Add support for encoding 8-bit bitmaps

src/ImageSharp/Formats/Bmp/BmpBitsPerPixel.cs

@@ -4,10 +4,15 @@
 namespace SixLabors.ImageSharp.Formats.Bmp
 {
     /// <summary>
-    /// Enumerates the available bits per pixel for bitmap.
+    /// Enumerates the available bits per pixel the bitmap encoder supports.
     /// </summary>
     public enum BmpBitsPerPixel : short
     {
+        /// <summary>
+        /// 8 bits per pixel. Each pixel consists of 1 byte.
+        /// </summary>
+        Pixel8 = 8,
+
         /// <summary>
         /// 16 bits per pixel. Each pixel consists of 2 bytes.
         /// </summary>

src/ImageSharp/Formats/Bmp/BmpEncoder.cs

@@ -10,7 +10,6 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// <summary>
     /// Image encoder for writing an image to a stream as a Windows bitmap.
     /// </summary>
-    /// <remarks>The encoder can currently only write 24-bit rgb images to streams.</remarks>
     public sealed class BmpEncoder : IImageEncoder, IBmpEncoderOptions
     {
         /// <summary>

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -9,6 +9,7 @@ using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
 using SixLabors.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Bmp
@@ -43,6 +44,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         private const int Rgba32BlueMask = 0xFF;
 
+        /// <summary>
+        /// The color palette for an 8 bit image will have 256 entry's with 4 bytes for each entry.
+        /// </summary>
+        private const int ColorPaletteSize8Bit = 1024;
+
         private readonly MemoryAllocator memoryAllocator;
 
         private Configuration configuration;
@@ -142,11 +148,13 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 infoHeader.Compression = BmpCompression.BitFields;
             }
 
+            int colorPaletteSize = this.bitsPerPixel == BmpBitsPerPixel.Pixel8 ? ColorPaletteSize8Bit : 0;
+
             var fileHeader = new BmpFileHeader(
                 type: BmpConstants.TypeMarkers.Bitmap,
                 fileSize: BmpFileHeader.Size + infoHeaderSize + infoHeader.ImageSize,
                 reserved: 0,
-                offset: BmpFileHeader.Size + infoHeaderSize);
+                offset: BmpFileHeader.Size + infoHeaderSize + colorPaletteSize);
 
 #if NETCOREAPP2_1
             Span<byte> buffer = stackalloc byte[infoHeaderSize];
@@ -198,6 +206,10 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 case BmpBitsPerPixel.Pixel16:
                     this.Write16Bit(stream, pixels);
                     break;
+
+                case BmpBitsPerPixel.Pixel8:
+                    this.Write8Bit(stream, image);
+                    break;
             }
         }
 
@@ -276,5 +288,51 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 }
             }
         }
+
+        /// <summary>
+        /// Writes an 8 Bit image with a color palette. The color palette has 256 entry's with 4 bytes for each entry.
+        /// </summary>
+        /// <typeparam name="TPixel">The type of the pixel.</typeparam>
+        /// <param name="stream">The <see cref="Stream"/> to write to.</param>
+        /// <param name="image"> The <see cref="ImageFrame{TPixel}"/> containing pixel data.</param>
+        private void Write8Bit<TPixel>(Stream stream, ImageFrame<TPixel> image)
+            where TPixel : struct, IPixel<TPixel>
+        {
+#if NETCOREAPP2_1
+            Span<byte> colorPalette = stackalloc byte[ColorPaletteSize8Bit];
+#else
+            byte[] colorPalette = new byte[ColorPaletteSize8Bit];
+#endif
+
+            var quantizer = new WuQuantizer(256);
+            QuantizedFrame<TPixel> quantized = quantizer.CreateFrameQuantizer<TPixel>(this.configuration).QuantizeFrame(image);
+
+            int idx = 0;
+            var color = default(Rgba32);
+            foreach (TPixel quantizedColor in quantized.Palette)
+            {
+                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;
+            }
+
+            stream.Write(colorPalette, 0, ColorPaletteSize8Bit);
+
+            for (int y = image.Height - 1; y >= 0; y--)
+            {
+                Span<byte> pixelSpan = quantized.GetRowSpan(y);
+                stream.Write(pixelSpan);
+
+                for (int i = 0; i < this.padding; i++)
+                {
+                    stream.WriteByte(0);
+                }
+            }
+        }
     }
 }

src/ImageSharp/Formats/Bmp/IBmpEncoderOptions.cs

@@ -6,7 +6,6 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// <summary>
     /// Configuration options for use during bmp encoding
     /// </summary>
-    /// <remarks>The encoder can currently only write 16-bit, 24-bit and 32-bit rgb images to streams.</remarks>
     internal interface IBmpEncoderOptions
     {
         /// <summary>

src/ImageSharp/Processing/Processors/Quantization/QuantizedFrame{TPixel}.cs

@@ -62,7 +62,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
 
         /// <summary>
         /// Gets the representation of the pixels as a <see cref="Span{T}"/> of contiguous memory
-        /// at row <paramref name="rowIndex"/> beginning from the the first pixel on that row.
+        /// at row <paramref name="rowIndex"/> beginning from the first pixel on that row.
         /// </summary>
         /// <param name="rowIndex">The row.</param>
         /// <returns>The <see cref="Span{T}"/></returns>

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

@@ -114,6 +114,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         [WithFile(WinBmpv3, PixelTypes.Rgb24, BmpBitsPerPixel.Pixel24)]
         [WithFile(Rgb16, PixelTypes.Bgra5551, BmpBitsPerPixel.Pixel16)]
         [WithFile(Bit16, PixelTypes.Bgra5551, BmpBitsPerPixel.Pixel16)]
+        [WithFile(Bit8Palette4, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel8)]
+        [WithFile(Bit8Gs, PixelTypes.Gray8, BmpBitsPerPixel.Pixel8)]
         public void Encode_WithV3Header_Works<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
             // if supportTransparency is false, a v3 bitmap header will be written
             where TPixel : struct, IPixel<TPixel> => TestBmpEncoderCore(provider, bitsPerPixel, supportTransparency: false);
@@ -126,11 +128,14 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         [WithFile(WinBmpv3, PixelTypes.Rgb24, BmpBitsPerPixel.Pixel24)]
         [WithFile(Rgb16, PixelTypes.Bgra5551, BmpBitsPerPixel.Pixel16)]
         [WithFile(Bit16, PixelTypes.Bgra5551, BmpBitsPerPixel.Pixel16)]
+        [WithFile(Bit8Palette4, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel8)]
+        [WithFile(Bit8Gs, PixelTypes.Gray8, BmpBitsPerPixel.Pixel8)]
         public void Encode_WithV4Header_Works<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
             where TPixel : struct, IPixel<TPixel> => TestBmpEncoderCore(provider, bitsPerPixel, supportTransparency: true);
 
         [Theory]
         [WithFile(TestImages.Png.GrayAlpha2BitInterlaced, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel32)]
+        [WithFile(Bit32Rgba, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel32)]
         public void Encode_PreservesAlpha<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
             where TPixel : struct, IPixel<TPixel> => TestBmpEncoderCore(provider, bitsPerPixel, supportTransparency: true);
 
@@ -139,8 +144,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                // There is no alpha in bmp with 24 bits per pixels, so the reference image will be made opaque.
+                // There is no alpha in bmp with less then 32 bits per pixels, so the reference image will be made opaque.
-                if (bitsPerPixel == BmpBitsPerPixel.Pixel24)
+                if (bitsPerPixel != BmpBitsPerPixel.Pixel32)
                 {
                     image.Mutate(c => c.MakeOpaque());
                 }

tests/ImageSharp.Tests/TestImages.cs

@@ -236,6 +236,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Bit1Pal1 = "Bmp/pal1p1.bmp";
             public const string Bit4 = "Bmp/pal4.bmp";
             public const string Bit8 = "Bmp/test8.bmp";
+            public const string Bit8Gs = "Bmp/pal8gs.bmp";
             public const string Bit8Inverted = "Bmp/test8-inverted.bmp";
             public const string Bit16 = "Bmp/test16.bmp";
             public const string Bit16Inverted = "Bmp/test16-inverted.bmp";

tests/ImageSharp.Tests/TestUtilities/PixelTypes.cs

@@ -60,6 +60,8 @@ namespace SixLabors.ImageSharp.Tests
 
         Bgra5551 = 1 << 22,
 
+        Gray8 = 1 << 23,
+
         // TODO: Add multi-flag entries by rules defined in PackedPixelConverterHelper
 
         // "All" is handled as a separate, individual case instead of using bitwise OR