From 093d84612c2901eb95a1043b700341281d8bed51 Mon Sep 17 00:00:00 2001
From: winscripter <142818255+winscripter@users.noreply.github.com>
Date: Mon, 24 Aug 2026 21:40:59 +0400
Subject: [PATCH] Finish inverse RCT

---
 .../Processing/Modular/Transforms/JxlRct.cs   | 83 ++++++++++++++++++-
 .../Modular/Transforms/JxlTransform.cs        | 28 +++++++
 2 files changed, 110 insertions(+), 1 deletion(-)

diff --git a/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlRct.cs b/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlRct.cs
index 35b865661..ab55b1297 100644
--- a/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlRct.cs
+++ b/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlRct.cs
@@ -10,10 +10,22 @@ namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Modular.Transforms;
 /// </summary>
 internal static class JxlRct
 {
-    public static void InverseRctRow(int transformType, Span<int> in0, Span<int> in1, Span<int> in2, Span<int> out0, Span<int> out1, Span<int> out2, int width)
+    /// <summary>
+    /// Performs Inverse Reversible Color Transform (RCT) on one row.
+    /// </summary>
+    /// <param name="transformType">The kind of RCT.</param>
+    /// <param name="in0">Input Y</param>
+    /// <param name="in1">Input Co</param>
+    /// <param name="in2">Input Cg</param>
+    /// <param name="out0">Output R</param>
+    /// <param name="out1">Output G</param>
+    /// <param name="out2">Output B</param>
+    private static void InverseRctRow(int transformType, Span<int> in0, Span<int> in1, Span<int> in2, Span<int> out0, Span<int> out1, Span<int> out2)
     {
         DebugGuard.MustBeBetweenOrEqualTo(transformType, 0, 6, nameof(transformType));
 
+        int width = in0.Length; // All input & output channels have equal widths
+
         int second = transformType >> 1;
         int third = transformType & 1;
 
@@ -112,4 +124,73 @@ internal static class JxlRct
             }
         }
     }
+
+    /// <summary>
+    /// Performs Inverse Reversible Color Transform (RCT) on an entire
+    /// image.
+    /// </summary>
+    /// <param name="configuration">
+    /// The configuration is used to access maximum degree of parallelism.
+    /// </param>
+    /// <param name="img">
+    /// Image to compute inverse RCT.
+    /// </param>
+    /// <param name="beginC">
+    /// Offset of the color channel for Y, Co, Cg.
+    /// </param>
+    /// <param name="rctType">
+    /// Type of Reversible Color Transform
+    /// </param>
+    /// <exception cref="InvalidOperationException">
+    /// Invoked when RCT/permutation is invalid.
+    /// </exception>
+    public static void InverseRct(Configuration configuration, JxlModularImage img, int beginC, int rctType)
+    {
+        JxlTransform.CheckEqualChannels(img, beginC, beginC + 2);
+
+        int m = beginC;
+        JxlModularChannel c0 = img.Channels[m + 0];
+        int w = c0.Width;
+        int h = c0.Height;
+
+        if (rctType == 0)
+        {
+            // No-op
+            return;
+        }
+
+        int permutation = rctType / 7;
+
+        if (permutation >= 7)
+        {
+            throw new InvalidOperationException("Permutation must be <= 6");
+        }
+
+        int custom = rctType % 7;
+
+        if (custom == 0)
+        {
+            // Permute-only.
+            JxlModularChannel ch0 = img.Channels[m];
+            JxlModularChannel ch1 = img.Channels[m + 1];
+            JxlModularChannel ch2 = img.Channels[m + 2];
+            img.Channels[m + (permutation % 3)] = ch0;
+            img.Channels[m + ((permutation + 1 + (permutation / 3)) % 3)] = ch1;
+            img.Channels[m + ((permutation + 2 - (permutation / 3)) % 3)] = ch2;
+            return;
+        }
+
+        _ = Parallel.For(0, configuration.MaxDegreeOfParallelism, y =>
+        {
+            Span<int> in0 = img.Channels[m].GetRow(y);
+            Span<int> in1 = img.Channels[m + 1].GetRow(y);
+            Span<int> in2 = img.Channels[m + 2].GetRow(y);
+
+            Span<int> out0 = img.Channels[m + (permutation % 3)].GetRow(y);
+            Span<int> out1 = img.Channels[m + ((permutation + 1 + (permutation / 3)) % 3)].GetRow(y);
+            Span<int> out2 = img.Channels[m + ((permutation + 2 - (permutation / 3)) % 3)].GetRow(y);
+
+            InverseRctRow(custom, in0, in1, in2, out0, out1, out2);
+        });
+    }
 }
diff --git a/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlTransform.cs b/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlTransform.cs
index 780301061..860adb469 100644
--- a/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlTransform.cs
+++ b/src/ImageSharp/Formats/Jxl/Processing/Modular/Transforms/JxlTransform.cs
@@ -8,4 +8,32 @@ namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Modular.Transforms;
 internal sealed class JxlTransform : IJxlFields
 {
     public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+
+    public static void CheckEqualChannels(JxlModularImage image, int c1, int c2)
+    {
+        int channelsCount = image.Channels.Count;
+
+        if (c1 > channelsCount || c2 >= channelsCount || c2 < c1)
+        {
+            throw new InvalidOperationException($"Invalid channel range: {c1}..{c2} (there are only {channelsCount} channels)");
+        }
+
+        if (c1 < image.NbMetaChannels && c2 >= image.NbMetaChannels)
+        {
+            throw new InvalidOperationException("Invalid: transforming mix of meta and nonmeta");
+        }
+
+        JxlModularChannel ch1 = image.Channels[c1];
+        for (int c = c1 + 1; c <= c2; c++)
+        {
+            JxlModularChannel ch2 = image.Channels[c];
+            if (ch1.Width != ch2.Width ||
+                ch1.Height != ch2.Height ||
+                ch1.HorizontalShift != ch2.HorizontalShift ||
+                ch1.VerticalShift != ch2.VerticalShift)
+            {
+                throw new InvalidOperationException($"Channel {c} is not equal");
+            }
+        }
+    }
 }