Ensure cdfMinSpan is cleared before use.

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

@@ -22,10 +22,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             bool clipHistogram,
             int clipLimit,
             int numberOfTiles)
-            : base(luminanceLevels, clipHistogram, clipLimit)
-        {
-            this.NumberOfTiles = numberOfTiles;
-        }
+            : base(luminanceLevels, clipHistogram, clipLimit) => this.NumberOfTiles = numberOfTiles;
 
         /// <summary>
         /// Gets the number of tiles the image is split into (horizontal and vertically) for the adaptive histogram equalization.
@@ -34,8 +31,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
 
         /// <inheritdoc />
         public override IImageProcessor<TPixel> CreatePixelSpecificProcessor<TPixel>(Configuration configuration, Image<TPixel> source, Rectangle sourceRectangle)
-        {
-            return new AdaptiveHistogramEqualizationProcessor<TPixel>(
+            => new AdaptiveHistogramEqualizationProcessor<TPixel>(
                 configuration,
                 this.LuminanceLevels,
                 this.ClipHistogram,
@@ -43,6 +39,5 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                 this.NumberOfTiles,
                 source,
                 sourceRectangle);
-        }
     }
 }

src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistogramEqualizationProcessor{TPixel}.cs

@@ -459,10 +459,14 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             private readonly Configuration configuration;
             private readonly MemoryAllocator memoryAllocator;
 
-            // Used for storing the minimum value for each CDF entry.
+            /// <summary>
+            /// Used for storing the minimum value for each CDF entry.
+            /// </summary>
             private readonly Buffer2D<int> cdfMinBuffer2D;
 
-            // Used for storing the LUT for each CDF entry.
+            /// <summary>
+            /// Used for storing the LUT for each CDF entry.
+            /// </summary>
             private readonly Buffer2D<int> cdfLutBuffer2D;
             private readonly int pixelsInTile;
             private readonly int sourceWidth;
@@ -596,6 +600,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                         int y = this.tileYStartPositions[index].y;
                         int endY = Math.Min(y + this.tileHeight, this.sourceHeight);
                         Span<int> cdfMinSpan = this.cdfMinBuffer2D.GetRowSpan(cdfY);
+                        cdfMinSpan.Clear();
 
                         using IMemoryOwner<int> histogramBuffer = this.allocator.Allocate<int>(this.luminanceLevels);
                         Span<int> histogram = histogramBuffer.GetSpan();
@@ -614,7 +619,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                                 for (int dx = x; dx < xlimit; dx++)
                                 {
                                     int luminance = GetLuminance(rowSpan[dx], this.luminanceLevels);
-                                    histogram[luminance]++;
+
+                                    // This is safe. The index maxes out to the span length.
+                                    Unsafe.Add(ref histogramBase, luminance)++;
                                 }
                             }
 

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

@@ -49,44 +49,18 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         /// </summary>
         /// <param name="options">The <see cref="HistogramEqualizationOptions"/>.</param>
         /// <returns>The <see cref="HistogramEqualizationProcessor"/>.</returns>
-        public static HistogramEqualizationProcessor FromOptions(HistogramEqualizationOptions options)
+        public static HistogramEqualizationProcessor FromOptions(HistogramEqualizationOptions options) => options.Method switch
         {
-            HistogramEqualizationProcessor processor;
+            HistogramEqualizationMethod.Global
+            => new GlobalHistogramEqualizationProcessor(options.LuminanceLevels, options.ClipHistogram, options.ClipLimit),
 
-            switch (options.Method)
-            {
-                case HistogramEqualizationMethod.Global:
-                    processor = new GlobalHistogramEqualizationProcessor(
-                        options.LuminanceLevels,
-                        options.ClipHistogram,
-                        options.ClipLimit);
-                    break;
+            HistogramEqualizationMethod.AdaptiveTileInterpolation
+            => new AdaptiveHistogramEqualizationProcessor(options.LuminanceLevels, options.ClipHistogram, options.ClipLimit, options.NumberOfTiles),
 
-                case HistogramEqualizationMethod.AdaptiveTileInterpolation:
-                    processor = new AdaptiveHistogramEqualizationProcessor(
-                        options.LuminanceLevels,
-                        options.ClipHistogram,
-                        options.ClipLimit,
-                        options.NumberOfTiles);
-                    break;
+            HistogramEqualizationMethod.AdaptiveSlidingWindow
+            => new AdaptiveHistogramEqualizationSlidingWindowProcessor(options.LuminanceLevels, options.ClipHistogram, options.ClipLimit, options.NumberOfTiles),
 
-                case HistogramEqualizationMethod.AdaptiveSlidingWindow:
-                    processor = new AdaptiveHistogramEqualizationSlidingWindowProcessor(
-                        options.LuminanceLevels,
-                        options.ClipHistogram,
-                        options.ClipLimit,
-                        options.NumberOfTiles);
-                    break;
-
-                default:
-                    processor = new GlobalHistogramEqualizationProcessor(
-                        options.LuminanceLevels,
-                        options.ClipHistogram,
-                        options.ClipLimit);
-                    break;
-            }
-
-            return processor;
-        }
+            _ => new GlobalHistogramEqualizationProcessor(options.LuminanceLevels, options.ClipHistogram, options.ClipLimit),
+        };
     }
 }

src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationProcessor{TPixel}.cs

@@ -142,6 +142,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         [MethodImpl(InliningOptions.ShortMethod)]
         public static int GetLuminance(TPixel sourcePixel, int luminanceLevels)
         {
+            // TODO: We need a bulk per span equivalent.
             var vector = sourcePixel.ToVector4();
             return ColorNumerics.GetBT709Luminance(ref vector, luminanceLevels);
         }

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

@@ -141,6 +141,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
         /// See: https://github.com/SixLabors/ImageSharp/pull/984
         /// </summary>
         /// <typeparam name="TPixel">The pixel type of the image.</typeparam>
+        /// <param name="provider">The test image provider.</param>
         [Theory]
         [WithTestPatternImages(110, 110, PixelTypes.Rgb24)]
         [WithTestPatternImages(170, 170, PixelTypes.Rgb24)]
@@ -162,5 +163,35 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
                 image.CompareToReferenceOutput(ValidatorComparer, provider);
             }
         }
+
+        [Theory]
+        [WithTestPatternImages(5120, 9234, PixelTypes.L16)]
+        public unsafe void Issue1640<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage();
+
+            // https://github.com/SixLabors/ImageSharp/discussions/1640
+            for (int i = 0; i < 2; i++)
+            {
+                var options = new HistogramEqualizationOptions
+                {
+                    Method = HistogramEqualizationMethod.AdaptiveTileInterpolation,
+                    LuminanceLevels = 4096,
+                    ClipHistogram = false,
+                    ClipLimit = 350,
+                    NumberOfTiles = 8
+                };
+
+                Image<TPixel> processed = image.Clone(ctx =>
+                {
+                    ctx.HistogramEqualization(options);
+                    ctx.Resize(image.Width / 4, image.Height / 4, KnownResamplers.Bicubic);
+                });
+
+                processed.DebugSave(provider);
+                processed.CompareToReferenceOutput(ValidatorComparer, provider);
+            }
+        }
     }
 }

tests/Images/External/ReferenceOutput/HistogramEqualizationTests/Issue1640_L16_TestPattern5120x9234.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2e6bff82eaedcd43932a5bd11d1feeea2143f00ab2ee5fe0654a403bba9ba2de
+size 424844