Minor improvements to diffusion

src/ImageSharp/Processing/Dithering/ErrorDiffusion/ErrorDiffuserBase.cs

@@ -78,6 +78,19 @@ namespace SixLabors.ImageSharp.Processing.Dithering.ErrorDiffusion
             // Calculate the error
             Vector4 error = source.ToVector4() - transformed.ToVector4();
 
+            // No error? Break out as there's nothing to pass.
+            if (error.Equals(Vector4.Zero))
+            {
+                return;
+            }
+
+            this.DoDither(image, x, y, minX, minY, maxX, maxY, error);
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void DoDither<TPixel>(ImageFrame<TPixel> image, int x, int y, int minX, int minY, int maxX, int maxY, Vector4 error)
+            where TPixel : struct, IPixel<TPixel>
+        {
             // Loop through and distribute the error amongst neighboring pixels.
             for (int row = 0; row < this.matrixHeight; row++)
             {

src/ImageSharp/Processing/Quantization/FrameQuantizers/FrameQuantizerBase{TPixel}.cs

@@ -125,9 +125,9 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
         protected byte GetClosestPixel(TPixel pixel, TPixel[] colorPalette, Dictionary<TPixel, byte> cache)
         {
             // Check if the color is in the lookup table
-            if (cache.ContainsKey(pixel))
+            if (cache.TryGetValue(pixel, out byte value))
             {
-                return cache[pixel];
+                return value;
             }
 
             return this.GetClosestPixelSlow(pixel, colorPalette, cache);

src/ImageSharp/Processing/Quantization/FrameQuantizers/OctreeFrameQuantizer{TPixel}.cs

@@ -223,11 +223,6 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             /// </summary>
             private readonly OctreeNode root;
 
-            /// <summary>
-            /// Array of reducible nodes
-            /// </summary>
-            private readonly OctreeNode[] reducibleNodes;
-
             /// <summary>
             /// Maximum number of significant bits in the image
             /// </summary>
@@ -253,7 +248,7 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             {
                 this.maxColorBits = maxColorBits;
                 this.Leaves = 0;
-                this.reducibleNodes = new OctreeNode[9];
+                this.ReducibleNodes = new OctreeNode[9];
                 this.root = new OctreeNode(0, this.maxColorBits, this);
                 this.previousColor = default;
                 this.previousNode = null;
@@ -262,12 +257,12 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             /// <summary>
             /// Gets or sets the number of leaves in the tree
             /// </summary>
-            private int Leaves { get; set; }
+            public int Leaves { get; set; }
 
             /// <summary>
             /// Gets the array of reducible nodes
             /// </summary>
-            private OctreeNode[] ReducibleNodes => this.reducibleNodes;
+            private OctreeNode[] ReducibleNodes { get; }
 
             /// <summary>
             /// Add a given color value to the Octree
@@ -354,14 +349,14 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             {
                 // Find the deepest level containing at least one reducible node
                 int index = this.maxColorBits - 1;
-                while ((index > 0) && (this.reducibleNodes[index] == null))
+                while ((index > 0) && (this.ReducibleNodes[index] == null))
                 {
                     index--;
                 }
 
                 // Reduce the node most recently added to the list at level 'index'
-                OctreeNode node = this.reducibleNodes[index];
-                this.reducibleNodes[index] = node.NextReducible;
+                OctreeNode node = this.ReducibleNodes[index];
+                this.ReducibleNodes[index] = node.NextReducible;
 
                 // Decrement the leaf count after reducing the node
                 this.Leaves -= node.Reduce();