using Parallel.For

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

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Numerics;
+using System.Threading.Tasks;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Memory;
@@ -28,6 +29,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         public AdaptiveHistEqualizationProcessor(int gridSize, int clipLimit, int luminanceLevels)
             : base(luminanceLevels)
         {
+            Guard.MustBeGreaterThan(gridSize, 8, nameof(gridSize));
+            Guard.MustBeGreaterThan(clipLimit, 1, nameof(clipLimit));
+
             this.GridSize = gridSize;
             this.ClipLimit = clipLimit;
         }
@@ -51,17 +55,22 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
 
             int pixelsInGrid = this.GridSize * this.GridSize;
             int halfGridSize = this.GridSize / 2;
+            using (Buffer2D<TPixel> targetPixels = configuration.MemoryAllocator.Allocate2D<TPixel>(source.Width, source.Height))
+            {
+                Parallel.For(
+                    0,
+                    source.Width,
+                    configuration.ParallelOptions,
+                    x =>
+                    {
                         using (IBuffer<int> histogramBuffer = memoryAllocator.AllocateClean<int>(this.LuminanceLevels))
                         using (IBuffer<int> histogramBufferCopy = memoryAllocator.AllocateClean<int>(this.LuminanceLevels))
                         using (IBuffer<int> cdfBuffer = memoryAllocator.AllocateClean<int>(this.LuminanceLevels))
-            using (Buffer2D<TPixel> targetPixels = configuration.MemoryAllocator.Allocate2D<TPixel>(source.Width, source.Height))
                         {
                             Span<int> histogram = histogramBuffer.GetSpan();
                             Span<int> histogramCopy = histogramBufferCopy.GetSpan();
                             Span<int> cdf = cdfBuffer.GetSpan();
 
-                for (int x = 0; x < source.Width; x++)
-                {
                             histogram.Clear();
 
                             // Build the histogram of grayscale values for the current grid.
@@ -81,7 +90,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                                 double numberOfPixelsMinusCdfMin = (double)(pixelsInGrid - cdfMin);
 
                                 // Map the current pixel to the new equalized value
-                        int luminance = this.GetLuminance(pixels[(y * source.Width) + x], this.LuminanceLevels);
+                                int luminance = this.GetLuminance(source[x, y], this.LuminanceLevels);
                                 double luminanceEqualized = cdf[luminance] / numberOfPixelsMinusCdfMin;
                                 targetPixels[x, y].PackFromVector4(new Vector4((float)luminanceEqualized));
 
@@ -94,6 +103,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                                 this.AddPixelsTooHistogram(rowSpan, histogram, this.LuminanceLevels);
                             }
                         }
+                    });
 
                 Buffer2D<TPixel>.SwapOrCopyContent(source.PixelBuffer, targetPixels);
             }