Partial adaptive quantizer encoder

src/ImageSharp/Formats/Jxl/Processing/Encoder/JxlAdaptiveQuantization.cs

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+// 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;
+    }
+}