added first attempt of histogram equalization

src/ImageSharp/Processing/Contrast/HistogramEqualizationExtension.cs

@@ -0,0 +1,23 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Contrast
+{
+    /// <summary>
+    /// Adds extension that allows applying an HistogramEqualization to the image.
+    /// </summary>
+    public static class HistogramEqualizationExtension
+    {
+        /// <summary>
+        /// Equalizes the histogram of an image to increases the global 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>
+        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

@@ -0,0 +1,64 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Contrast
+{
+    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);
+            int numberOfPixels = source.Width * source.Height;
+
+            // build the histogram of the grayscale levels
+            int luminanceLevels = 256;
+            int[] histogram = new int[luminanceLevels];
+            for (int y = 0; y < source.Height; y++)
+            {
+                Span<TPixel> row = source.GetPixelRowSpan(y);
+                for (int x = 0; x < source.Width; x++)
+                {
+                    TPixel sourcePixel = row[x];
+                    sourcePixel.ToRgb24(ref rgb);
+
+                    // 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
+            double[] cdf = new double[luminanceLevels];
+            double sum = 0.0d;
+            for (int i = 0; i < histogram.Length; i++)
+            {
+                double p = (double)histogram[i] / numberOfPixels;
+                sum += p;
+                cdf[i] = sum;
+            }
+
+            // apply the cdf to each pixel of the image
+            for (int y = 0; y < source.Height; y++)
+            {
+                Span<TPixel> row = source.GetPixelRowSpan(y);
+                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));
+                    byte luminanceEqualized = (byte)(cdf[luminance] * luminance);
+
+                    row[x].PackFromRgba32(new Rgba32(luminanceEqualized, luminanceEqualized, luminanceEqualized));
+                }
+            }
+        }
+    }
+}