From c4c4bf292298c026df470db1a17ff826515fd95d Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Sat, 5 Sep 2026 01:26:41 +1000
Subject: [PATCH] Clamp histogram luminance indices

---
 .../Common/Helpers/ColorNumerics.cs           | 12 +++-
 .../HistogramEqualizationTests.cs             | 55 +++++++++++++++++++
 2 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/src/ImageSharp/Common/Helpers/ColorNumerics.cs b/src/ImageSharp/Common/Helpers/ColorNumerics.cs
index 0b88aa5b07..d13eead453 100644
--- a/src/ImageSharp/Common/Helpers/ColorNumerics.cs
+++ b/src/ImageSharp/Common/Helpers/ColorNumerics.cs
@@ -27,7 +27,17 @@ internal static class ColorNumerics
     /// </param>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static int GetBT709Luminance(Vector4 vector, int luminanceLevels)
-        => (int)MathF.Round(Vector4.Dot(vector, Bt709) * (luminanceLevels - 1));
+    {
+        float luminance = Vector4.Dot(vector, Bt709);
+        if (float.IsNaN(luminance))
+        {
+            return 0;
+        }
+
+        // Floating-point pixel formats can contain values outside their normalized range.
+        luminance = Math.Clamp(luminance, 0F, 1F);
+        return (int)MathF.Round(luminance * (luminanceLevels - 1));
+    }
 
     /// <summary>
     /// Gets the luminance from the rgb components using the formula
diff --git a/tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs b/tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs
index c39648d014..917d68d025 100644
--- a/tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs
+++ b/tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs
@@ -14,6 +14,18 @@ public class HistogramEqualizationTests
 {
     private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.0456F);
 
+    [Fact]
+    public void GlobalHistogramEqualization_ClampsDecodedFloatingPointSamples()
+    {
+        byte[] data = BuildFloatTiff(8, 2F);
+
+        using Image<HalfVector4> image = Image.Load<HalfVector4>(data);
+        image.Mutate(x => x.HistogramEqualization());
+
+        Assert.Equal(8, image.Width);
+        Assert.Equal(1, image.Height);
+    }
+
     [Theory]
     [InlineData(256)]
     [InlineData(65536)]
@@ -238,4 +250,47 @@ public class HistogramEqualizationTests
 
         ValidatorComparer.VerifySimilarity(referenceResult, processed);
     }
+
+    private static byte[] BuildFloatTiff(int width, float sample)
+    {
+        const int headerLength = 8;
+        const int entryCount = 10;
+        const int ifdLength = 2 + (entryCount * 12) + 4;
+        uint pixelOffset = headerLength + ifdLength;
+
+        using MemoryStream stream = new();
+        using BinaryWriter writer = new(stream);
+
+        writer.Write((byte)0x49);
+        writer.Write((byte)0x49);
+        writer.Write((ushort)42);
+        writer.Write((uint)headerLength);
+        writer.Write((ushort)entryCount);
+        WriteEntry(writer, 256, 4, 1, (uint)width);
+        WriteEntry(writer, 257, 4, 1, 1);
+        WriteEntry(writer, 258, 3, 1, 32);
+        WriteEntry(writer, 259, 3, 1, 1);
+        WriteEntry(writer, 262, 3, 1, 1);
+        WriteEntry(writer, 273, 4, 1, pixelOffset);
+        WriteEntry(writer, 277, 3, 1, 1);
+        WriteEntry(writer, 278, 4, 1, 1);
+        WriteEntry(writer, 279, 4, 1, (uint)(width * sizeof(float)));
+        WriteEntry(writer, 339, 3, 1, 3);
+        writer.Write(0U);
+
+        for (int i = 0; i < width; i++)
+        {
+            writer.Write(sample);
+        }
+
+        return stream.ToArray();
+
+        static void WriteEntry(BinaryWriter writer, ushort tag, ushort type, uint count, uint value)
+        {
+            writer.Write(tag);
+            writer.Write(type);
+            writer.Write(count);
+            writer.Write(value);
+        }
+    }
 }