Use float threshold for dither

src/ImageSharp/Dithering/Ordered/IOrderedDither.cs

@@ -21,7 +21,7 @@ namespace SixLabors.ImageSharp.Dithering
         /// <param name="x">The column index.</param>
         /// <param name="y">The row index.</param>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
-        void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, byte threshold, int x, int y)
+        void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, float threshold, int x, int y)
             where TPixel : struct, IPixel<TPixel>;
     }
 }

src/ImageSharp/Dithering/Ordered/OrderedDither.cs

@@ -41,7 +41,7 @@ namespace SixLabors.ImageSharp.Dithering
         }
 
         /// <inheritdoc />
-        public void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, byte threshold, int x, int y)
+        public void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, float threshold, int x, int y)
             where TPixel : struct, IPixel<TPixel>
         {
             image[x, y] = this.thresholdMatrix[y % this.modulusY, x % this.modulusX] >= threshold ? lower : upper;

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

@@ -93,7 +93,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
             sourcePixel.ToRgba32(ref rgba);
 
             // Convert to grayscale using ITU-R Recommendation BT.709 if required
-            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+            float luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
             for (int y = startY; y < endY; y++)
             {
@@ -108,7 +108,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                     if (!previousPixel.Equals(sourcePixel))
                     {
                         sourcePixel.ToRgba32(ref rgba);
-                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+                        luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
                         // Setup the previous pointer
                         previousPixel = sourcePixel;

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

@@ -74,7 +74,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
             sourcePixel.ToRgba32(ref rgba);
 
             // Convert to grayscale using ITU-R Recommendation BT.709 if required
-            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+            float luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
             for (int y = startY; y < endY; y++)
             {
@@ -89,7 +89,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                     if (!previousPixel.Equals(sourcePixel))
                     {
                         sourcePixel.ToRgba32(ref rgba);
-                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+                        luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
                         // Setup the previous pointer
                         previousPixel = sourcePixel;

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

@@ -82,7 +82,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
             sourcePixel.ToRgba32(ref rgba);
 
             // Convert to grayscale using ITU-R Recommendation BT.709 if required
-            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+            float luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
             for (int y = startY; y < endY; y++)
             {
@@ -98,7 +98,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                     {
                         pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
                         sourcePixel.ToRgba32(ref rgba);
-                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+                        luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
                         // Setup the previous pointer
                         previousPixel = sourcePixel;

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

@@ -63,7 +63,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
             sourcePixel.ToRgba32(ref rgba);
 
             // Convert to grayscale using ITU-R Recommendation BT.709 if required
-            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+            float luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
             for (int y = startY; y < endY; y++)
             {
@@ -79,7 +79,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                     {
                         pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
                         sourcePixel.ToRgba32(ref rgba);
-                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+                        luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
 
                         // Setup the previous pointer
                         previousPixel = sourcePixel;