From e0305d78c5dc0986a860034e3c5d859c1a406528 Mon Sep 17 00:00:00 2001
From: winscripter <142818255+winscripter@users.noreply.github.com>
Date: Wed, 2 Sep 2026 21:58:14 +0400
Subject: [PATCH] Partial adaptive quantizer encoder

---
 .../Encoder/JxlAdaptiveQuantization.cs        | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 src/ImageSharp/Formats/Jxl/Processing/Encoder/JxlAdaptiveQuantization.cs

diff --git a/src/ImageSharp/Formats/Jxl/Processing/Encoder/JxlAdaptiveQuantization.cs b/src/ImageSharp/Formats/Jxl/Processing/Encoder/JxlAdaptiveQuantization.cs
new file mode 100644
index 0000000000..00150b3dd3
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/Processing/Encoder/JxlAdaptiveQuantization.cs
@@ -0,0 +1,44 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Encoder;
+
+/// <summary>
+/// Adaptive quantization encoder
+/// </summary>
+internal static class JxlAdaptiveQuantization
+{
+    // Scaling differences between JPEG XL and Butteraugli
+    private const float SGMul = 226.77216153508914f;
+    private const float SGMul2 = 1f / 73.377132366608819f;
+
+    // Includes correlation factor for std::log -> log2
+    private const float SGRetMul = SGMul2 * 18.6580932135f * JxlMath.InverseLog2E;
+    private const float SGVOffset = 7.7825991679894591f;
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static float ComputeMaskForAcStrategyUse(float outputValue)
+    {
+        const float multiplier = 1f;
+        const float offset = 0.001f;
+
+        return multiplier / (outputValue + offset);
+    }
+
+    public static float RatioOfDerivativesOfCubicRootToSimpleGamma(float v, bool invert = false)
+    {
+        float epsilon = 1e-2f;
+        v = Math.Max(0, v); // cannot be < 0
+        const float numMul = SGRetMul * 3 * SGMul;
+        float voffset = (SGVOffset * JxlMath.InverseLog2E) + epsilon;
+        const float denMul = JxlMath.InverseLog2E * SGMul;
+
+        float v2 = v * v;
+        float num = (numMul * v2) + epsilon;
+        float den = ((denMul * v) * v2) + voffset;
+
+        return invert ? num / den : den / num;
+    }
+}