 +146,34 @@ internal class Av1SymbolEncoder : IDisposable
     /// <param name="configuration">The configuration providing output and temporary memory.</param>
     /// <param name="initialSize">The initial output buffer size in bytes.</param>
     /// <param name="qIndex">The frame base quantizer index.</param>
-    public Av1SymbolEncoder(Configuration configuration, int initialSize, int qIndex)
+    /// <param name="updateCdf">A value indicating whether encoded symbols adapt their tile distributions.</param>
+    public Av1SymbolEncoder(Configuration configuration, int initialSize, int qIndex, bool updateCdf = true)
     {
-        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);
+        // Encoding and decoding must begin from equivalent tile-local models. Copying the defaults also prevents
+        // one encoded image from changing the probabilities used by later encoder or decoder instances.
+        this.tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy.CreateCopy();
+        this.tilePartitionTypes = Av1Distribution.CreateCopy(Av1DefaultDistributions.PartitionTypes);
+        this.keyFrameYMode = Av1Distribution.CreateCopy(Av1DefaultDistributions.KeyFrameYMode);
+        this.uvMode = Av1Distribution.CreateCopy(Av1DefaultDistributions.UvMode);
+        this.filterIntra = Av1Distribution.CreateCopy(Av1DefaultDistributions.FilterIntra);
+        this.filterIntraMode = Av1DefaultDistributions.FilterIntraMode.CreateCopy();
+        this.deltaQuantizerAbsolute = Av1DefaultDistributions.DeltaQuantizerAbsolute.CreateCopy();
+        this.intraExtendedTransform = Av1Distribution.CreateCopy(Av1DefaultDistributions.IntraExtendedTransform);
+        this.segmentId = Av1Distribution.CreateCopy(Av1DefaultDistributions.SegmentId);
+        this.angleDelta = Av1Distribution.CreateCopy(Av1DefaultDistributions.AngleDelta);
+        this.skip = Av1Distribution.CreateCopy(Av1DefaultDistributions.Skip);
+        this.skipMode = Av1Distribution.CreateCopy(Av1DefaultDistributions.SkipMode);
+        this.chromaFromLumaSign = Av1DefaultDistributions.ChromaFromLumaSign.CreateCopy();
+        this.chromaFromLumaAlpha = Av1Distribution.CreateCopy(Av1DefaultDistributions.ChromaFromLumaAlpha);
+        this.transformBlockSkip = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetTransformBlockSkip(qIndex));
+        this.endOfBlockFlag = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetEndOfBlockFlag(qIndex));
+        this.coefficientsBaseRange = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetCoefficientsBaseRange(qIndex));
+        this.coefficientsBase = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetCoefficientsBase(qIndex));
+        this.coefficientsBaseEndOfBlock = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetBaseEndOfBlock(qIndex));
+        this.dcSign = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetDcSign(qIndex));
+        this.endOfBlockExtra = Av1Distribution.CreateCopy(Av1DefaultDistributions.GetEndOfBlockExtra(qIndex));
         this.configuration = configuration;
-        this.writer = new(configuration, initialSize);
+        this.writer = new(configuration, initialSize, updateCdf);
         this.baseQIndex = qIndex;
     }
 

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1SymbolReader.cs

@@ -23,6 +23,11 @@ internal ref struct Av1SymbolReader
     /// </summary>
     private readonly Span<byte> buffer;
 
+    /// <summary>
+    /// Indicates whether decoded symbols adapt their distributions.
+    /// </summary>
+    private readonly bool updateCdf;
+
     /// <summary>
     /// The next byte position to load into the code-value window.
     /// </summary>
@@ -51,9 +56,11 @@ internal ref struct Av1SymbolReader
     /// Initializes a new instance of the <see cref="Av1SymbolReader"/> struct over one entropy-coded span.
     /// </summary>
     /// <param name="span">The bounded entropy-coded bytes.</param>
-    public Av1SymbolReader(Span<byte> span)
+    /// <param name="updateCdf">A value indicating whether decoded symbols adapt their distributions.</param>
+    public Av1SymbolReader(Span<byte> span, bool updateCdf = true)
     {
         this.buffer = span;
+        this.updateCdf = updateCdf;
         this.position = 0;
         this.difference = (1U << (DecoderWindowsSize - 1)) - 1;
         this.range = 0x8000;
@@ -62,7 +69,7 @@ internal ref struct Av1SymbolReader
     }
 
     /// <summary>
-    /// Reads one symbol and adapts its distribution.
+    /// Reads one symbol and adapts its distribution when CDF updates are enabled.
     /// </summary>
     /// <param name="distribution">The inverse cumulative distribution for the symbol alphabet.</param>
     /// <returns>The decoded zero-based symbol.</returns>
@@ -70,8 +77,12 @@ internal ref struct Av1SymbolReader
     {
         int value = this.DecodeIntegerQ15(distribution);
 
-        // Decoder and encoder must adapt after the same symbol so their subsequent intervals remain identical.
-        distribution.Update(value);
+        // disable_cdf_update freezes every tile distribution while leaving range decoding unchanged.
+        if (this.updateCdf)
+        {
+            distribution.Update(value);
+        }
+
         return value;
     }
 

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

@@ -39,6 +39,11 @@ internal class Av1SymbolWriter : IDisposable
     /// </summary>
     private readonly AutoExpandingMemory<ushort> memory;
 
+    /// <summary>
+    /// Indicates whether encoded symbols adapt their distributions.
+    /// </summary>
+    private readonly bool updateCdf;
+
     /// <summary>
     /// The next pre-carry output position.
     /// </summary>
@@ -49,10 +54,12 @@ internal class Av1SymbolWriter : IDisposable
     /// </summary>
     /// <param name="configuration">The configuration that supplies output allocation.</param>
     /// <param name="initialSize">The estimated encoded size in bytes.</param>
-    public Av1SymbolWriter(Configuration configuration, int initialSize)
+    /// <param name="updateCdf">A value indicating whether encoded symbols adapt their distributions.</param>
+    public Av1SymbolWriter(Configuration configuration, int initialSize, bool updateCdf = true)
     {
         this.configuration = configuration;
         this.memory = new AutoExpandingMemory<ushort>(configuration, (initialSize + 1) >> 1);
+        this.updateCdf = updateCdf;
     }
 
     /// <summary>
@@ -61,7 +68,7 @@ internal class Av1SymbolWriter : IDisposable
     public void Dispose() => this.memory.Dispose();
 
     /// <summary>
-    /// Writes one binary symbol and adapts its distribution.
+    /// Writes one binary symbol and adapts its distribution when CDF updates are enabled.
     /// </summary>
     /// <param name="symbol">The binary symbol.</param>
     /// <param name="distribution">The inverse cumulative distribution for the binary alphabet.</param>
@@ -69,7 +76,7 @@ internal class Av1SymbolWriter : IDisposable
         => this.WriteSymbol(symbol ? 1 : 0, distribution);
 
     /// <summary>
-    /// Writes one symbol and adapts its distribution.
+    /// Writes one symbol and adapts its distribution when CDF updates are enabled.
     /// </summary>
     /// <param name="symbol">The zero-based symbol.</param>
     /// <param name="distribution">The inverse cumulative distribution for the symbol alphabet.</param>
@@ -80,7 +87,12 @@ internal class Av1SymbolWriter : IDisposable
         DebugGuard.IsTrue(distribution[distribution.NumberOfSymbols - 1] == 0, "Last entry in Probabilities table needs to be zero.");
 
         this.EncodeIntegerQ15(symbol, distribution);
-        distribution.Update(symbol);
+
+        // disable_cdf_update freezes every tile distribution while leaving range encoding unchanged.
+        if (this.updateCdf)
+        {
+            distribution.Update(symbol);
+        }
     }
 
     /// <summary>

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

@@ -168,7 +168,14 @@ internal class Av1TileReader : IAv1TileReader
     /// <remarks>Corresponds to <c>parse_tile</c> in SVT-AV1.</remarks>
     public void ReadTile(Span<byte> tileData, int tileNum)
     {
-        Av1SymbolDecoder reader = new(this.configuration, tileData, this.FrameHeader.QuantizationParameters.BaseQIndex);
+        // The frame syntax exposes a disable flag, while the range reader follows libaom's positive
+        // allow_update_cdf convention.
+        Av1SymbolDecoder reader = new(
+            this.configuration,
+            tileData,
+            this.FrameHeader.QuantizationParameters.BaseQIndex,
+            !this.FrameHeader.DisableCdfUpdate);
+
         int tileColumnIndex = tileNum % this.FrameHeader.TilesInfo.TileColumnCount;
         int tileRowIndex = tileNum / this.FrameHeader.TilesInfo.TileColumnCount;