Add headers & aspect ratio helpers

src/ImageSharp/Formats/Jxl/JxlAspectRatioHelpers.cs

@@ -0,0 +1,38 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl;
+
+internal static class JxlAspectRatioHelpers
+{
+    private static readonly SignedRational[] Ratios =
+    [
+        new(1, 1),
+        new(12, 10),
+        new(4, 3),
+        new(3, 2),
+        new(16, 9),
+        new(5, 4),
+        new(2, 1)
+    ];
+
+    public static SignedRational FixedAspectRatios(int ratio) => Ratios[ratio - 1];
+
+    public static int FindAspectRatio(int x, int y)
+    {
+        for (int i = 0; i < 7; i++)
+        {
+            if (x == MultiplyTruncate(Ratios[i], y))
+            {
+                return i;
+            }
+        }
+
+        return 0;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static int MultiplyTruncate(SignedRational rational, int multiplicand) => (multiplicand * rational.Numerator) / rational.Denominator;
+}

src/ImageSharp/Formats/Jxl/Metadata/JxlAnimationHeader.cs

@@ -0,0 +1,19 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.IO;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Metadata;
+
+internal sealed class JxlAnimationHeader : IJxlFields
+{
+    public int TpsNumerator { get; set; }
+
+    public int TpsDenominator { get; set; }
+
+    public int LoopCount { get; set; }
+
+    public bool ContainsTimecodes { get; set; }
+
+    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+}

src/ImageSharp/Formats/Jxl/Metadata/JxlImageMetadata.cs

@@ -123,4 +123,6 @@ internal sealed class JxlImageMetadata : IJxlFields
 
         this.Modular16BitBufferSufficient = bits <= 12;
     }
+
+    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
 }

src/ImageSharp/Formats/Jxl/Metadata/JxlPreviewHeader.cs

@@ -0,0 +1,68 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.IO;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Metadata;
+
+internal sealed class JxlPreviewHeader : IJxlFields
+{
+    private bool div8;
+    private int ySizeDiv8;
+    private int ySize;
+    private int ratio;
+    private int xSizeDiv8;
+    private int xSize;
+
+    public int YSize => this.div8 ? (this.ySizeDiv8 * 8) : this.ySize;
+
+    public int XSize
+    {
+        get
+        {
+            if (this.ratio != 0)
+            {
+                SignedRational signedRational = JxlAspectRatioHelpers.FixedAspectRatios(this.ratio);
+
+                return JxlAspectRatioHelpers.MultiplyTruncate(signedRational, this.YSize);
+            }
+
+            return this.div8 ? (this.xSizeDiv8 * 8) : this.xSize;
+        }
+    }
+
+    public void Set(int x, int y)
+    {
+        if (x == 0 || y == 0)
+        {
+            throw new ArgumentException("Empty preview");
+        }
+
+        this.div8 = ((x % JxlFrameDimensions.BlockDimensions) | (y % JxlFrameDimensions.BlockDimensions)) == 0;
+
+        if (this.div8)
+        {
+            this.ySizeDiv8 = y / 8;
+        }
+        else
+        {
+            this.ySize = y;
+        }
+
+        this.ratio = JxlAspectRatioHelpers.FindAspectRatio(x, y);
+
+        if (this.ratio == 0)
+        {
+            if (this.div8)
+            {
+                this.xSizeDiv8 = x / 8;
+            }
+            else
+            {
+                this.xSize = x;
+            }
+        }
+    }
+
+    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+}

src/ImageSharp/Formats/Jxl/Metadata/JxlSizeHeader.cs

@@ -0,0 +1,73 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.IO;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Metadata;
+
+internal sealed class JxlSizeHeader : IJxlFields
+{
+    private bool isSmall;
+    private int ySizeDiv8Minus1;
+    private int ySize;
+    private int ratio;
+    private int xSizeDiv8Minus1;
+    private int xSize;
+
+    public int YSize => this.isSmall ? ((this.ySizeDiv8Minus1 + 1) * 8) : this.ySize;
+
+    public int XSize
+    {
+        get
+        {
+            if (this.ratio != 0)
+            {
+                SignedRational aspectRatio = JxlAspectRatioHelpers.FixedAspectRatios(this.ratio);
+
+                return JxlAspectRatioHelpers.MultiplyTruncate(aspectRatio, this.YSize);
+            }
+
+            return this.isSmall ? ((this.xSizeDiv8Minus1 + 1) * 8) : this.xSize;
+        }
+    }
+
+    public void Set(int x, int y)
+    {
+        if (x > int.MaxValue || y > int.MaxValue)
+        {
+            throw new ArgumentException("Image too large");
+        }
+
+        if (x == 0 || y == 0)
+        {
+            throw new ArgumentException("Empty image");
+        }
+
+        this.ratio = JxlAspectRatioHelpers.FindAspectRatio(x, y);
+        this.isSmall = y < 256 && (y % JxlFrameDimensions.BlockDimensions) == 0
+                    && (this.ratio != 0 || (x <= 256 && (x % JxlFrameDimensions.BlockDimensions) == 0));
+
+        if (this.isSmall)
+        {
+            this.ySizeDiv8Minus1 = (y / 8) - 1;
+        }
+        else
+        {
+            this.ySize = y;
+        }
+
+        if (this.ratio == 0)
+        {
+            if (this.isSmall)
+            {
+                this.xSizeDiv8Minus1 = (x / 8) - 1;
+            }
+            else
+            {
+                this.xSize = x;
+            }
+        }
+    }
+
+    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+}