Merge pull request #1428 from SixLabors/bp/bmp8BitBulkConv

Use bulk conversion in Write8BitColor

Author: JimBobSquarePants

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -6,7 +6,6 @@
 using System.IO;
 using System.Runtime.InteropServices;
 using System.Threading;
-using System.Threading.Tasks;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Memory;
@@ -342,20 +341,11 @@ private void Write8BitColor<TPixel>(Stream stream, ImageFrame<TPixel> image, Spa
             using IndexedImageFrame<TPixel> quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(image, image.Bounds());
 
             ReadOnlySpan<TPixel> quantizedColors = quantized.Palette.Span;
-            var color = default(Rgba32);
-
-            // TODO: Use bulk conversion here for better perf
-            int idx = 0;
-            foreach (TPixel quantizedColor in quantizedColors)
+            PixelOperations<TPixel>.Instance.ToBgra32(this.configuration, quantizedColors, MemoryMarshal.Cast<byte, Bgra32>(colorPalette));
+            Span<uint> colorPaletteAsUInt = MemoryMarshal.Cast<byte, uint>(colorPalette);
+            for (int i = 0; i < colorPaletteAsUInt.Length; i++)
             {
-                quantizedColor.ToRgba32(ref color);
-                colorPalette[idx] = color.B;
-                colorPalette[idx + 1] = color.G;
-                colorPalette[idx + 2] = color.R;
-
-                // Padding byte, always 0.
-                colorPalette[idx + 3] = 0;
-                idx += 4;
+                colorPaletteAsUInt[i] = colorPaletteAsUInt[i] & 0x00FFFFFF; // Padding byte, always 0.
             }
 
             stream.Write(colorPalette);