Begin optimizing the global histogram

src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistEqualizationProcessor.cs

@@ -484,11 +484,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                     using (IMemoryOwner<int> histogramBuffer = this.memoryAllocator.Allocate<int>(luminanceLevels))
                     {
                         Span<int> histogram = histogramBuffer.GetSpan();
+                        ref int histogramBase = ref MemoryMarshal.GetReference(histogram);
 
                         for (int x = 0; x < sourceWidth; x += tileWidth)
                         {
                             histogram.Clear();
-                            Span<int> cdf = this.GetCdfLutSpan(cdfX, index);
+                            ref int cdfBase = ref MemoryMarshal.GetReference(this.GetCdfLutSpan(cdfX, index));
 
                             int xlimit = Math.Min(x + tileWidth, sourceWidth);
                             for (int dy = y; dy < endY; dy++)
@@ -506,7 +507,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                                 processor.ClipHistogram(histogram, processor.ClipLimitPercentage, this.pixelsInTile);
                             }
 
-                            Unsafe.Add(ref cdfMinBase, cdfX) = processor.CalculateCdf(cdf, histogram, histogram.Length - 1);
+                            Unsafe.Add(ref cdfMinBase, cdfX) = processor.CalculateCdf(ref cdfBase, ref histogramBase, histogram.Length - 1);
 
                             cdfX++;
                         }

src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistEqualizationSWProcessor.cs

@@ -2,7 +2,10 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers;
 using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using System.Threading.Tasks;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
@@ -59,21 +62,24 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                     parallelOptions,
                     x =>
                     {
-                        using (System.Buffers.IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
-                        using (System.Buffers.IMemoryOwner<int> histogramBufferCopy = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
-                        using (System.Buffers.IMemoryOwner<int> cdfBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
-                        using (System.Buffers.IMemoryOwner<TPixel> pixelRowBuffer = memoryAllocator.Allocate<TPixel>(tileWidth, AllocationOptions.Clean))
+                        using (IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
+                        using (IMemoryOwner<int> histogramBufferCopy = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
+                        using (IMemoryOwner<int> cdfBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
+                        using (IMemoryOwner<TPixel> pixelRowBuffer = memoryAllocator.Allocate<TPixel>(tileWidth, AllocationOptions.Clean))
                         {
                             Span<int> histogram = histogramBuffer.GetSpan();
+                            ref int histogramBase = ref MemoryMarshal.GetReference(histogram);
                             Span<int> histogramCopy = histogramBufferCopy.GetSpan();
-                            Span<int> cdf = cdfBuffer.GetSpan();
+                            ref int histogramCopyBase = ref MemoryMarshal.GetReference(histogramCopy);
+                            ref int cdfBase = ref MemoryMarshal.GetReference(cdfBuffer.GetSpan());
+
                             Span<TPixel> pixelRow = pixelRowBuffer.GetSpan();
                             int maxHistIdx = 0;
 
                             // Build the histogram of grayscale values for the current tile.
                             for (int dy = -halfTileWith; dy < halfTileWith; dy++)
                             {
-                                Span<TPixel> rowSpan = this.GetPixelRow(source, pixelRow, (int)x - halfTileWith, dy, tileWidth);
+                                Span<TPixel> rowSpan = this.GetPixelRow(source, pixelRow, x - halfTileWith, dy, tileWidth);
                                 int maxIdx = this.AddPixelsToHistogram(rowSpan, histogram, this.LuminanceLevels);
                                 if (maxIdx > maxHistIdx)
                                 {
@@ -91,12 +97,15 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                                 }
 
                                 // Calculate the cumulative distribution function, which will map each input pixel in the current tile to a new value.
-                                int cdfMin = this.ClipHistogramEnabled ? this.CalculateCdf(cdf, histogramCopy, maxHistIdx) : this.CalculateCdf(cdf, histogram, maxHistIdx);
+                                int cdfMin = this.ClipHistogramEnabled
+                                ? this.CalculateCdf(ref cdfBase, ref histogramCopyBase, maxHistIdx)
+                                : this.CalculateCdf(ref cdfBase, ref histogramBase, maxHistIdx);
+
                                 float numberOfPixelsMinusCdfMin = pixeInTile - cdfMin;
 
                                 // Map the current pixel to the new equalized value
                                 int luminance = GetLuminance(source[x, y], this.LuminanceLevels);
-                                float luminanceEqualized = cdf[luminance] / numberOfPixelsMinusCdfMin;
+                                float luminanceEqualized = Unsafe.Add(ref cdfBase, luminance) / numberOfPixelsMinusCdfMin;
                                 targetPixels[x, y].FromVector4(new Vector4(luminanceEqualized, luminanceEqualized, luminanceEqualized, source[x, y].ToVector4().W));
 
                                 // Remove top most row from the histogram, mirroring rows which exceeds the borders.
@@ -218,7 +227,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                 // If the histogram at the maximum index has changed to 0, search for the next smaller value.
                 if (luminance == maxHistIdx && histogram[luminance] == 0)
                 {
-                    for (int j = luminance; j >= 0;  j--)
+                    for (int j = luminance; j >= 0; j--)
                     {
                         maxHistIdx = j;
                         if (histogram[j] != 0)

src/ImageSharp/Processing/Processors/Normalization/GlobalHistogramEqualizationProcessor.cs

@@ -2,7 +2,9 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers;
 using System.Numerics;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
@@ -37,11 +39,13 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             int numberOfPixels = source.Width * source.Height;
             Span<TPixel> pixels = source.GetPixelSpan();
 
-            using (System.Buffers.IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
-            using (System.Buffers.IMemoryOwner<int> cdfBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
+            using (IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
+            using (IMemoryOwner<int> cdfBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean))
             {
                 // Build the histogram of the grayscale levels.
                 Span<int> histogram = histogramBuffer.GetSpan();
+                ref int histogramBase = ref MemoryMarshal.GetReference(histogram);
+
                 for (int i = 0; i < pixels.Length; i++)
                 {
                     TPixel sourcePixel = pixels[i];
@@ -56,7 +60,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
 
                 // Calculate the cumulative distribution function, which will map each input pixel to a new value.
                 Span<int> cdf = cdfBuffer.GetSpan();
-                int cdfMin = this.CalculateCdf(cdf, histogram, histogram.Length - 1);
+                ref int cdfBase = ref MemoryMarshal.GetReference(cdf);
+                int cdfMin = this.CalculateCdf(ref cdfBase, ref histogramBase, histogram.Length - 1);
 
                 // Apply the cdf to each pixel of the image
                 float numberOfPixelsMinusCdfMin = numberOfPixels - cdfMin;

src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationProcessor.cs

@@ -53,17 +53,15 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         /// <summary>
         /// Calculates the cumulative distribution function.
         /// </summary>
-        /// <param name="cdf">The array holding the cdf.</param>
-        /// <param name="histogram">The histogram of the input image.</param>
+        /// <param name="cdfBase">The reference to the array holding the cdf.</param>
+        /// <param name="histogramBase">The reference to the histogram of the input image.</param>
         /// <param name="maxIdx">Index of the maximum of the histogram.</param>
         /// <returns>The first none zero value of the cdf.</returns>
-        public int CalculateCdf(Span<int> cdf, Span<int> histogram, int maxIdx)
+        public int CalculateCdf(ref int cdfBase, ref int histogramBase, int maxIdx)
         {
             int histSum = 0;
             int cdfMin = 0;
             bool cdfMinFound = false;
-            ref int cdfBase = ref MemoryMarshal.GetReference(cdf);
-            ref int histogramBase = ref MemoryMarshal.GetReference(histogram);
 
             for (int i = 0; i <= maxIdx; i++)
             {

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

@@ -37,7 +37,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
                 {
                     for (int x = 0; x < 8; x++)
                     {
-                        byte luminance = pixels[y * 8 + x];
+                        byte luminance = pixels[(y * 8) + x];
                         image[x, y] = new Rgba32(luminance, luminance, luminance);
                     }
                 }
@@ -66,9 +66,9 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
                     for (int x = 0; x < 8; x++)
                     {
                         Rgba32 actual = image[x, y];
-                        Assert.Equal(expected[y * 8 + x], actual.R);
-                        Assert.Equal(expected[y * 8 + x], actual.G);
-                        Assert.Equal(expected[y * 8 + x], actual.B);
+                        Assert.Equal(expected[(y * 8) + x], actual.R);
+                        Assert.Equal(expected[(y * 8) + x], actual.G);
+                        Assert.Equal(expected[(y * 8) + x], actual.B);
                     }
                 }
             }