Add histogram equalization benchmark

tests/ImageSharp.Benchmarks/Processing/HistogramEqualization.cs

@@ -0,0 +1,53 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.IO;
+using BenchmarkDotNet.Attributes;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Normalization;
+using SixLabors.ImageSharp.Tests;
+
+namespace SixLabors.ImageSharp.Benchmarks.Processing
+{
+    [Config(typeof(Config.ShortClr))]
+    public class HistogramEqualization : BenchmarkBase
+    {
+        private Image<Rgba32> image;
+
+        [GlobalSetup]
+        public void ReadImages()
+        {
+            if (this.image == null)
+            {
+                this.image = Image.Load<Rgba32>(File.OpenRead(Path.Combine(TestEnvironment.InputImagesDirectoryFullPath, TestImages.Jpeg.Baseline.HistogramEqImage)));
+            }
+        }
+
+        [GlobalCleanup]
+        public void Cleanup()
+        {
+            this.image.Dispose();
+        }
+
+        [Benchmark(Description = "Global Histogram Equalization")]
+        public void GlobalHistogramEqualization()
+        {
+            this.image.Mutate(img => img.HistogramEqualization(new HistogramEqualizationOptions()
+            {
+                LuminanceLevels = 256,
+                Method = HistogramEqualizationMethod.Global
+            }));
+        }
+
+        [Benchmark(Description = "AdaptiveHistogramEqualization (Tile interpolation)")]
+        public void AdaptiveHistogramEqualization()
+        {
+            this.image.Mutate(img => img.HistogramEqualization(new HistogramEqualizationOptions()
+            {
+                LuminanceLevels = 256,
+                Method = HistogramEqualizationMethod.AdaptiveTileInterpolation
+            }));
+        }
+    }
+}