Misc tile parsing fixes

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

@@ -9,6 +9,7 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Symbol;
 internal ref struct Av1SymbolDecoder
 {
     private static readonly int[] IntraModeContext = [0, 1, 2, 3, 4, 4, 4, 4, 3, 0, 1, 2, 0];
+    private static readonly int[] AlphaVContexts = [-1, 0, 3, -1, 1, 4, -1, 2, 5];
 
     private readonly Av1Distribution tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy;
     private readonly Av1Distribution[] tilePartitionTypes = Av1DefaultDistributions.PartitionTypes;
@@ -244,17 +245,17 @@ internal ref struct Av1SymbolDecoder
         return r.ReadSymbol(this.chromeForLumaSign);
     }
 
-    public int ReadChromaFromLumaAlphaU(int jointSign)
+    public int ReadChromaFromLumaAlphaU(int jointSignPlus1)
     {
         ref Av1SymbolReader r = ref this.reader;
-        int context = jointSign + 1 - 3;
+        int context = jointSignPlus1 - 3;
         return r.ReadSymbol(this.chromeForLumaAlpha[context]);
     }
 
-    public int ReadChromaFromLumaAlphaV(int jointSign)
+    public int ReadChromaFromLumaAlphaV(int jointSignPlus1)
     {
         ref Av1SymbolReader r = ref this.reader;
-        int context = (((jointSign + 1) % 3) * 3) + ((jointSign + 1) / 3) - 3;
+        int context = AlphaVContexts[jointSignPlus1];
         return r.ReadSymbol(this.chromeForLumaAlpha[context]);
     }
 

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileDecoder.cs

@@ -154,25 +154,29 @@ internal class Av1TileDecoder : IAv1TileDecoder
         int quarterBlock4x4Size = halfBlock4x4Size >> 1;
         bool hasRows = (modeInfoLocation.Y + halfBlock4x4Size) < this.FrameInfo.ModeInfoRowCount;
         bool hasColumns = (modeInfoLocation.X + halfBlock4x4Size) < this.FrameInfo.ModeInfoColumnCount;
-        int ctx = this.GetPartitionPlaneContext(modeInfoLocation, blockSize, tileInfo, superblockInfo);
-        Av1PartitionType partitionType = Av1PartitionType.Split;
-        if (blockSize < Av1BlockSize.Block8x8)
-        {
-            partitionType = Av1PartitionType.None;
-        }
-        else if (hasRows && hasColumns)
-        {
-            partitionType = reader.ReadPartitionType(ctx);
-        }
-        else if (hasColumns)
-        {
-            bool splitOrVertical = reader.ReadSplitOrVertical(blockSize, ctx);
-            partitionType = splitOrVertical ? Av1PartitionType.Split : Av1PartitionType.Horizontal;
-        }
-        else if (hasRows)
+        Av1PartitionType partitionType = Av1PartitionType.None;
+        if (blockSize >= Av1BlockSize.Block8x8)
         {
-            bool splitOrHorizontal = reader.ReadSplitOrHorizontal(blockSize, ctx);
-            partitionType = splitOrHorizontal ? Av1PartitionType.Split : Av1PartitionType.Vertical;
+            int ctx = this.GetPartitionPlaneContext(modeInfoLocation, blockSize, tileInfo, superblockInfo);
+            partitionType = Av1PartitionType.Split;
+            if (blockSize < Av1BlockSize.Block8x8)
+            {
+                partitionType = Av1PartitionType.None;
+            }
+            else if (hasRows && hasColumns)
+            {
+                partitionType = reader.ReadPartitionType(ctx);
+            }
+            else if (hasColumns)
+            {
+                bool splitOrVertical = reader.ReadSplitOrVertical(blockSize, ctx);
+                partitionType = splitOrVertical ? Av1PartitionType.Split : Av1PartitionType.Horizontal;
+            }
+            else if (hasRows)
+            {
+                bool splitOrHorizontal = reader.ReadSplitOrHorizontal(blockSize, ctx);
+                partitionType = splitOrHorizontal ? Av1PartitionType.Split : Av1PartitionType.Vertical;
+            }
         }
 
         Av1BlockSize subSize = partitionType.GetBlockSubSize(blockSize);
@@ -1525,19 +1529,19 @@ internal class Av1TileDecoder : IAv1TileDecoder
     /// </summary>
     private static void ReadChromaFromLumaAlphas(ref Av1SymbolDecoder reader, Av1BlockModeInfo modeInfo)
     {
-        int jointSign = reader.ReadChromFromLumaSign();
+        int jointSignPlus1 = reader.ReadChromFromLumaSign() + 1;
         int index = 0;
-        if (jointSign + 1 != 0)
+        if (jointSignPlus1 >= 3)
         {
-            index = reader.ReadChromaFromLumaAlphaU(jointSign) << Av1Constants.ChromaFromLumaAlphabetSizeLog2;
+            index = reader.ReadChromaFromLumaAlphaU(jointSignPlus1) << Av1Constants.ChromaFromLumaAlphabetSizeLog2;
         }
 
-        if ((jointSign + 1) % 3 != 0)
+        if (jointSignPlus1 % 3 != 0)
         {
-            index += reader.ReadChromaFromLumaAlphaV(jointSign);
+            index += reader.ReadChromaFromLumaAlphaV(jointSignPlus1);
         }
 
-        modeInfo.ChromaFromLumaAlphaSign = jointSign;
+        modeInfo.ChromaFromLumaAlphaSign = jointSignPlus1 - 1;
         modeInfo.ChromaFromLumaAlphaIndex = index;
     }
 

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1BitStreamTests.cs

@@ -46,11 +46,12 @@ public class Av1BitStreamTests
     {
         // arrange
         // Three 32-bit values with MSB set.
-        byte[] buffer = {
-                          0xff, 0xff, 0xff, 0xff, // 4294967295
-                          0x80, 0xff, 0xee, 0xdd, // 2164256477
-                          0xa0, 0xaa, 0xbb, 0xcc // 2695543756
-                        };
+        byte[] buffer =
+            [
+                0xff, 0xff, 0xff, 0xff, // 4294967295
+                0x80, 0xff, 0xee, 0xdd, // 2164256477
+                0xa0, 0xaa, 0xbb, 0xcc // 2695543756
+            ];
         uint expected0 = 4294967295;
         uint expected1 = 2164256477;
         uint expected2 = 2695543756;