Superblock info exposes its mode info position

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

@@ -82,8 +82,16 @@ internal partial class Av1FrameInfo
         this.deltaLoopFilter = new int[superblockCount << this.deltaLoopFactorLog2];
     }
 
+    /// <summary>
+    /// Gets the number of mode info blocks in a single superblock.
+    /// </summary>
     public int ModeInfoCount => this.modeInfos.Length;
 
+    /// <summary>
+    /// Gets the Width or height of a single superblock, counted in mode info blocks.
+    /// </summary>
+    public int SuperblockModeInfoSize => this.modeInfoSizePerSuperblock;
+
     public Av1SuperblockInfo GetSuperblock(Point index)
     {
         Span<Av1SuperblockInfo> span = this.superblockInfos;

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1ParseLeftNeighbor4x4Context.cs

@@ -66,7 +66,7 @@ internal class Av1ParseLeftNeighbor4x4Context
 
     public void UpdatePartition(Point modeInfoLocation, Av1SuperblockInfo superblockInfo, Av1BlockSize subSize, Av1BlockSize blockSize)
     {
-        int startIndex = (modeInfoLocation.Y - superblockInfo.Position.Y) & Av1PartitionContext.Mask;
+        int startIndex = (modeInfoLocation.Y - superblockInfo.ModeInfoPosition.Y) & Av1PartitionContext.Mask;
         int bh = blockSize.Get4x4HighCount();
         int value = Av1PartitionContext.GetLeftContext(subSize);
         DebugGuard.MustBeLessThanOrEqualTo(startIndex, this.LeftTransformHeight.Length - bh, nameof(startIndex));
@@ -75,7 +75,7 @@ internal class Av1ParseLeftNeighbor4x4Context
 
     public void UpdateTransformation(Point modeInfoLocation, Av1SuperblockInfo superblockInfo, Av1TransformSize transformSize, Av1BlockSize blockSize, bool skip)
     {
-        int startIndex = modeInfoLocation.Y - superblockInfo.Position.Y;
+        int startIndex = modeInfoLocation.Y - superblockInfo.ModeInfoPosition.Y;
         int transformHeight = transformSize.GetHeight();
         int n4h = blockSize.Get4x4HighCount();
         if (skip)

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1SuperblockInfo.cs

@@ -18,6 +18,11 @@ internal class Av1SuperblockInfo
     /// </summary>
     public Point Position { get; }
 
+    /// <summary>
+    /// Gets the position of this superblock inside the tile, counted in mode info blocks (of 4x4 pixels).
+    /// </summary>
+    public Point ModeInfoPosition => this.Position * this.frameInfo.SuperblockModeInfoSize;
+
     public ref int SuperblockDeltaQ => ref this.frameInfo.GetDeltaQuantizationIndex(this.Position);
 
     public Av1BlockModeInfo SuperblockModeInfo => this.GetModeInfo(new Point(0, 0));

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

@@ -372,7 +372,7 @@ internal class Av1TileReader : IAv1TileReader
             int txsWide = planeBlockSize.GetWidth() >> 2;
             int txsHigh = planeBlockSize.GetHeight() >> 2;
             int aboveOffset = (partitionInfo.ColumnIndex - tileInfo.ModeInfoColumnStart) >> subX;
-            int leftOffset = (partitionInfo.RowIndex - partitionInfo.SuperblockInfo.Position.Y) >> subY;
+            int leftOffset = (partitionInfo.RowIndex - partitionInfo.SuperblockInfo.ModeInfoPosition.Y) >> subY;
             this.aboveNeighborContext.ClearContext(i, aboveOffset, txsWide);
             this.leftNeighborContext.ClearContext(i, leftOffset, txsHigh);
         }
@@ -1267,7 +1267,7 @@ internal class Av1TileReader : IAv1TileReader
         Av1TransformSize maxTransformSize = partitionInfo.ModeInfo.BlockSize.GetMaximumTransformSize();
         int aboveWidth = this.aboveNeighborContext.AboveTransformWidth[partitionInfo.ColumnIndex - tileInfo.ModeInfoColumnStart];
         int above = (aboveWidth >= maxTransformSize.GetWidth()) ? 1 : 0;
-        int leftHeight = this.leftNeighborContext.LeftTransformHeight[partitionInfo.RowIndex - superblockInfo.Position.Y];
+        int leftHeight = this.leftNeighborContext.LeftTransformHeight[partitionInfo.RowIndex - superblockInfo.ModeInfoPosition.Y];
         int left = (leftHeight >= maxTransformSize.GetHeight()) ? 1 : 0;
         bool hasAbove = partitionInfo.AvailableAbove;
         bool hasLeft = partitionInfo.AvailableLeft;
@@ -1880,7 +1880,7 @@ internal class Av1TileReader : IAv1TileReader
     {
         // Maximum partition point is 8x8. Offset the log value occordingly.
         int aboveCtx = this.aboveNeighborContext.AbovePartitionWidth[location.X - tileInfo.ModeInfoColumnStart];
-        int leftCtx = this.leftNeighborContext.LeftPartitionHeight[(location.Y - superblockInfo.Position.Y) & Av1PartitionContext.Mask];
+        int leftCtx = this.leftNeighborContext.LeftPartitionHeight[(location.Y - superblockInfo.ModeInfoPosition.Y) & Av1PartitionContext.Mask];
         int blockSizeLog = blockSize.Get4x4WidthLog2() - Av1BlockSize.Block8x8.Get4x4WidthLog2();
         int above = (aboveCtx >> blockSizeLog) & 0x1;
         int left = (leftCtx >> blockSizeLog) & 0x1;

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1FrameDecoderStub.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
+using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
+
+internal class Av1FrameDecoderStub : IAv1FrameDecoder
+{
+    private readonly List<Av1SuperblockInfo> superblocks = [];
+
+    public void DecodeSuperblock(Point modeInfoPosition, Av1SuperblockInfo superblockInfo, Av1TileInfo tileInfo)
+        => this.superblocks.Add(superblockInfo);
+
+    public int SuperblockCount => this.superblocks.Count;
+}

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

@@ -3,7 +3,6 @@
 
 using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
-using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 
 namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
@@ -12,9 +11,9 @@ namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
 public class Av1TilingTests
 {
     [Theory]
-    [InlineData(TestImages.Heif.XnConvert, 0x010E, 0x03CC, 18)]
-    // [InlineData(TestImages.Heif.Orange4x4, 0x010E, 0x001d, 21)]
-    public void ReadFirstTile(string filename, int dataOffset, int dataSize, int tileOffset)
+    [InlineData(TestImages.Heif.XnConvert, 0x010E, 0x03CC, 18, 16)]
+    [InlineData(TestImages.Heif.Orange4x4, 0x010E, 0x001d, 21, 1)]
+    public void ReadFirstTile(string filename, int dataOffset, int dataSize, int tileOffset, int superblockCount)
     {
         // Assign
         string filePath = Path.Combine(TestEnvironment.InputImagesDirectoryFullPath, filename);
@@ -27,7 +26,7 @@ public class Av1TilingTests
         obuReader.ReadAll(ref bitStreamReader, dataSize, stub);
         Av1FrameBuffer frameBuffer = new(Configuration.Default, obuReader.SequenceHeader, Av1ColorFormat.Yuv444, false);
         Av1FrameInfo frameInfo = new(obuReader.SequenceHeader);
-        Av1FrameDecoder frameDecoder = new(obuReader.SequenceHeader, obuReader.FrameHeader, frameInfo, frameBuffer);
+        Av1FrameDecoderStub frameDecoder = new();
         Av1TileReader tileReader = new(Configuration.Default, obuReader.SequenceHeader, obuReader.FrameHeader, frameDecoder);
 
         // Act
@@ -35,5 +34,6 @@ public class Av1TilingTests
 
         // Assert
         Assert.Equal(dataSize * 8, bitStreamReader.BitPosition);
+        Assert.Equal(superblockCount, frameDecoder.SuperblockCount);
     }
 }