Smarter dithering.

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

@@ -75,15 +75,14 @@ namespace SixLabors.ImageSharp.Processing.Dithering.ErrorDiffusion
         {
             image[x, y] = transformed;
 
-            // Calculate the error
-            Vector4 error = source.ToVector4() - transformed.ToVector4();
-
-            // No error? Break out as there's nothing to pass.
-            if (error.Equals(Vector4.Zero))
+            // Equal? Break out as there's nothing to pass.
+            if (source.Equals(transformed))
             {
                 return;
             }
 
+            // Calculate the error
+            Vector4 error = source.ToVector4() - transformed.ToVector4();
             this.DoDither(image, x, y, minX, minY, maxX, maxY, error);
         }
 

src/ImageSharp/Processing/Dithering/Processors/ErrorDiffusionPaletteProcessor.cs

@@ -97,6 +97,13 @@ namespace SixLabors.ImageSharp.Processing.Dithering.Processors
                     if (!previousPixel.Equals(sourcePixel))
                     {
                         pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
+
+                        // No error to spread, exact match.
+                        if (sourcePixel.Equals(pair.First))
+                        {
+                            continue;
+                        }
+
                         sourcePixel.ToRgba32(ref rgba);
                         luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 

src/ImageSharp/Processing/Dithering/Processors/OrderedDitherPaletteProcessor.cs

@@ -78,6 +78,13 @@ namespace SixLabors.ImageSharp.Processing.Dithering.Processors
                     if (!previousPixel.Equals(sourcePixel))
                     {
                         pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
+
+                        // No error to spread, exact match.
+                        if (sourcePixel.Equals(pair.First))
+                        {
+                            continue;
+                        }
+
                         sourcePixel.ToRgba32(ref rgba);
                         luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 

src/ImageSharp/Processing/Dithering/Processors/PaletteDitherProcessorBase.cs

@@ -37,11 +37,17 @@ namespace SixLabors.ImageSharp.Processing.Dithering.Processors
         protected PixelPair<TPixel> GetClosestPixelPair(ref TPixel pixel, TPixel[] colorPalette)
         {
             // Check if the color is in the lookup table
-            if (this.cache.ContainsKey(pixel))
+            if (this.cache.TryGetValue(pixel, out PixelPair<TPixel> value))
             {
-                return this.cache[pixel];
+                return value;
             }
 
+            return this.GetClosestPixelPairSlow(ref pixel, colorPalette);
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private PixelPair<TPixel> GetClosestPixelPairSlow(ref TPixel pixel, TPixel[] colorPalette)
+        {
             // Not found - loop through the palette and find the nearest match.
             float leastDistance = float.MaxValue;
             float secondLeastDistance = float.MaxValue;
@@ -51,19 +57,19 @@ namespace SixLabors.ImageSharp.Processing.Dithering.Processors
             TPixel secondClosest = default;
             for (int index = 0; index < colorPalette.Length; index++)
             {
-                TPixel temp = colorPalette[index];
-                float distance = Vector4.DistanceSquared(vector, temp.ToVector4());
+                ref TPixel candidate = ref colorPalette[index];
+                float distance = Vector4.DistanceSquared(vector, candidate.ToVector4());
 
                 if (distance < leastDistance)
                 {
                     leastDistance = distance;
                     secondClosest = closest;
-                    closest = temp;
+                    closest = candidate;
                 }
                 else if (distance < secondLeastDistance)
                 {
                     secondLeastDistance = distance;
-                    secondClosest = temp;
+                    secondClosest = candidate;
                 }
             }
 

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

@@ -51,7 +51,7 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             : base(quantizer, false)
         {
             this.colors = (byte)quantizer.MaxColors;
-            this.octree = new Octree(this.GetBitsNeededForColorDepth(this.colors));
+            this.octree = new Octree(ImageMaths.GetBitsNeededForColorDepth(this.colors).Clamp(1, 8));
         }
 
         /// <inheritdoc/>
@@ -189,20 +189,6 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
             return (byte)this.octree.GetPaletteIndex(ref pixel, ref rgba);
         }
 
-        /// <summary>
-        /// Returns how many bits are required to store the specified number of colors.
-        /// Performs a Log2() on the value.
-        /// </summary>
-        /// <param name="colorCount">The number of colors.</param>
-        /// <returns>
-        /// The <see cref="int"/>
-        /// </returns>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private int GetBitsNeededForColorDepth(int colorCount)
-        {
-            return (int)Math.Ceiling(Math.Log(colorCount, 2));
-        }
-
         /// <summary>
         /// Class which does the actual quantization
         /// </summary>

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

@@ -30,7 +30,7 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
         public PaletteFrameQuantizer(PaletteQuantizer quantizer, TPixel[] colors)
             : base(quantizer, true)
         {
-            Guard.MustBeLessThanOrEqualTo(256, colors.Length, "Maximum color count must be 256.");
+            Guard.MustBeBetweenOrEqualTo(colors.Length, 1, 255, nameof(colors));
             this.colors = colors;
         }
 

tests/ImageSharp.Tests/Formats/GeneralFormatTests.cs

@@ -83,8 +83,7 @@ namespace SixLabors.ImageSharp.Tests
 
             using (Image<TPixel> image = provider.GetImage())
             {
-                image.Mutate(c => c.Quantize(quantizer));
-                image.DebugSave(provider, new PngEncoder() { ColorType = PngColorType.Palette }, testOutputDetails: quantizerName);
+                image.DebugSave(provider, new PngEncoder() { ColorType = PngColorType.Palette, Quantizer = quantizer }, testOutputDetails: quantizerName);
             }
 
             provider.Configuration.MemoryAllocator.ReleaseRetainedResources();