Add 3 missing methods to JxlFrameDecoder, add spline and spotcolor render pipeline stages

src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlFrameDecoder.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Jxl.IO;
 using SixLabors.ImageSharp.Formats.Jxl.IO.FrameHeader;
 using SixLabors.ImageSharp.Formats.Jxl.IO.Metadata;
 using SixLabors.ImageSharp.Formats.Jxl.Processing.Image;
@@ -133,4 +134,28 @@ internal sealed class JxlFrameDecoder
 
         frameDecoder.FinalizeFrame();
     }
+
+    private static int BytesPerChannel(JxlDataType dataType) =>
+        dataType == JxlDataType.Byte ? 1
+           : dataType == JxlDataType.Single
+              ? 4
+              : 2;
+
+    private int GetStorageLocation(int thread, int task) => this.useTaskId ? task : thread;
+
+    private void PrepareStorage(int numThreads, int numTasks)
+    {
+        int storageSize = Math.Min(numThreads, numTasks);
+        if (storageSize > this.groupDecoderCaches.Count)
+        {
+            this.groupDecoderCaches = [.. this.groupDecoderCaches.Take(storageSize)];
+        }
+
+        this.useTaskId = numThreads > numTasks;
+        bool useNoise = (this.frameHeader.Flags & (int)JxlFrameHeaderFlags.Noise) != 0;
+        bool useGroupIds = this.modularFrameDecoder.UsesFullImage && (this.frameHeader.Encoding == JxlFrameEncoding.VarDct || useNoise);
+
+        this.decoderState.RenderPipeline?.PrepareForThreads(storageSize, useGroupIds);
+        this.decoderState.Upsampler8x.PrepareForThreads(numThreads);
+    }
 }

src/ImageSharp/Formats/Jxl/Processing/RenderPipeline/SplineStage.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Memory;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.RenderPipeline;
+
+internal sealed class SplineStage(Configuration configuration, JxlSplines splines) : RenderPipelineStageBase(configuration)
+{
+    public override string Name => "Splines";
+
+    public override void ProcessRow(Buffer2D<Memory<float>> inputRows, Buffer2D<Memory<float>> outputRows, int xExtraLeft, int xExtraRight, int width, int xPos, int yPos)
+    {
+        Span<float> rowX = this.GetInputRow(inputRows, 0, 0, xExtraLeft);
+        Span<float> rowY = this.GetInputRow(inputRows, 1, 0, xExtraLeft);
+        Span<float> rowB = this.GetInputRow(inputRows, 2, 0, xExtraLeft);
+        splines.AddToRow(rowX, rowY, rowB, yPos, xPos - xExtraLeft, xPos + width + xExtraRight);
+    }
+
+    public override RenderPipelineChannelMode GetChannelMode(int channel) => channel < 3 ? RenderPipelineChannelMode.InPlace : RenderPipelineChannelMode.Ignored;
+}

src/ImageSharp/Formats/Jxl/Processing/RenderPipeline/SpotColorStage.cs

@@ -0,0 +1,37 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Memory;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.RenderPipeline;
+
+internal sealed class SpotColorStage(Configuration configuration, int spotColorOffset, Memory<float> spotColor)
+    : RenderPipelineStageBase(configuration)
+{
+    private readonly int spotC = 3 + spotColorOffset;
+
+    public override string Name => "Spot";
+
+    public override void ProcessRow(Buffer2D<Memory<float>> inputRows, Buffer2D<Memory<float>> outputRows, int xExtraLeft, int xExtraRight, int width, int xPos, int yPos)
+    {
+        Span<float> spotColors = spotColor.Span;
+
+        float scale = 0;
+        for (int c = 0; c < 3; c++)
+        {
+            Span<float> p = this.GetInputRow(inputRows, c, 0);
+            Span<float> s = this.GetInputRow(inputRows, this.spotC, 0);
+
+            for (int x = 0; x < width; x++)
+            {
+                float mix = scale * s[x];
+                p[x] = (mix * spotColors[c]) + ((1.0f - mix) * p[x]);
+            }
+        }
+    }
+
+    public override RenderPipelineChannelMode GetChannelMode(int channel)
+        => channel < 3 ? RenderPipelineChannelMode.InPlace
+            : channel == this.spotC ? RenderPipelineChannelMode.Input
+            : RenderPipelineChannelMode.Ignored;
+}