Optimize dither

README.md

@@ -119,7 +119,7 @@ using (Image<Rgba32> image = new Image<Rgba32>(400, 400)
 For optimized access within a loop it is recommended that the following methods are used.
 
 1. `image.GetRowSpan(y)`
-2. `image.GetRowSPan(x, y)`
+2. `image.GetRowSpan(x, y)`
 
 For advanced pixel format usage there are multiple [PixelFormat implementations](https://github.com/JimBobSquarePants/ImageSharp/tree/master/src/ImageSharp/PixelFormats) available allowing developers to implement their own color models in the same manner as Microsoft XNA Game Studio and MonoGame. 
 

src/ImageSharp/Processing/Processors/Binarization/BinaryThresholdProcessor.cs

@@ -83,25 +83,22 @@ namespace ImageSharp.Processing.Processors
                 startY = 0;
             }
 
-            using (PixelAccessor<TPixel> sourcePixels = source.Lock())
+            Parallel.For(
-            {
+                minY,
-                Parallel.For(
+                maxY,
-                    minY,
+                this.ParallelOptions,
-                    maxY,
+                y =>
-                    this.ParallelOptions,
+                {
-                    y =>
+                    Span<TPixel> row = source.GetRowSpan(y - startY);
+
+                    for (int x = minX; x < maxX; x++)
                     {
-                        int offsetY = y - startY;
+                        ref TPixel color = ref row[x - startX];
-                        for (int x = minX; x < maxX; x++)
+
-                        {
+                        // Any channel will do since it's Grayscale.
-                            int offsetX = x - startX;
+                        color = color.ToVector4().X >= threshold ? upper : lower;
-                            TPixel color = sourcePixels[offsetX, offsetY];
+                    }
-
+                });
-                            // Any channel will do since it's Grayscale.
-                            sourcePixels[offsetX, offsetY] = color.ToVector4().X >= threshold ? upper : lower;
-                        }
-                    });
-            }
         }
     }
 }

src/ImageSharp/Processing/Processors/Binarization/OrderedDitherProcessor.cs

@@ -97,11 +97,12 @@ namespace ImageSharp.Processing.Processors
             for (int y = minY; y < maxY; y++)
             {
                 int offsetY = y - startY;
+                Span<TPixel> row = source.GetRowSpan(offsetY);
 
                 for (int x = minX; x < maxX; x++)
                 {
                     int offsetX = x - startX;
-                    TPixel sourceColor = source[offsetX, offsetY];
+                    TPixel sourceColor = row[offsetX];
                     this.Dither.Dither(source, sourceColor, this.UpperColor, this.LowerColor, bytes, this.Index, offsetX, offsetY, maxX, maxY);
                 }
             }