Fix converting pixel to gray in histogram equalization

src/ImageSharp/Common/Helpers/ImageMaths.cs

@@ -1,7 +1,8 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Numerics;
 using System.Runtime.CompilerServices;
 
 using SixLabors.ImageSharp.PixelFormats;
@@ -14,6 +15,20 @@ namespace SixLabors.ImageSharp
     /// </summary>
     internal static class ImageMaths
     {
+        /// <summary>
+        /// Vector for converting pixel to gray value as specified by ITU-R Recommendation BT.709.
+        /// </summary>
+        private static readonly Vector4 Bt709 = new Vector4(.2126f, .7152f, .0722f, 0.0f);
+
+        /// <summary>
+        /// Convert a pixel value to grayscale using ITU-R Recommendation BT.709.
+        /// </summary>
+        /// <param name="vector">The vector to get the luminance from.</param>
+        /// <param name="luminanceLevels">The number of luminance levels (256 for 8 bit, 65536 for 16 bit grayscale images)</param>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public static int GetBT709Luminance(ref Vector4 vector, int luminanceLevels)
+            => (int)MathF.Round(Vector4.Dot(vector, Bt709) * (luminanceLevels - 1));
+
         /// <summary>
         /// Gets the luminance from the rgb components using the formula as specified by ITU-R Recommendation BT.709.
         /// </summary>

src/ImageSharp/Formats/Tga/TgaEncoderCore.cs

@@ -45,11 +45,6 @@ namespace SixLabors.ImageSharp.Formats.Tga
         /// </summary>
         private readonly TgaCompression compression;
 
-        /// <summary>
-        /// Vector for converting pixel to gray value.
-        /// </summary>
-        private static readonly Vector4 Bt709 = new Vector4(.2126f, .7152f, .0722f, 0.0f);
-
         /// <summary>
         /// Initializes a new instance of the <see cref="TgaEncoderCore"/> class.
         /// </summary>
@@ -347,15 +342,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
             where TPixel : struct, IPixel<TPixel>
         {
             var vector = sourcePixel.ToVector4();
-            return GetLuminance(ref vector);
+            return ImageMaths.GetBT709Luminance(ref vector, 256);
         }
-
-        /// <summary>
-        /// Convert the pixel values to grayscale using ITU-R Recommendation BT.709.
-        /// </summary>
-        /// <param name="vector">The vector to get the luminance from.</param>
-        [MethodImpl(InliningOptions.ShortMethod)]
-        public static int GetLuminance(ref Vector4 vector)
-            => (int)MathF.Round(Vector4.Dot(vector, Bt709) * (256 - 1));
     }
 }

src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistogramEqualizationSlidingWindowProcessor{TPixel}.cs

@@ -349,7 +349,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         {
             for (int idx = 0; idx < length; idx++)
             {
-                int luminance = GetLuminance(ref Unsafe.Add(ref greyValuesBase, idx), luminanceLevels);
+                int luminance = ImageMaths.GetBT709Luminance(ref Unsafe.Add(ref greyValuesBase, idx), luminanceLevels);
                 Unsafe.Add(ref histogramBase, luminance)++;
             }
         }
@@ -366,7 +366,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         {
             for (int idx = 0; idx < length; idx++)
             {
-                int luminance = GetLuminance(ref Unsafe.Add(ref greyValuesBase, idx), luminanceLevels);
+                int luminance = ImageMaths.GetBT709Luminance(ref Unsafe.Add(ref greyValuesBase, idx), luminanceLevels);
                 Unsafe.Add(ref histogramBase, luminance)--;
             }
         }

src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationProcessor{TPixel}.cs

@@ -143,16 +143,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         public static int GetLuminance(TPixel sourcePixel, int luminanceLevels)
         {
             var vector = sourcePixel.ToVector4();
-            return GetLuminance(ref vector, luminanceLevels);
+            return ImageMaths.GetBT709Luminance(ref vector, luminanceLevels);
         }
-
-        /// <summary>
-        /// Convert the pixel values to grayscale using ITU-R Recommendation BT.709.
-        /// </summary>
-        /// <param name="vector">The vector to get the luminance from</param>
-        /// <param name="luminanceLevels">The number of luminance levels (256 for 8 bit, 65536 for 16 bit grayscale images)</param>
-        [MethodImpl(InliningOptions.ShortMethod)]
-        public static int GetLuminance(ref Vector4 vector, int luminanceLevels)
-            => (int)MathF.Round(((.2126F * vector.X) + (.7152F * vector.Y) + (.0722F * vector.Y)) * (luminanceLevels - 1));
     }
 }