Add Y'Cb'Cr chroma subsampling as part of the frame header

See frame_header.h

src/ImageSharp/Formats/Jxl/IO/JxlYCbCrChromaSubsampling.cs

@@ -0,0 +1,147 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.Fields;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+/// <summary>
+/// Gets the Y'Cb'Cr chroma subsampling information as part
+/// of the JPEG XL Frame Header.
+/// </summary>
+internal sealed class JxlYCbCrChromaSubsampling : IJxlFields
+{
+    private readonly int[] channelMode = new int[3];
+
+    private static ReadOnlySpan<byte> HShiftData => [0, 1, 1, 0];
+
+    private static ReadOnlySpan<byte> VShiftData => [0, 1, 0, 1];
+
+    public byte MaxHShift { get; private set; }
+
+    public byte MaxVShift { get; private set; }
+
+    /// <summary>
+    /// Gets a value indicating whether this 4:4:4 chroma subsampling.
+    /// </summary>
+    public bool Is444 =>
+            this.HShift(0) == 0 && this.VShift(0) == 0 && // Cb
+            this.HShift(2) == 0 && this.VShift(2) == 0 && // Cr
+            this.HShift(1) == 0 && this.VShift(1) == 0;   // Y;
+
+    /// <summary>
+    /// Gets a value indicating whether this 4:2:0 chroma subsampling.
+    /// </summary>
+    public bool Is420 =>
+            this.HShift(0) == 1 && this.VShift(0) == 1 && // Cb
+            this.HShift(2) == 1 && this.VShift(2) == 1 && // Cr
+            this.HShift(1) == 0 && this.VShift(1) == 0;   // Y
+
+    /// <summary>
+    /// Gets a value indicating whether this 4:2:2 chroma subsampling.
+    /// </summary>
+    public bool Is422 =>
+            this.HShift(0) == 1 && this.VShift(0) == 0 && // Cb
+            this.HShift(2) == 1 && this.VShift(2) == 0 && // Cr
+            this.HShift(1) == 0 && this.VShift(1) == 0;   // Y
+
+    /// <summary>
+    /// Gets a value indicating whether this 4:4:0 chroma subsampling.
+    /// </summary>
+    public bool Is440 =>
+            this.HShift(0) == 0 && this.VShift(0) == 1 && // Cb
+            this.HShift(2) == 0 && this.VShift(2) == 1 && // Cr
+            this.HShift(1) == 0 && this.VShift(1) == 0;    // Y
+
+    public byte RawHShift(int c) => HShiftData[this.channelMode[c]];
+
+    public byte RawVShift(int c) => VShiftData[this.channelMode[c]];
+
+    public byte HShift(int c) => (byte)(this.MaxHShift - HShiftData[this.channelMode[c]]);
+
+    public byte VShift(int c) => (byte)(this.MaxVShift - VShiftData[this.channelMode[c]]);
+
+    private void Recompute()
+    {
+        this.MaxHShift = 0;
+        this.MaxVShift = 0;
+
+        for (int i = 0; i < 3; i++)
+        {
+            int ch = this.channelMode[i];
+
+            this.MaxHShift = Math.Max(this.MaxHShift, HShiftData[ch]);
+            this.MaxVShift = Math.Max(this.MaxVShift, VShiftData[ch]);
+        }
+    }
+
+    public bool Set(ReadOnlySpan<byte> hsample, ReadOnlySpan<byte> vsample)
+    {
+        for (int c = 0; c < 3; c++)
+        {
+            int cjpeg = c < 2 ? (c ^ 1) : c;
+            int i = 0;
+
+            for (; i < 4; i++)
+            {
+                if (1 << HShiftData[i] == hsample[cjpeg] && 1 << VShiftData[i] == vsample[cjpeg])
+                {
+                    this.channelMode[c] = i;
+                    break;
+                }
+            }
+
+            if (i == 4)
+            {
+                return false;
+            }
+        }
+
+        this.Recompute();
+        return true;
+    }
+
+    public override string ToString()
+    {
+        if (this.Is444)
+        {
+            return "4:4:4";
+        }
+        else if (this.Is420)
+        {
+            return "4:2:0";
+        }
+        else if (this.Is422)
+        {
+            return "4:2:2";
+        }
+        else if (this.Is440)
+        {
+            return "4:4:0";
+        }
+        else
+        {
+            return $"[Custom] {this.channelMode[0]}:{this.channelMode[1]}:{this.channelMode[2]}";
+        }
+    }
+
+    public bool Visit(JxlVisitor visitor)
+    {
+        for (int i = 0; i < 3; i++)
+        {
+            int channel = this.channelMode[i];
+
+            uint unsignedChannel = (uint)channel;
+            bool wroteSuccessfully = visitor.Bits(2, 0, ref unsignedChannel);
+
+            if (!wroteSuccessfully)
+            {
+                return false;
+            }
+
+            this.channelMode[i] = (int)unsignedChannel;
+        }
+
+        return true;
+    }
+}