added support for 16 bit greyscale

src/ImageSharp/Processing/Contrast/HistogramEqualizationExtension.cs

@@ -15,7 +15,7 @@ namespace SixLabors.ImageSharp.Processing.Contrast
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
-        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        /// <returns>A histogram equalized grayscale image.</returns>
         public static IImageProcessingContext<TPixel> HistogramEqualization<TPixel>(this IImageProcessingContext<TPixel> source)
             where TPixel : struct, IPixel<TPixel>
             => source.ApplyProcessor(new HistogramEqualizationProcessor<TPixel>());

src/ImageSharp/Processing/Contrast/HistogramEqualizationProcessor.cs

@@ -9,17 +9,23 @@ using SixLabors.Primitives;
 
 namespace SixLabors.ImageSharp.Processing.Contrast
 {
+    /// <summary>
+    /// Applies a global histogram equalization to the image.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
     internal class HistogramEqualizationProcessor<TPixel> : ImageProcessor<TPixel>
         where TPixel : struct, IPixel<TPixel>
     {
         /// <inheritdoc/>
         protected override void OnFrameApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
         {
-            var rgb = default(Rgb24);
+            var rgb48 = default(Rgb48);
+            var rgb24 = default(Rgb24);
             int numberOfPixels = source.Width * source.Height;
+            bool is16bitPerChannel = typeof(TPixel) == typeof(Rgb48) || typeof(TPixel) == typeof(Rgba64);
 
             // build the histogram of the grayscale levels
-            int luminanceLevels = 256;
+            int luminanceLevels = is16bitPerChannel ? 65536 : 256;
             int[] histogram = new int[luminanceLevels];
             for (int y = 0; y < source.Height; y++)
             {
@@ -27,15 +33,12 @@ namespace SixLabors.ImageSharp.Processing.Contrast
                 for (int x = 0; x < source.Width; x++)
                 {
                     TPixel sourcePixel = row[x];
-                    sourcePixel.ToRgb24(ref rgb);
+                    int luminance = this.GetLuminance(sourcePixel, is16bitPerChannel, ref rgb24, ref rgb48);
-
-                    // Convert to grayscale using ITU-R Recommendation BT.709 if required
-                    int luminance = (int)((.2126F * rgb.R) + (.7152F * rgb.G) + (.0722F * rgb.B));
                     histogram[luminance]++;
                 }
             }
 
-            // calculate the cumulative distribution function which will be the cumulative histogram
+            // calculate the cumulative distribution function (which will be the cumulative histogram)
             int[] cdf = new int[luminanceLevels];
             int histSum = 0;
             for (int i = 0; i < histogram.Length; i++)
@@ -69,13 +72,46 @@ namespace SixLabors.ImageSharp.Processing.Contrast
                 for (int x = 0; x < source.Width; x++)
                 {
                     TPixel sourcePixel = row[x];
-                    sourcePixel.ToRgb24(ref rgb);
+
-                    int luminance = (int)((.2126F * rgb.R) + (.7152F * rgb.G) + (.0722F * rgb.B));
+                    int luminance = this.GetLuminance(sourcePixel, is16bitPerChannel, ref rgb24, ref rgb48);
                     double luminanceEqualized = (lut[luminance] / numberOfPixelsMinusCdfMin) * luminanceLevelsMinusOne;
                     luminanceEqualized = Math.Round(luminanceEqualized);
-                    row[x].PackFromRgba32(new Rgba32((byte)luminanceEqualized, (byte)luminanceEqualized, (byte)luminanceEqualized));
+
+                    if (is16bitPerChannel)
+                    {
+                        row[x].PackFromRgb48(new Rgb48((ushort)luminanceEqualized, (ushort)luminanceEqualized, (ushort)luminanceEqualized));
+                    }
+                    else
+                    {
+                        row[x].PackFromRgba32(new Rgba32((byte)luminanceEqualized, (byte)luminanceEqualized, (byte)luminanceEqualized));
+                    }
                 }
             }
         }
+
+        /// <summary>
+        /// Convert the pixel values to grayscale using ITU-R Recommendation BT.709.
+        /// </summary>
+        /// <param name="sourcePixel">The pixel to get the luminance from</param>
+        /// <param name="is16bitPerChannel">Flag indicates, if its 16 bits per channel, otherwise its 8</param>
+        /// <param name="rgb24">Will store the pixel values in case of 8 bit per channel</param>
+        /// <param name="rgb48">Will store the pixel values in case of 16 bit per channel</param>
+        private int GetLuminance(TPixel sourcePixel, bool is16bitPerChannel, ref Rgb24 rgb24, ref Rgb48 rgb48)
+        {
+            // Convert to grayscale using ITU-R Recommendation BT.709
+            int luminance;
+            if (is16bitPerChannel)
+            {
+                sourcePixel.ToRgb48(ref rgb48);
+                luminance = (int)((.2126F * rgb48.R) + (.7152F * rgb48.G) + (.0722F * rgb48.B));
+            }
+            else
+            {
+                sourcePixel.ToRgb24(ref rgb24);
+                luminance = (int)((.2126F * rgb24.R) + (.7152F * rgb24.G) + (.0722F * rgb24.B));
+            }
+
+            return luminance;
+        }
     }
 }

tests/ImageSharp.Tests/Processing/Contrast/HistogramEqualizationTests.cs

@@ -37,14 +37,14 @@ namespace SixLabors.ImageSharp.Tests.Processing.Contrast
 
             byte[] expected = new byte[]
             {
-                0, 12, 53, 32, 146, 53, 174, 53,
+                0,    12,   53,   32,  146,   53,  174,   53,
-                57, 32, 12, 227, 219, 202, 32, 154,
+                57,   32,   12,  227,  219,  202,   32,  154,
-                65, 85, 93, 239, 251, 227, 65, 158,
+                65,   85,   93,  239,  251,  227,   65,  158,
-                73, 146, 146, 247, 255, 235, 154, 130,
+                73,  146,  146,  247,  255,  235,  154,  130,
-                97, 166, 117, 231, 243, 210, 117, 117,
+                97,  166,  117,  231,  243,  210,  117,  117,
-                117, 190, 36, 190, 178, 93, 20, 170,
+                117, 190,   36,  190,  178,   93,   20,  170,
-                130, 202, 73, 20, 12, 53, 85, 194,
+                130, 202,   73,   20,   12,   53,   85,  194,
-                146, 206, 130, 117, 85, 166, 182, 215
+                146, 206,  130,  117,   85,  166,  182,  215
             };
 
             // act
@@ -58,7 +58,6 @@ namespace SixLabors.ImageSharp.Tests.Processing.Contrast
                     Rgba32 actual = image[x, y];
                     int diff = expected[y * 8 + x] - actual.R;
                     Assert.True(diff == 0);
-                    int foo = 2;
                 }
             }
         }