Add support for encoding 1 bit per pixel bitmaps

src/ImageSharp/Formats/Bmp/BmpBitsPerPixel.cs

@@ -8,6 +8,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// </summary>
     public enum BmpBitsPerPixel : short
     {
+        /// <summary>
+        /// 1 bit per pixel.
+        /// </summary>
+        Pixel1 = 1,
+
         /// <summary>
         /// 4 bits per pixel.
         /// </summary>

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -1303,16 +1303,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             short bitsPerPixel = this.infoHeader.BitsPerPixel;
             this.bmpMetadata = this.metadata.GetBmpMetadata();
             this.bmpMetadata.InfoHeaderType = infoHeaderType;
-
-            // We can only encode at these bit rates so far (1 bit per pixel is still missing).
-            if (bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel4)
-                || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel8)
-                || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel16)
-                || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel24)
-                || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel32))
-            {
-                this.bmpMetadata.BitsPerPixel = (BmpBitsPerPixel)bitsPerPixel;
-            }
+            this.bmpMetadata.BitsPerPixel = (BmpBitsPerPixel)bitsPerPixel;
         }
 
         /// <summary>

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -56,6 +56,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         private const int ColorPaletteSize4Bit = 64;
 
+        /// <summary>
+        /// The color palette for an 1 bit image will have 2 entry's with 4 bytes for each entry.
+        /// </summary>
+        private const int ColorPaletteSize1Bit = 8;
+
         /// <summary>
         /// Used for allocating memory during processing operations.
         /// </summary>
@@ -79,7 +84,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         private readonly bool writeV4Header;
 
         /// <summary>
-        /// The quantizer for reducing the color count for 8-Bit images.
+        /// The quantizer for reducing the color count for 8-Bit, 4-Bit and 1-Bit images.
         /// </summary>
         private readonly IQuantizer quantizer;
 
@@ -180,6 +185,10 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             {
                 colorPaletteSize = ColorPaletteSize4Bit;
             }
+            else if (this.bitsPerPixel == BmpBitsPerPixel.Pixel1)
+            {
+                colorPaletteSize = ColorPaletteSize1Bit;
+            }
 
             var fileHeader = new BmpFileHeader(
                 type: BmpConstants.TypeMarkers.Bitmap,
@@ -241,6 +250,10 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 case BmpBitsPerPixel.Pixel4:
                     this.Write4BitColor(stream, image);
                     break;
+
+                case BmpBitsPerPixel.Pixel1:
+                    this.Write1BitColor(stream, image);
+                    break;
             }
         }
 
@@ -325,7 +338,7 @@ 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.
+        /// 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>
@@ -349,7 +362,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         }
 
         /// <summary>
-        /// Writes an 8 Bit color image with a color palette. The color palette has 256 entry's with 4 bytes for each entry.
+        /// Writes an 8 bit color 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>
@@ -377,7 +390,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         }
 
         /// <summary>
-        /// Writes an 8 Bit gray image with a color palette. The color palette has 256 entry's with 4 bytes for each entry.
+        /// Writes an 8 bit gray 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>
@@ -415,7 +428,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         }
 
         /// <summary>
-        /// Writes an 4 Bit color image with a color palette. The color palette has 16 entry's with 4 bytes for each entry.
+        /// Writes an 4 bit color image with a color palette. The color palette has 16 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>
@@ -458,6 +471,52 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             }
         }
 
+        /// <summary>
+        /// Writes a 1 bit image with a color palette. The color palette has 2 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 Write1BitColor<TPixel>(Stream stream, ImageFrame<TPixel> image)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using IQuantizer<TPixel> frameQuantizer = this.quantizer.CreatePixelSpecificQuantizer<TPixel>(this.configuration, new QuantizerOptions()
+            {
+                MaxColors = 2
+            });
+            using IndexedImageFrame<TPixel> quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(image, image.Bounds());
+            using IMemoryOwner<byte> colorPaletteBuffer = this.memoryAllocator.AllocateManagedByteBuffer(ColorPaletteSize1Bit, AllocationOptions.Clean);
+
+            Span<byte> colorPalette = colorPaletteBuffer.GetSpan();
+            ReadOnlySpan<TPixel> quantizedColorPalette = quantized.Palette.Span;
+            this.WriteColorPalette(stream, quantizedColorPalette, colorPalette);
+
+            ReadOnlySpan<byte> quantizedPixelRow = quantized.GetPixelRowSpan(0);
+            int rowPadding = quantizedPixelRow.Length % 8 != 0 ? this.padding - 1 : this.padding;
+            for (int y = image.Height - 1; y >= 0; y--)
+            {
+                quantizedPixelRow = quantized.GetPixelRowSpan(y);
+
+                int endIdx = quantizedPixelRow.Length % 8 == 0 ? quantizedPixelRow.Length : quantizedPixelRow.Length - 8;
+                for (int i = 0; i < endIdx; i += 8)
+                {
+                    Write1BitPalette(stream, i, i + 8, quantizedPixelRow);
+                }
+
+                if (quantizedPixelRow.Length % 8 != 0)
+                {
+                    int startIdx = quantizedPixelRow.Length - 7;
+                    endIdx = quantizedPixelRow.Length;
+                    Write1BitPalette(stream, startIdx, endIdx, quantizedPixelRow);
+                }
+
+                for (int i = 0; i < rowPadding; i++)
+                {
+                    stream.WriteByte(0);
+                }
+            }
+        }
+
         /// <summary>
         /// Writes the color palette to the stream. The color palette has 4 bytes for each entry.
         /// </summary>
@@ -478,5 +537,25 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             stream.Write(colorPalette);
         }
+
+        /// <summary>
+        /// Writes a 1-bit palette.
+        /// </summary>
+        /// <param name="stream">The stream to write the palette to.</param>
+        /// <param name="startIdx">The start index.</param>
+        /// <param name="endIdx">The end index.</param>
+        /// <param name="quantizedPixelRow">A quantized pixel row.</param>
+        private static void Write1BitPalette(Stream stream, int startIdx, int endIdx, ReadOnlySpan<byte> quantizedPixelRow)
+        {
+            int shift = 7;
+            byte indices = 0;
+            for (int j = startIdx; j < endIdx; j++)
+            {
+                indices = (byte)(indices | ((byte)(quantizedPixelRow[j] & 1) << shift));
+                shift--;
+            }
+
+            stream.WriteByte(indices);
+        }
     }
 }

src/ImageSharp/Formats/Bmp/IBmpEncoderOptions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 using SixLabors.ImageSharp.Processing.Processors.Quantization;
@@ -24,8 +24,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         bool SupportTransparency { get; }
 
         /// <summary>
-        /// Gets the quantizer for reducing the color count for 8-Bit images.
+        /// Gets the quantizer for reducing the color count for 8-Bit, 4-Bit, and 1-Bit images.
+        /// Defaults to the Octree Quantizer.
         /// </summary>
         IQuantizer Quantizer { get; }
     }
-}
+}

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

@@ -40,6 +40,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         public static readonly TheoryData<string, BmpBitsPerPixel> BmpBitsPerPixelFiles =
         new TheoryData<string, BmpBitsPerPixel>
         {
+            { Bit1, BmpBitsPerPixel.Pixel1 },
             { Bit4, BmpBitsPerPixel.Pixel4 },
             { Bit8, BmpBitsPerPixel.Pixel8 },
             { Rgb16, BmpBitsPerPixel.Pixel16 },
@@ -201,6 +202,42 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
             }
         }
 
+        [Theory]
+        [WithFile(Bit1, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel1)]
+        public void Encode_1Bit_WithV3Header_Works<TPixel>(
+            TestImageProvider<TPixel> provider,
+            BmpBitsPerPixel bitsPerPixel)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            // The Magick Reference Decoder can not decode 1-Bit bitmaps, so only execute this on windows.
+            if (TestEnvironment.IsWindows)
+            {
+                TestBmpEncoderCore(
+                    provider,
+                    bitsPerPixel,
+                    supportTransparency: false,
+                    quantizer: KnownQuantizers.Wu);  // using the wu quantizer, because the octree seems to have problems with just two colors.
+            }
+        }
+
+        [Theory]
+        [WithFile(Bit1, PixelTypes.Rgba32, BmpBitsPerPixel.Pixel1)]
+        public void Encode_1Bit_WithV4Header_Works<TPixel>(
+            TestImageProvider<TPixel> provider,
+            BmpBitsPerPixel bitsPerPixel)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            // The Magick Reference Decoder can not decode 1-Bit bitmaps, so only execute this on windows.
+            if (TestEnvironment.IsWindows)
+            {
+                TestBmpEncoderCore(
+                    provider,
+                    bitsPerPixel,
+                    supportTransparency: true,
+                    quantizer: KnownQuantizers.Wu); // using the wu quantizer, because the octree seems to have problems with just two colors.
+            }
+        }
+
         [Theory]
         [WithFile(Bit8Gs, PixelTypes.L8, BmpBitsPerPixel.Pixel8)]
         public void Encode_8BitGray_WithV4Header_Works<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
@@ -297,7 +334,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         private static void TestBmpEncoderCore<TPixel>(
             TestImageProvider<TPixel> provider,
             BmpBitsPerPixel bitsPerPixel,
-            bool supportTransparency = true,
+            bool supportTransparency = true, // if set to true, will write a V4 header, otherwise a V3 header.
+            IQuantizer quantizer = null,
             ImageComparer customComparer = null)
             where TPixel : unmanaged, IPixel<TPixel>
         {
@@ -309,7 +347,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
                     image.Mutate(c => c.MakeOpaque());
                 }
 
-                var encoder = new BmpEncoder { BitsPerPixel = bitsPerPixel, SupportTransparency = supportTransparency };
+                var encoder = new BmpEncoder
+                {
+                    BitsPerPixel = bitsPerPixel,
+                    SupportTransparency = supportTransparency,
+                    Quantizer = quantizer ?? KnownQuantizers.Octree
+                };
 
                 // Does DebugSave & load reference CompareToReferenceInput():
                 image.VerifyEncoder(provider, "bmp", bitsPerPixel, encoder, customComparer);