Implement Read Chroma from Luma syntax

src/ImageSharp/Formats/Heif/Av1/Av1Constants.cs

@@ -168,4 +168,9 @@ internal static class Av1Constants
     public const int BaseRangeSizeMinus1 = 3;
 
     public const int MaxBaseRange = 15;
+
+    /// <summary>
+    /// Log2 of number of values for ChromaFromLuma Alpha U and ChromaFromLuma Alpha V.
+    /// </summary>
+    public const int ChromaFromLumaAlphabetSizeLog2 = 4;
 }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1DefaultDistributions.cs

@@ -1994,6 +1994,18 @@ internal static class Av1DefaultDistributions
             ]
         ];
 
+    public static Av1Distribution ChromeForLumaSign => new(1418, 2123, 13340, 18405, 26972, 28343, 32294);
+
+    public static Av1Distribution[] ChromeForLumaAlpha =>
+        [
+            new(7637, 20719, 31401, 32481, 32657, 32688, 32692, 32696, 32700, 32704, 32708, 32712, 32716, 32720, 32724),
+            new(14365, 23603, 28135, 31168, 32167, 32395, 32487, 32573, 32620, 32647, 32668, 32672, 32676, 32680, 32684),
+            new(11532, 22380, 28445, 31360, 32349, 32523, 32584, 32649, 32673, 32677, 32681, 32685, 32689, 32693, 32697),
+            new(26990, 31402, 32282, 32571, 32692, 32696, 32700, 32704, 32708, 32712, 32716, 32720, 32724, 32728, 32732),
+            new(17248, 26058, 28904, 30608, 31305, 31877, 32126, 32321, 32394, 32464, 32516, 32560, 32576, 32593, 32622),
+            new(14738, 21678, 25779, 27901, 29024, 30302, 30980, 31843, 32144, 32413, 32520, 32594, 32622, 32656, 32660)
+        ];
+
     public static Av1Distribution[][][] GetEndOfBlockFlag(int baseQIndex)
     {
         int qContext = GetQContext(baseQIndex);

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

@@ -79,6 +79,11 @@ internal class Av1Distribution
     {
     }
 
+    public Av1Distribution(uint p0, uint p1, uint p2, uint p3, uint p4, uint p5, uint p6, uint p7, uint p8, uint p9, uint p10, uint p11, uint p12, uint p13, uint p14)
+        : this([p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, 0], 2)
+    {
+    }
+
     private Av1Distribution(uint[] props, int speed)
     {
         // this.probabilities = props;

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

@@ -29,6 +29,8 @@ internal ref struct Av1SymbolDecoder
     private readonly Av1Distribution[][][] coefficientsBaseRange;
     private readonly Av1Distribution[][] transformBlockSkip;
     private readonly Av1Distribution[][][] endOfBlockExtra;
+    private readonly Av1Distribution chromeForLumaSign = Av1DefaultDistributions.ChromeForLumaSign;
+    private readonly Av1Distribution[] chromeForLumaAlpha = Av1DefaultDistributions.ChromeForLumaAlpha;
     private Av1SymbolReader reader;
 
     public Av1SymbolDecoder(Span<byte> tileData, int qIndex)
@@ -236,6 +238,26 @@ internal ref struct Av1SymbolDecoder
         return r.ReadSymbol(this.coefficientsBase[(int)transformSizeContext][(int)planeType][coefficientContext]);
     }
 
+    public int ReadChromFromLumaSign()
+    {
+        ref Av1SymbolReader r = ref this.reader;
+        return r.ReadSymbol(this.chromeForLumaSign);
+    }
+
+    public int ReadChromaFromLumaAlphaU(int jointSign)
+    {
+        ref Av1SymbolReader r = ref this.reader;
+        int context = jointSign + 1 - 3;
+        return r.ReadSymbol(this.chromeForLumaAlpha[context]);
+    }
+
+    public int ReadChromaFromLumaAlphaV(int jointSign)
+    {
+        ref Av1SymbolReader r = ref this.reader;
+        int context = (((jointSign + 1) % 3) * 3) + ((jointSign + 1) / 3) - 3;
+        return r.ReadSymbol(this.chromeForLumaAlpha[context]);
+    }
+
     private static uint GetElementProbability(Av1Distribution probability, Av1PartitionType element)
         => probability[(int)element - 1] - probability[(int)element];
 }

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

@@ -1449,7 +1449,7 @@ internal class Av1TileDecoder : IAv1TileDecoder
                 partitionInfo.ModeInfo.UvMode = reader.ReadIntraModeUv(partitionInfo.ModeInfo.YMode, this.IsChromaForLumaAllowed(partitionInfo));
                 if (partitionInfo.ModeInfo.UvMode == Av1PredictionMode.UvChromaFromLuma)
                 {
-                    this.ReadChromaFromLumaAlphas(ref reader, partitionInfo);
+                    ReadChromaFromLumaAlphas(ref reader, partitionInfo.ModeInfo);
                 }
 
                 // 5.11.43.Intra angle info chroma syntax.
@@ -1523,10 +1523,23 @@ internal class Av1TileDecoder : IAv1TileDecoder
     /// <summary>
     /// 5.11.45. Read CFL alphas syntax.
     /// </summary>
-    private void ReadChromaFromLumaAlphas(ref Av1SymbolDecoder reader, Av1PartitionInfo partitionInfo) =>
+    private static void ReadChromaFromLumaAlphas(ref Av1SymbolDecoder reader, Av1BlockModeInfo modeInfo)
+    {
+        int jointSign = reader.ReadChromFromLumaSign();
+        int index = 0;
+        if (jointSign + 1 != 0)
+        {
+            index = reader.ReadChromaFromLumaAlphaU(jointSign) << Av1Constants.ChromaFromLumaAlphabetSizeLog2;
+        }
 
-        // TODO: Implement.
-        throw new NotImplementedException();
+        if ((jointSign + 1) % 3 != 0)
+        {
+            index += reader.ReadChromaFromLumaAlphaV(jointSign);
+        }
+
+        modeInfo.ChromaFromLumaAlphaSign = jointSign;
+        modeInfo.ChromaFromLumaAlphaIndex = index;
+    }
 
     /// <summary>
     /// 5.11.42. and 5.11.43.