Complete AV1 difference-weighted compound decoding checkpoint

HEIF_IMPLEMENTATION_PLAN.md

@@ -30,7 +30,7 @@ Reference checkout evidence on 2026-08-31:
 Reconciled with the worktree on 2026-08-31.
 
 - [~] The bounded container reader, still-image path, sequence parser, AV1 decoder, color pipeline, presentation pipeline, and broad AV1 test suite exist locally.
-- [~] The inter-frame decoder has verified checkpoints through wedge compound prediction. Difference-weighted compound prediction, OBMC, scaled references, local warped motion, and global motion exist locally but remain open until their ordered checkpoints below are completed.
+- [~] The inter-frame decoder has verified checkpoints through difference-weighted compound prediction. OBMC, scaled references, local warped motion, and global motion exist locally but remain open until their ordered checkpoints below are completed.
 - [~] Loop filtering, CDEF, super-resolution, restoration, film grain, layered presentation, alpha composition, and color conversion exist locally. Shared-source cleanup changed the current tree, so final production-path verification is open.
 - [~] AV1 writer primitives, forward transforms, symbol encoding, and tile-writing source exist locally, but they are not connected to the public encoder.
 - [ ] The public AV1 encoder is not implemented. HeifEncoderCore.Encode throws NotSupportedException when AV1 is selected.
@@ -180,8 +180,8 @@ The single-reference syntax, buffer, reconstruction, and ownership foundation is
 - [x] Inter-intra prediction.
 - [x] Distance-weighted compound prediction.
 - [x] Wedge compound prediction.
-- [~] Difference-weighted compound prediction. Current item.
-- [~] OBMC.
+- [x] Difference-weighted compound prediction.
+- [~] OBMC. Current item.
 - [~] Scaled-reference prediction.
 - [~] Local warped prediction.
 - [~] Non-translational global prediction.
@@ -322,6 +322,42 @@ Verified wedge compound checkpoint evidence on 2026-08-31:
 - [x] The focused Release checkpoint set passes 35/35 on net10.0 and 35/35 on net11.0, with zero
   failures or skips. Scoped analyzer and whitespace verification pass for every changed C# file.
   Roslynk reports zero compiler errors, `git diff --check` passes, and `.gitattributes` is unchanged.
+- [x] The completed checkpoint was committed as `9883a24dc319e16b471f68be632d4f62f2c1cd5e`
+  with author and committer `James Jackson-South <james_south@hotmail.com>`.
+
+Verified difference-weighted compound checkpoint evidence on 2026-08-31:
+
+- [x] Audited syntax against current libaom `av1/decoder/decodemv.c`. ImageSharp applies the same
+  masked-compound enable and block-size gates, selects difference-weighted compound directly when wedge
+  is unavailable, and reads the same one-bit type-38 mask orientation.
+- [x] Audited mask generation and reconstruction against current libaom `av1/common/reconinter.c` and
+  `aom_dsp/blend_a64_mask.c`. The d16 path rounds the absolute intermediate difference by the
+  convolution and bit-depth shift, scales it by 1/16, adds the type-38 base, clamps or inverts the mask,
+  and then blends the original no-round intermediates before final rounding and clipping. Chroma reuses
+  the luma-derived mask through rounded subsampling.
+- [x] Corrected the production 10/12-bit subpixel eligibility gate, which previously rounded both
+  references before difference-mask construction and blending. Difference-weighted blocks now use the
+  existing semantic intermediate mask-builder and mask-blend predictor/operator families through the
+  sole final rounding step. No new operator family, per-block allocation, or copy was introduced.
+- [x] Renamed the stale “pinned formula” test and extended FeatureTestRunner's independent oracle across
+  current-libaom regular and d16 mask arithmetic, both mask orientations, 8/10/12-bit samples, widths
+  that cross every Vector512, Vector256, Vector128, and scalar boundary, subpixel phases, and row-padding
+  sentinels.
+- [x] Added a complete `Av1BlockDecoder.DecodeBlock` regression for 10/12-bit half-sample prediction
+  and both type-38 orientations. Its expected mask and reconstruction are calculated directly from the
+  current-libaom equations, independently of the production mask builder and finalizer.
+- [x] Extracted the fixture's 5,358-byte AV1 `mdat` payload and decoded it with refreshed current
+  libaom `aomdec`, using one thread with row threading disabled. All 19 frames decoded. The final
+  19,200 YUV444 samples have SHA-256
+  `E8CAA650F1571C5B9CACAF8C06E1DDF5F5D2ED35F65F1C34377076C573425899` and match the retained native
+  reference with zero differing samples.
+- [x] The real 19-frame production sequence requires both difference-mask orientations, compares final
+  native Y, Cb, and Cr planes exactly, compares final RGBA presentation through ImageSharp's established
+  reference-output API, and repeats the complete decode with a 1,024-byte constrained tracked allocator
+  and exactly-once return checks.
+- [x] The focused Release checkpoint set passes 37/37 on net10.0 and 37/37 on net11.0, with zero
+  failures or skips. Scoped analyzer and whitespace verification pass for every changed C# file.
+  Roslynk reports zero compiler errors, `git diff --check` passes, and `.gitattributes` is unchanged.
 
 For every item:
 

src/ImageSharp/Formats/Heif/Av1/Transform/Av1BlockDecoder.cs

@@ -375,7 +375,8 @@ internal sealed class Av1BlockDecoder : IDisposable
                     modeInfo.CompoundType is
                         Av1CompoundType.Average or
                         Av1CompoundType.DistanceWeighted or
-                        Av1CompoundType.Wedge &&
+                        Av1CompoundType.Wedge or
+                        Av1CompoundType.DifferenceWeighted &&
                     modeInfo.MotionMode != Av1MotionMode.Warped &&
                     modeInfo.YMode != Av1PredictionMode.GlobalGlobalMotionVector;
 
@@ -763,6 +764,43 @@ internal sealed class Av1BlockDecoder : IDisposable
                                     subY: 0,
                                     this.frameBuffer.BitDepth.GetBitCount());
                             }
+                            else if (modeInfo.CompoundType == Av1CompoundType.DifferenceWeighted)
+                            {
+                                int lumaWidth = blockSize.GetWidth();
+                                if (plane == 0)
+                                {
+                                    // Difference-weighted chroma reuses the luma-derived segment mask. Building it
+                                    // only for plane zero preserves that decoded contract before chroma subsampling.
+                                    Av1CompoundIntermediateDifferenceWeightedMaskBuilder.FillDifferenceWeightedIntermediateMask(
+                                        compoundMask,
+                                        lumaWidth,
+                                        first,
+                                        predictionWidth,
+                                        highBitDepthSecondPrediction,
+                                        predictionWidth,
+                                        predictionWidth,
+                                        predictionHeight,
+                                        this.frameBuffer.BitDepth.GetBitCount(),
+                                        modeInfo.DifferenceWeightedMaskType);
+                                }
+
+                                // The d16 mask and final blend consume the same no-round intermediates. Rounding
+                                // either reference first changes both the derived mask and the reconstructed sample.
+                                Av1CompoundIntermediateMaskBlendPredictor.BlendIntermediate(
+                                    highBitDepthDestination,
+                                    reconstructionStride,
+                                    first,
+                                    predictionWidth,
+                                    highBitDepthSecondPrediction,
+                                    predictionWidth,
+                                    compoundMask,
+                                    lumaWidth,
+                                    predictionWidth,
+                                    predictionHeight,
+                                    subX,
+                                    subY,
+                                    this.frameBuffer.BitDepth.GetBitCount());
+                            }
                             else
                             {
                                 Av1CompoundIntermediateAveragePredictor.AverageIntermediate(

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1CompoundBlockDecoderTests.cs

@@ -150,6 +150,15 @@ public class Av1CompoundBlockDecoderTests
             ValidateSubpixelHighBitDepthWedgeCompoundPrediction,
             CompoundPredictionConfigurations);
 
+    /// <summary>
+    /// Verifies that high-bit-depth subpixel predictors retain no-round precision through difference masking and blending.
+    /// </summary>
+    [Fact]
+    public void DecodeBlockReconstructsSubpixelHighBitDepthDifferenceWeightedCompoundPrediction()
+        => FeatureTestRunner.RunWithHwIntrinsicsFeature(
+            ValidateSubpixelHighBitDepthDifferenceWeightedCompoundPrediction,
+            CompoundPredictionConfigurations);
+
     /// <summary>
     /// Verifies that both references of a GLOBAL_GLOBALMV block use their complete matrix before compound averaging.
     /// </summary>
@@ -693,14 +702,33 @@ public class Av1CompoundBlockDecoderTests
         }
     }
 
+    /// <summary>
+    /// Reconstructs both high-bit-depth subpixel difference-mask orientations at every supported source precision.
+    /// </summary>
+    private static void ValidateSubpixelHighBitDepthDifferenceWeightedCompoundPrediction()
+    {
+        foreach (Av1BitDepth bitDepth in new[] { Av1BitDepth.TenBit, Av1BitDepth.TwelveBit })
+        {
+            foreach (Av1DifferenceWeightedMaskType maskType in Enum.GetValues<Av1DifferenceWeightedMaskType>())
+            {
+                ValidateSubpixelHighBitDepthCompoundPredictionAtBitDepth(
+                    bitDepth,
+                    Av1CompoundType.DifferenceWeighted,
+                    maskType);
+            }
+        }
+    }
+
     /// <summary>
     /// Reconstructs one high-bit-depth half-sample compound block and compares it with the scalar no-round pipeline.
     /// </summary>
     /// <param name="bitDepth">The native sample depth.</param>
     /// <param name="compoundType">The final compound operation.</param>
+    /// <param name="differenceWeightedMaskType">The difference-mask orientation when that compound operation is selected.</param>
     private static void ValidateSubpixelHighBitDepthCompoundPredictionAtBitDepth(
         Av1BitDepth bitDepth,
-        Av1CompoundType compoundType)
+        Av1CompoundType compoundType,
+        Av1DifferenceWeightedMaskType differenceWeightedMaskType = Av1DifferenceWeightedMaskType.Type38)
     {
         const int frameSize = 32;
         const int blockOrigin = 8;
@@ -734,6 +762,7 @@ public class Av1CompoundBlockDecoderTests
             CompoundType = compoundType,
             CompoundWedgeIndex = 0,
             CompoundWedgeSign = true,
+            DifferenceWeightedMaskType = differenceWeightedMaskType,
         };
 
         modeInfo.ReferenceFrames[0] = Av1ReferenceFrameType.Last;
@@ -832,6 +861,54 @@ public class Av1CompoundBlockDecoderTests
 
             Assert.NotEqual((ushort)60, expected[0]);
         }
+        else if (compoundType == Av1CompoundType.DifferenceWeighted)
+        {
+            int bitCount = bitDepth.GetBitCount();
+            int intermediateRange = bitCount + 7 - 3 + 2;
+            int round0 = 3 + Math.Max(intermediateRange - 16, 0);
+            int roundBits = 14 - round0 - 7;
+            int offsetBits = bitCount + 14 - round0;
+            int roundOffset = (1 << (offsetBits - 7)) + (1 << (offsetBits - 8));
+            int differenceRound = roundBits + bitCount - 8;
+            int maximum = (1 << bitCount) - 1;
+            byte[] expectedMask = new byte[blockSize * blockSize];
+            byte[] actualMask = new byte[blockSize * blockSize];
+
+            for (int index = 0; index < expected.Length; index++)
+            {
+                int difference = Math.Abs(expectedFirst[index] - expectedSecond[index]);
+                difference = (difference + (1 << (differenceRound - 1))) >> differenceRound;
+                int alpha = Math.Min(64, 38 + (difference >> 4));
+                if (differenceWeightedMaskType == Av1DifferenceWeightedMaskType.Type38Inverse)
+                {
+                    alpha = 64 - alpha;
+                }
+
+                expectedMask[index] = (byte)alpha;
+                int result = ((alpha * expectedFirst[index]) + ((64 - alpha) * expectedSecond[index])) >> 6;
+                result -= roundOffset;
+                if (roundBits != 0)
+                {
+                    result = (result + (1 << (roundBits - 1))) >> roundBits;
+                }
+
+                expected[index] = (ushort)Math.Clamp(result, 0, maximum);
+            }
+
+            Av1CompoundIntermediateDifferenceWeightedMaskBuilder.FillDifferenceWeightedIntermediateMask(
+                actualMask,
+                blockSize,
+                expectedFirst,
+                blockSize,
+                expectedSecond,
+                blockSize,
+                blockSize,
+                blockSize,
+                bitCount,
+                differenceWeightedMaskType);
+
+            Assert.Equal(expectedMask, actualMask);
+        }
         else if (compoundType == Av1CompoundType.Wedge)
         {
             ReadOnlySpan<byte> wedgeMask =

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1CompoundInterPredictorTests.cs

@@ -170,14 +170,14 @@ public class Av1CompoundInterPredictorTests
     }
 
     /// <summary>
-    /// Verifies both difference-mask orientations at each supported bit depth.
+    /// Verifies the current libaom difference-mask formula in both orientations at each supported bit depth.
     /// </summary>
     [Fact]
-    public void DifferenceWeightedMasksMatchPinnedFormula()
+    public void DifferenceWeightedMasksMatchCurrentLibaomFormula()
         => FeatureTestRunner.RunWithHwIntrinsicsFeature(ValidateDifferenceWeightedMasks, PredictorConfigurations);
 
     /// <summary>
-    /// Applies the pinned difference-mask formula at every bit depth and intrinsic width.
+    /// Applies the current libaom difference-mask formula at every bit depth and intrinsic width.
     /// </summary>
     private static void ValidateDifferenceWeightedMasks()
     {
@@ -634,6 +634,48 @@ public class Av1CompoundInterPredictorTests
                         bitDepth);
 
                     Assert.Equal(expectedMasked, actualMasked);
+
+                    int differenceRound = roundBits + bitDepth - 8;
+                    foreach (Av1DifferenceWeightedMaskType maskType in Enum.GetValues<Av1DifferenceWeightedMaskType>())
+                    {
+                        byte[] expectedDifferenceMask = new byte[maskStride * height];
+                        byte[] actualDifferenceMask = new byte[maskStride * height];
+                        expectedDifferenceMask.AsSpan().Fill(0xA5);
+                        actualDifferenceMask.AsSpan().Fill(0xA5);
+
+                        for (int row = 0; row < height; row++)
+                        {
+                            for (int column = 0; column < width; column++)
+                            {
+                                int intermediateIndex = (row * intermediateStride) + column;
+                                int difference = Math.Abs(
+                                    expectedFirst[intermediateIndex] - expectedSecond[intermediateIndex]);
+
+                                difference = (difference + (1 << (differenceRound - 1))) >> differenceRound;
+                                int alpha = Math.Min(64, 38 + (difference >> 4));
+                                if (maskType == Av1DifferenceWeightedMaskType.Type38Inverse)
+                                {
+                                    alpha = 64 - alpha;
+                                }
+
+                                expectedDifferenceMask[(row * maskStride) + column] = (byte)alpha;
+                            }
+                        }
+
+                        Av1CompoundIntermediateDifferenceWeightedMaskBuilder.FillDifferenceWeightedIntermediateMask(
+                            actualDifferenceMask,
+                            maskStride,
+                            actualFirst,
+                            intermediateStride,
+                            actualSecond,
+                            intermediateStride,
+                            width,
+                            height,
+                            bitDepth,
+                            maskType);
+
+                        Assert.Equal(expectedDifferenceMask, actualDifferenceMask);
+                    }
                 }
             }
         }

tests/Images/Input/Heif/Av1/Conformance/README.md

@@ -154,13 +154,18 @@ Current `aomdec` produced all 19 YUV444 frames. The final frame's 19,200 native
 `E8CAA650F1571C5B9CACAF8C06E1DDF5F5D2ED35F65F1C34377076C573425899` and match the retained Y4M
 with zero differing samples.
 
+The difference-weighted fixture's 5,358-byte AV1 `mdat` payload was decoded under the same current-libaom
+conditions. Current `aomdec` produced all 19 YUV444 frames. The final frame's 19,200 native samples have
+SHA-256 `E8CAA650F1571C5B9CACAF8C06E1DDF5F5D2ED35F65F1C34377076C573425899` and match the retained
+Y4M with zero differing samples.
+
 The production tests independently require their decoded mode states. The distance-weighted input must
-exercise distance-weighted compound prediction, the wedge input must exercise both mask orientations,
-and the inter-intra input must exercise both smooth and wedge inter-intra prediction. The tests decode all
-preceding samples, compare final native Y, Cb, and Cr planes exactly, compare final RGBA presentation
-through ImageSharp's established reference-output API, and repeat reconstruction with constrained tracked
-allocation. The retained PNG files are presentation evidence only and are not AV1 reconstruction
-references.
+exercise distance-weighted compound prediction, the wedge and difference-weighted inputs must each
+exercise both mask orientations, and the inter-intra input must exercise both smooth and wedge inter-intra
+prediction. The tests decode all preceding samples, compare final native Y, Cb, and Cr planes exactly,
+compare final RGBA presentation through ImageSharp's established reference-output API, and repeat
+reconstruction with constrained tracked allocation. The retained PNG files are presentation evidence only
+and are not AV1 reconstruction references.
 
 ## Overlapping motion-compensation fixture