Fix and optimize error diffusion

src/ImageSharp/Dithering/ErrorDiffusion/ErrorDiffuserBase.cs

@@ -58,7 +58,7 @@ namespace SixLabors.ImageSharp.Dithering.Base
             this.startingOffset = 0;
             for (int i = 0; i < this.matrixWidth; i++)
             {
-                // Good to disable here as we are not comparing matematical output.
+                // Good to disable here as we are not comparing mathematical output.
                 // ReSharper disable once CompareOfFloatsByEqualityOperator
                 if (matrix[0, i] != 0)
                 {
@@ -90,7 +90,7 @@ namespace SixLabors.ImageSharp.Dithering.Base
             // Calculate the error
             Vector4 error = source.ToVector4() - transformed.ToVector4();
 
-            // Loop through and distribute the error amongst neighbouring pixels.
+            // Loop through and distribute the error amongst neighboring pixels.
             for (int row = 0; row < this.matrixHeight; row++)
             {
                 int matrixY = y + row;
@@ -115,10 +115,8 @@ namespace SixLabors.ImageSharp.Dithering.Base
 
                             ref TPixel pixel = ref rowSpan[matrixX];
                             var offsetColor = pixel.ToVector4();
-                            var coefficientVector = new Vector4(coefficient);
 
-                            Vector4 result = ((error * coefficientVector) / this.divisorVector) + offsetColor;
-                            result.W = offsetColor.W;
+                            Vector4 result = ((error * coefficient) / this.divisorVector) + offsetColor;
                             pixel.PackFromVector4(result);
                         }
                     }

src/ImageSharp/Formats/Png/PngEncoder.cs

@@ -18,7 +18,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         public bool IgnoreMetadata { get; set; }
 
         /// <summary>
-        /// Gets or sets the size of the color palette to use. Set to zero to leav png encoding to use pixel data.
+        /// Gets or sets the size of the color palette to use. Set to zero to leave png encoding to use pixel data.
         /// </summary>
         public int PaletteSize { get; set; } = 0;
 

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -148,7 +148,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <summary>
         /// Initializes a new instance of the <see cref="PngEncoderCore"/> class.
         /// </summary>
-        /// <param name="options">The options for influancing the encoder</param>
+        /// <param name="options">The options for influencing the encoder</param>
         public PngEncoderCore(IPngEncoderOptions options)
         {
             this.ignoreMetadata = options.IgnoreMetadata;

src/ImageSharp/Quantizers/QuantizerBase{TPixel}.cs

@@ -40,7 +40,7 @@ namespace SixLabors.ImageSharp.Quantizers.Base
         }
 
         /// <inheritdoc />
-        public bool Dither { get; set; } = false;
+        public bool Dither { get; set; } = true;
 
         /// <inheritdoc />
         public IErrorDiffuser DitherType { get; set; } = new FloydSteinbergDiffuser();

src/ImageSharp/Quantizers/WuQuantizer{TPixel}.cs

@@ -281,7 +281,7 @@ namespace SixLabors.ImageSharp.Quantizers
         }
 
         /// <summary>
-        /// Gets the index index of the given color in the palette.
+        /// Gets the index of the given color in the palette.
         /// </summary>
         /// <param name="r">The red value.</param>
         /// <param name="g">The green value.</param>
@@ -827,7 +827,7 @@ namespace SixLabors.ImageSharp.Quantizers
         {
             if (this.Dither)
             {
-                // The colors have changed so we need to use Euclidean distance caclulation to find the closest value.
+                // The colors have changed so we need to use Euclidean distance calculation to find the closest value.
                 // This palette can never be null here.
                 return this.GetClosestPixel(pixel, this.palette, this.colorMap);
             }