Correct AV1 mode neighbor lookup

HEIF_IMPLEMENTATION_PLAN.md

@@ -58,7 +58,7 @@ This snapshot pins or classifies the available references and failures; it does
 | --- | --- | --- | --- |
 | `Av1YuvConverter.ConvertToRgb`, `ConvertFromRgb`, scalar row conversion, and chroma reconstruction | H.273 formulas 20-31 and the identity, YCgCo, and non-constant-luminance matrix formulas; AV1 section 6.4.2 chroma sample positions | libavif `src/reformat.c` and `src/colr.c` at `092276ce89098ead06db80975173191e5fee1826`; libaom `aom/aom_image.h` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Scalar behavioral oracle for 8-bit full/limited-range conversion. Decode covers monochrome, YUV 4:2:0, 4:2:2, and 4:4:4 with AV1 chroma sample positioning; encode remains YUV 4:4:4 at this snapshot. Later high-bit-depth and SIMD paths must match it. |
 | `Av1FrameBuffer` high-bit-depth sample layout and `Av1YuvConverter` 10/12-bit output conversion | AV1 section 6.4.1 bit depth and H.273 sample-range scaling | libaom `aom_scale/yv12config.h`, `av1/common/idct.c`, and `av1/common/reconintra.c` at `03087864cf4bea6abb0d28f95cf7843511413d8f`; libavif `src/avif.c` and `src/reformat.c` at `092276ce89098ead06db80975173191e5fee1826` | Establish two-byte native sample storage with sample-unit strides for 10/12-bit reconstruction and use the same scalar color model at every supported bit depth. |
-| `Av1PredictionDecoder`, `Av1HighBitDepthPredictor`, `Av1ChromaFromLumaContext`, and the scalar DC, directional, Paeth, smooth, filter-intra, and chroma-from-luma predictors | AV1 sections 7.11.2 and 7.11.2.3 intra prediction | libaom `aom_dsp/intrapred.c`, `av1/common/reconintra.c`, `av1/common/cfl.c`, and `av1/common/cfl.h` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Behavioral oracle for neighbor addressing, directional upsampling, Paeth selection, smooth normalization, filter-intra taps, high-bit-depth clipping, chroma-from-luma storage/subsampling, and chroma-from-luma row strides. Existing managed scalar tables and predictors remain the implementation base. The WIP rectangular byte-pipeline smooth digest expectations encode width/height-swapped weights and must be replaced only from an independently generated oracle, not regenerated from this implementation. |
+| `Av1PredictionDecoder`, `Av1HighBitDepthPredictor`, `Av1ChromaFromLumaContext`, `Av1PartitionInfo`, and the scalar DC, directional, Paeth, smooth, filter-intra, and chroma-from-luma predictors | AV1 sections 7.11.2 and 7.11.2.3 intra prediction | libaom `aom_dsp/intrapred.c`, `av1/common/reconintra.c`, `av1/common/av1_common_int.h`, `av1/common/cfl.c`, and `av1/common/cfl.h` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Behavioral oracle for luma/chroma mode-neighbor addressing, directional upsampling, Paeth selection, smooth normalization, filter-intra taps, high-bit-depth clipping, chroma-from-luma storage/subsampling, and chroma-from-luma row strides. Existing managed scalar tables and predictors remain the implementation base. The WIP rectangular byte-pipeline smooth digest expectations encode width/height-swapped weights and must be replaced only from an independently generated oracle, not regenerated from this implementation. |
 | `Av1Inverse2dTransformer` and `Av1InverseTransformerFactory` | AV1 section 7.11.2 inverse transform and reconstruction | libaom `av1/common/av1_inv_txfm1d.c`, `av1/common/av1_inv_txfm2d.c`, and `av1/common/idct.c` at `03087864cf4bea6abb0d28f95cf7843511413d8f` | Scalar transform oracle for coefficient-row traversal, intermediate layout, stage ranges, clipping, and high-bit-depth sample addition. The managed 16-bit overload is also used as a parity oracle for the byte overload. |
 
 This table is intentionally incomplete. Add a row before each additional AV1 or HEVC algorithm is ported or materially reshaped.

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1FrameInfo.cs

@@ -48,7 +48,7 @@ internal partial class Av1FrameInfo
         // Allocate the arrays.
         this.superblockInfos = new Av1SuperblockInfo[superblockCount];
         this.modeInfos = new Av1BlockModeInfo[superblockCount * this.modeInfoCountPerSuperblock];
-        this.modeInfoMap = new Av1FrameModeInfoMap(new Size(this.modeInfoSizePerSuperblock * this.superblockColumnCount, this.modeInfoSizePerSuperblock * this.superblockRowCount), superblockSizeLog2);
+        this.modeInfoMap = new Av1FrameModeInfoMap(new Size(this.modeInfoSizePerSuperblock * this.superblockColumnCount, this.modeInfoSizePerSuperblock * this.superblockRowCount));
         this.transformInfosY = new Av1TransformInfo[superblockCount * this.modeInfoCountPerSuperblock];
         this.transformInfosUv = new Av1TransformInfo[2 * superblockCount * this.modeInfoCountPerSuperblock];
 
@@ -105,12 +105,7 @@ internal partial class Av1FrameInfo
         return span[i];
     }
 
-    public Av1BlockModeInfo GetModeInfo(Point superblockIndex)
-    {
-        Span<Av1BlockModeInfo> span = this.modeInfos;
-        int superblock = (superblockIndex.Y * this.superblockColumnCount) + superblockIndex.X;
-        return span[superblock * this.modeInfoCountPerSuperblock];
-    }
+    public Av1BlockModeInfo GetModeInfo(Point superblockIndex) => this.GetModeInfo(superblockIndex, Point.Empty);
 
     public Av1BlockModeInfo GetModeInfo(Point superblockIndex, Point modeInfoIndex)
     {
@@ -119,6 +114,11 @@ internal partial class Av1FrameInfo
         return this.modeInfos[index];
     }
 
+    /// <summary>
+    /// Gets the mode information record covering the specified frame-relative mode information position.
+    /// </summary>
+    public Av1BlockModeInfo GetModeInfoAt(Point modeInfoPosition) => this.modeInfos[this.modeInfoMap[modeInfoPosition]];
+
     /// <summary>
     /// Gets the mode information records parsed for the specified superblock in bitstream order.
     /// </summary>
@@ -223,8 +223,8 @@ internal partial class Av1FrameInfo
 
     private Point GetModeInfoPosition(Point superblockPosition, Point positionInSuperblock)
     {
-        int x = (superblockPosition.X * this.modeInfoCountPerSuperblock) + positionInSuperblock.X;
-        int y = (superblockPosition.Y * this.modeInfoCountPerSuperblock) + positionInSuperblock.Y;
+        int x = (superblockPosition.X * this.modeInfoSizePerSuperblock) + positionInSuperblock.X;
+        int y = (superblockPosition.Y * this.modeInfoSizePerSuperblock) + positionInSuperblock.Y;
         return new Point(x, y);
     }
 }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1FrameModeInfoMap.cs

@@ -14,13 +14,11 @@ internal partial class Av1FrameInfo
     public class Av1FrameModeInfoMap
     {
         private readonly ushort[] offsets;
-        private Size alignedModeInfoCount;
+        private readonly Size alignedModeInfoCount;
 
-        public Av1FrameModeInfoMap(Size modeInfoCount, int superblockSizeLog2)
+        public Av1FrameModeInfoMap(Size modeInfoCount)
         {
-            this.alignedModeInfoCount = new Size(
-                modeInfoCount.Width * (1 << (superblockSizeLog2 - Av1Constants.ModeInfoSizeLog2)),
-                modeInfoCount.Height * (1 << (superblockSizeLog2 - Av1Constants.ModeInfoSizeLog2)));
+            this.alignedModeInfoCount = modeInfoCount;
             this.NextIndex = 0;
             this.offsets = new ushort[this.alignedModeInfoCount.Width * this.alignedModeInfoCount.Height];
         }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1PartitionInfo.cs

@@ -120,6 +120,40 @@ internal class Av1PartitionInfo
         this.HeightInPixels[2] = Math.Max(1, bh4 >> subY) * modeInfoSize;
     }
 
+    public void PopulateModeInfoNeighbors(Av1FrameInfo frameInfo, ObuColorConfig colorConfig)
+    {
+        if (this.AvailableAbove)
+        {
+            this.AboveModeInfo = frameInfo.GetModeInfoAt(new Point(this.ColumnIndex, this.RowIndex - 1));
+        }
+
+        if (this.AvailableLeft)
+        {
+            this.LeftModeInfo = frameInfo.GetModeInfoAt(new Point(this.ColumnIndex - 1, this.RowIndex));
+        }
+
+        if (!this.IsChroma)
+        {
+            return;
+        }
+
+        int subX = colorConfig.SubSamplingX ? 1 : 0;
+        int subY = colorConfig.SubSamplingY ? 1 : 0;
+        int chromaBaseColumn = this.ColumnIndex - (this.ColumnIndex & subX);
+        int chromaBaseRow = this.RowIndex - (this.RowIndex & subY);
+
+        // Chroma neighbors refer to the bottom-right luma mode covered by each adjacent chroma block.
+        if (this.AvailableAboveForChroma)
+        {
+            this.AboveModeInfoForChroma = frameInfo.GetModeInfoAt(new Point(chromaBaseColumn + subX, chromaBaseRow - 1));
+        }
+
+        if (this.AvailableLeftForChroma)
+        {
+            this.LeftModeInfoForChroma = frameInfo.GetModeInfoAt(new Point(chromaBaseColumn - 1, chromaBaseRow + subY));
+        }
+    }
+
     public int GetMaxBlockWide(Av1BlockSize blockSize, bool subX)
     {
         int maxBlockWide = blockSize.GetWidth();

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

@@ -290,8 +290,6 @@ internal class Av1TileReader : IAv1TileReader
         int block4x4Width = blockSize.Get4x4WideCount();
         int block4x4Height = blockSize.Get4x4HighCount();
         int planesCount = this.SequenceHeader.ColorConfig.PlaneCount;
-        int subX = this.SequenceHeader.ColorConfig.SubSamplingX ? 1 : 0;
-        int subY = this.SequenceHeader.ColorConfig.SubSamplingY ? 1 : 0;
         Point superblockLocation = superblockInfo.Position * this.SequenceHeader.SuperblockModeInfoSize;
         Point locationInSuperblock = new Point(modeInfoLocation.X - superblockLocation.X, modeInfoLocation.Y - superblockLocation.Y);
         Av1BlockModeInfo blockModeInfo = new(planesCount, blockSize, locationInSuperblock);
@@ -317,25 +315,7 @@ internal class Av1TileReader : IAv1TileReader
             }
         }
 
-        if (partitionInfo.AvailableAbove)
-        {
-            partitionInfo.AboveModeInfo = superblockInfo.GetModeInfo(new Point(rowIndex - 1, columnIndex));
-        }
-
-        if (partitionInfo.AvailableLeft)
-        {
-            partitionInfo.LeftModeInfo = superblockInfo.GetModeInfo(new Point(rowIndex, columnIndex - 1));
-        }
-
-        if (partitionInfo.AvailableAboveForChroma)
-        {
-            partitionInfo.AboveModeInfoForChroma = superblockInfo.GetModeInfo(new Point(rowIndex & ~subY, columnIndex | subX));
-        }
-
-        if (partitionInfo.AvailableLeftForChroma)
-        {
-            partitionInfo.LeftModeInfoForChroma = superblockInfo.GetModeInfo(new Point(rowIndex | subY, columnIndex & ~subX));
-        }
+        partitionInfo.PopulateModeInfoNeighbors(this.FrameInfo, this.SequenceHeader.ColorConfig);
 
         this.ReadModeInfo(ref reader, partitionInfo);
         ReadPaletteTokens(ref reader, partitionInfo);

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

@@ -83,6 +83,8 @@ internal class Av1BlockDecoder
             }
         }
 
+        partitionInfo.PopulateModeInfoNeighbors(this.frameInfo, colorConfig);
+
         int maxBlocksWide = partitionInfo.GetMaxBlockWide(blockSize, false);
         int maxBlocksHigh = partitionInfo.GetMaxBlockHigh(blockSize, false);
 

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1TilingTests.cs

@@ -142,6 +142,23 @@ public class Av1TilingTests
 
                 Assert.Equal(superblockInfo.BlockCount, modeInfos.Length);
                 Assert.DoesNotContain(modeInfos.ToArray(), modeInfo => modeInfo is null);
+                Assert.Same(modeInfos[0], tileReader.FrameInfo.GetModeInfo(superblockPosition));
+
+                foreach (Av1BlockModeInfo modeInfo in modeInfos)
+                {
+                    Point modeInfoPosition = new(
+                        superblockInfo.ModeInfoPosition.X + modeInfo.PositionInSuperblock.X,
+                        superblockInfo.ModeInfoPosition.Y + modeInfo.PositionInSuperblock.Y);
+
+                    for (int y = 0; y < modeInfo.BlockSize.Get4x4HighCount(); y++)
+                    {
+                        for (int x = 0; x < modeInfo.BlockSize.Get4x4WideCount(); x++)
+                        {
+                            Assert.Same(modeInfo, tileReader.FrameInfo.GetModeInfoAt(new Point(modeInfoPosition.X + x, modeInfoPosition.Y + y)));
+                        }
+                    }
+                }
+
                 parsedModeInfoCount += modeInfos.Length;
             }
         }