Use DistanceSquared

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

@@ -42,8 +42,8 @@ namespace SixLabors.ImageSharp.Processing.Dithering.Processors
             }
 
             // Not found - loop through the palette and find the nearest match.
-            float leastDistance = int.MaxValue;
-            float secondLeastDistance = int.MaxValue;
+            float leastDistance = float.MaxValue;
+            float secondLeastDistance = float.MaxValue;
             var vector = pixel.ToVector4();
 
             TPixel closest = default;

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

@@ -125,11 +125,10 @@ 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.TryGetValue(pixel, out byte value))
-            {
-                return value;
-            }
-
+            // if (cache.TryGetValue(pixel, out byte value))
+            // {
+            //  return value;
+            // }
             return this.GetClosestPixelSlow(pixel, colorPalette, cache);
         }
 
@@ -137,34 +136,31 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
         private byte GetClosestPixelSlow(TPixel pixel, TPixel[] colorPalette, Dictionary<TPixel, byte> cache)
         {
             // Loop through the palette and find the nearest match.
-            byte colorIndex = 0;
-            float leastDistance = int.MaxValue;
+            int colorIndex = 0;
+            float leastDistance = float.MaxValue;
             var vector = pixel.ToVector4();
 
             for (int index = 0; index < colorPalette.Length; index++)
             {
-                float distance = Vector4.Distance(vector, colorPalette[index].ToVector4());
+                ref TPixel candidate = ref colorPalette[index];
+                float distance = Vector4.DistanceSquared(vector, candidate.ToVector4());
 
-                // Greater... Move on.
-                if (!(distance < leastDistance))
+                if (distance < leastDistance)
                 {
-                    continue;
+                    colorIndex = index;
+                    leastDistance = distance;
                 }
 
-                colorIndex = (byte)index;
-                leastDistance = distance;
-
-                // And if it's an exact match, exit the loop
-                if (MathF.Abs(distance) < Constants.Epsilon)
+                // If it's an exact match, exit the loop
+                if (distance == 0)
                 {
                     break;
                 }
             }
 
             // Now I have the index, pop it into the cache for next time
-            cache.Add(pixel, colorIndex);
-
-            return colorIndex;
+            // cache.Add(pixel, colorIndex);
+            return (byte)colorIndex;
         }
     }
 }

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
@@ -18,11 +19,6 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
     internal sealed class OctreeFrameQuantizer<TPixel> : FrameQuantizerBase<TPixel>
         where TPixel : struct, IPixel<TPixel>
     {
-        /// <summary>
-        /// A lookup table for colors
-        /// </summary>
-        private readonly Dictionary<TPixel, byte> colorMap = new Dictionary<TPixel, byte>();
-
         /// <summary>
         /// Maximum allowed color depth
         /// </summary>
@@ -33,6 +29,11 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
         /// </summary>
         private readonly Octree octree;
 
+        /// <summary>
+        /// A lookup table for colors
+        /// </summary>
+        private Dictionary<TPixel, byte> colorMap = new Dictionary<TPixel, byte>();
+
         /// <summary>
         /// The reduced image palette
         /// </summary>
@@ -476,7 +477,6 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
                                     | ((rgba.R & Mask[level]) >> shift);
 
                         OctreeNode child = this.children[index];
-
                         if (child == null)
                         {
                             // Create a new child node and store it in the array
@@ -501,12 +501,13 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
                     // Loop through all children and add their information to this node
                     for (int index = 0; index < 8; index++)
                     {
-                        if (this.children[index] != null)
+                        OctreeNode child = this.children[index];
+                        if (child != null)
                         {
-                            this.red += this.children[index].red;
-                            this.green += this.children[index].green;
-                            this.blue += this.children[index].blue;
-                            this.pixelCount += this.children[index].pixelCount;
+                            this.red += child.red;
+                            this.green += child.green;
+                            this.blue += child.blue;
+                            this.pixelCount += child.pixelCount;
                             ++childNodes;
                             this.children[index] = null;
                         }
@@ -528,14 +529,10 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
                 {
                     if (this.leaf)
                     {
-                        // This seems faster than using Vector4
-                        byte r = (this.red / this.pixelCount).ToByte();
-                        byte g = (this.green / this.pixelCount).ToByte();
-                        byte b = (this.blue / this.pixelCount).ToByte();
-
-                        // And set the color of the palette entry
+                        // Set the color of the palette entry
+                        var vector = Vector3.Clamp(new Vector3(this.red, this.green, this.blue) / this.pixelCount, Vector3.Zero, new Vector3(255));
                         TPixel pixel = default;
-                        pixel.PackFromRgba32(new Rgba32(r, g, b, 255));
+                        pixel.PackFromRgba32(new Rgba32((byte)vector.X, (byte)vector.Y, (byte)vector.Z, byte.MaxValue));
                         palette[index] = pixel;
 
                         // Consume the next palette index
@@ -573,13 +570,14 @@ namespace SixLabors.ImageSharp.Processing.Quantization.FrameQuantizers
                                          | ((rgba.G & Mask[level]) >> (shift - 1))
                                          | ((rgba.R & Mask[level]) >> shift);
 
-                        if (this.children[pixelIndex] != null)
+                        OctreeNode child = this.children[pixelIndex];
+                        if (child != null)
                         {
-                            index = this.children[pixelIndex].GetPaletteIndex(ref pixel, level + 1, ref rgba);
+                            index = child.GetPaletteIndex(ref pixel, level + 1, ref rgba);
                         }
                         else
                         {
-                            throw new Exception($"Cannot retrive a pixel at the given index {pixelIndex}.");
+                            throw new Exception($"Cannot retrieve a pixel at the given index {pixelIndex}.");
                         }
                     }