fixed issue with the border tiles, when tile width != tile height

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

@@ -100,25 +100,27 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             this.ProcessBorderColumn(source, pixels, cdfData, 0, tileWidth, tileHeight, xStart: 0, xEnd: halfTileWidth);
 
             // fix right column
-            this.ProcessBorderColumn(source, pixels, cdfData, this.Tiles - 1, tileWidth, tileHeight, xStart: source.Width - halfTileWidth, xEnd: source.Width);
+            int rightBorderStartX = ((this.Tiles - 1) * tileWidth) + halfTileWidth;
+            this.ProcessBorderColumn(source, pixels, cdfData, this.Tiles - 1, tileWidth, tileHeight, xStart: rightBorderStartX, xEnd: source.Width);
 
             // fix top row
             this.ProcessBorderRow(source, pixels, cdfData, 0, tileWidth, tileHeight, yStart: 0, yEnd: halfTileHeight);
 
             // fix bottom row
-            this.ProcessBorderRow(source, pixels, cdfData, this.Tiles - 1, tileWidth, tileHeight, yStart: source.Height - halfTileHeight, yEnd: source.Height);
+            int bottomBorderStartY = ((this.Tiles - 1) * tileHeight) + halfTileHeight;
+            this.ProcessBorderRow(source, pixels, cdfData, this.Tiles - 1, tileWidth, tileHeight, yStart: bottomBorderStartY, yEnd: source.Height);
 
             // left top corner
             this.ProcessCornerTile(source, pixels, cdfData[0, 0], xStart: 0, xEnd: halfTileWidth, yStart: 0, yEnd: halfTileHeight, pixelsInTile: pixelsInTile);
 
             // left bottom corner
-            this.ProcessCornerTile(source, pixels, cdfData[0, this.Tiles - 1], xStart: 0, xEnd: halfTileWidth, yStart: source.Height - halfTileHeight, yEnd: source.Height, pixelsInTile: pixelsInTile);
+            this.ProcessCornerTile(source, pixels, cdfData[0, this.Tiles - 1], xStart: 0, xEnd: halfTileWidth, yStart: bottomBorderStartY, yEnd: source.Height, pixelsInTile: pixelsInTile);
 
             // right top corner
-            this.ProcessCornerTile(source, pixels, cdfData[this.Tiles - 1, 0], xStart: source.Width - halfTileWidth, xEnd: source.Width, yStart: 0, yEnd: halfTileHeight, pixelsInTile: pixelsInTile);
+            this.ProcessCornerTile(source, pixels, cdfData[this.Tiles - 1, 0], xStart: rightBorderStartX, xEnd: source.Width, yStart: 0, yEnd: halfTileHeight, pixelsInTile: pixelsInTile);
 
             // right bottom corner
-            this.ProcessCornerTile(source, pixels, cdfData[this.Tiles - 1, this.Tiles - 1], xStart: source.Width - halfTileWidth, xEnd: source.Width, yStart: source.Height - halfTileHeight, yEnd: source.Height, pixelsInTile: pixelsInTile);
+            this.ProcessCornerTile(source, pixels, cdfData[this.Tiles - 1, this.Tiles - 1], xStart: rightBorderStartX, xEnd: source.Width, yStart: bottomBorderStartY, yEnd: source.Height, pixelsInTile: pixelsInTile);
         }
 
         /// <summary>
@@ -162,7 +164,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             int pixelsInTile = tileWidth * tileHeight;
 
             int cdfY = 0;
-            for (int y = halfTileWidth; y < source.Height - halfTileWidth; y += tileHeight)
+            for (int y = halfTileHeight; y < source.Height - halfTileHeight; y += tileHeight)
             {
                 int yLimit = Math.Min(y + tileHeight, source.Height - 1);
                 int tileY = 0;

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

@@ -130,8 +130,6 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         /// <returns>A pixel row of the length of the tile width.</returns>
         private Span<TPixel> GetPixelRow(ImageFrame<TPixel> source, Span<TPixel> rowPixels, int x, int y, int tileWidth)
         {
-            rowPixels.Clear();
-
             if (y < 0)
             {
                 y = Math.Abs(y);
@@ -145,6 +143,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             // Special cases for the left and the right border where GetPixelRowSpan can not be used
             if (x < 0)
             {
+                rowPixels.Clear();
                 int idx = 0;
                 for (int dx = x; dx < x + tileWidth; dx++)
                 {
@@ -156,6 +155,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             }
             else if (x + tileWidth > source.Width)
             {
+                rowPixels.Clear();
                 int idx = 0;
                 for (int dx = x; dx < x + tileWidth; dx++)
                 {

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

@@ -19,7 +19,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         AdaptiveTileInterpolation,
 
         /// <summary>
-        /// Adaptive sliding window histogram equalization.
+        /// Adaptive histogram equalization using sliding window. Slower then the tile interpolation mode, but can yield to better results.
         /// </summary>
         AdaptiveSlidingWindow,
     }

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

@@ -20,19 +20,19 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         public int LuminanceLevels { get; set; } = 256;
 
         /// <summary>
-        /// Gets or sets a value indicating whether to clip the histogram bins at a specific value. Defaults to false.
+        /// Gets or sets a value indicating whether to clip the histogram bins at a specific value. Defaults to true.
         /// </summary>
-        public bool ClipHistogram { get; set; } = false;
+        public bool ClipHistogram { get; set; } = true;
 
         /// <summary>
-        /// Gets or sets the histogram clip limit in percent of the total pixels in the grid. Histogram bins which exceed this limit, will be capped at this value.
+        /// Gets or sets the histogram clip limit in percent of the total pixels in a tile. Histogram bins which exceed this limit, will be capped at this value.
         /// Defaults to 0.35.
         /// </summary>
         public float ClipLimitPercentage { get; set; } = 0.035f;
 
         /// <summary>
-        /// Gets or sets the number of tiles the image is split into (horizontal and vertically) for the adaptive histogram equalization. Defaults to 8.
+        /// Gets or sets the number of tiles the image is split into (horizontal and vertically) for the adaptive histogram equalization. Defaults to 10.
         /// </summary>
-        public int Tiles { get; set; } = 8;
+        public int Tiles { get; set; } = 10;
     }
 }