Add tests for quantization of 32 bit bitmap to 8 bit color image

tests/ImageSharp.Tests/Formats/Bmp/BmpEncoderTests.cs

@@ -3,10 +3,12 @@
 
 using System.IO;
 
+using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 
 using Xunit;
@@ -180,6 +182,48 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
                 bitsPerPixel,
                 supportTransparency: true);
 
+        [Theory]
+        [WithFile(Bit32Rgb, PixelTypes.Rgba32)]
+        public void Encode_8BitColor_WithWuQuantizer<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                var encoder = new BmpEncoder
+                                  {
+                                    BitsPerPixel = BmpBitsPerPixel.Pixel8,
+                                    Quantizer = new WuQuantizer(256)
+                                  };
+                string actualOutputFile = provider.Utility.SaveTestOutputFile(image, "bmp", encoder, appendPixelTypeToFileName: false);
+                IImageDecoder referenceDecoder = TestEnvironment.GetReferenceDecoder(actualOutputFile);
+                using (var referenceImage = Image.Load<TPixel>(actualOutputFile, referenceDecoder))
+                {
+                    referenceImage.CompareToReferenceOutput(ImageComparer.Exact, provider, extension: "bmp", appendPixelTypeToFileName: false);
+                }
+            }
+        }
+
+        [Theory]
+        [WithFile(Bit32Rgb, PixelTypes.Rgba32)]
+        public void Encode_8BitColor_WithOctreeQuantizer<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                var encoder = new BmpEncoder
+                                  {
+                                      BitsPerPixel = BmpBitsPerPixel.Pixel8,
+                                      Quantizer = new OctreeQuantizer(256)
+                                  };
+                string actualOutputFile = provider.Utility.SaveTestOutputFile(image, "bmp", encoder, appendPixelTypeToFileName: false);
+                IImageDecoder referenceDecoder = TestEnvironment.GetReferenceDecoder(actualOutputFile);
+                using (var referenceImage = Image.Load<TPixel>(actualOutputFile, referenceDecoder))
+                {
+                    referenceImage.CompareToReferenceOutput(ImageComparer.Exact, provider, extension: "bmp", appendPixelTypeToFileName: false);
+                }
+            }
+        }
+
         [Theory]
         [WithFile(TestImages.Png.GrayAlpha2BitInterlaced, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel32)]
         [WithFile(Bit32Rgba, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel32)]

tests/Images/External

@@ -1 +1 @@
-Subproject commit c057090b4402120a83a8efe251aa5b691db9c0dc
+Subproject commit 42b3b980ed07afd7b6603a5bfa6ffb91d6c8a124