Add support for Decoding BI_ALPHABITFIELDS (#832)

* Adds support for BI_ALPHABITFIELDS

* Fix for decoding bitmaps with a less than full sized palette

src/ImageSharp/Formats/Bmp/BmpCompression.cs

@@ -34,7 +34,6 @@ internal enum BmpCompression : int
         /// If the first byte is zero, the record has different meanings, depending
         /// on the second byte. If the second byte is zero, it is the end of the row,
         /// if it is one, it is the end of the image.
-        /// Not supported at the moment.
         /// </summary>
         RLE4 = 2,
 
@@ -54,6 +53,13 @@ internal enum BmpCompression : int
         /// The bitmap contains a PNG image.
         /// Not supported at the moment.
         /// </summary>
-        PNG = 5
+        PNG = 5,
+
+        /// <summary>
+        /// Introduced with Windows CE.
+        /// Specifies that the bitmap is not compressed and that the color table consists of four DWORD color
+        /// masks that specify the red, green, blue, and alpha components of each pixel.
+        /// </summary>
+        BI_ALPHABITFIELDS = 6
     }
 }

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -75,7 +75,6 @@ internal sealed class BmpDecoderCore
 
         /// <summary>
         /// The file header containing general information.
-        /// TODO: Why is this not used? We advance the stream but do not use the values parsed.
         /// </summary>
         private BmpFileHeader fileHeader;
 
@@ -163,6 +162,7 @@ public Image<TPixel> Decode<TPixel>(Stream stream)
                         break;
 
                     case BmpCompression.BitFields:
+                    case BmpCompression.BI_ALPHABITFIELDS:
                         this.ReadBitFields(pixels, inverted);
 
                         break;
@@ -947,7 +947,7 @@ private void ReadInfoHeader()
                 infoHeaderType = BmpInfoHeaderType.WinVersion3;
                 this.infoHeader = BmpInfoHeader.ParseV3(buffer);
 
-                // if the info header is BMP version 3 and the compression type is BITFIELDS,
+                // If the info header is BMP version 3 and the compression type is BITFIELDS,
                 // color masks for each color channel follow the info header.
                 if (this.infoHeader.Compression == BmpCompression.BitFields)
                 {
@@ -958,6 +958,16 @@ private void ReadInfoHeader()
                     this.infoHeader.GreenMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4));
                     this.infoHeader.BlueMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4));
                 }
+                else if (this.infoHeader.Compression == BmpCompression.BI_ALPHABITFIELDS)
+                {
+                    byte[] bitfieldsBuffer = new byte[16];
+                    this.stream.Read(bitfieldsBuffer, 0, 16);
+                    Span<byte> data = bitfieldsBuffer.AsSpan<byte>();
+                    this.infoHeader.RedMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(0, 4));
+                    this.infoHeader.GreenMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4));
+                    this.infoHeader.BlueMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4));
+                    this.infoHeader.AlphaMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(12, 4));
+                }
             }
             else if (headerSize == BmpInfoHeader.AdobeV3Size)
             {
@@ -1078,6 +1088,9 @@ private int ReadImageHeaders(Stream stream, out bool inverted, out byte[] palett
                     int colorMapSizeBytes = this.fileHeader.Offset - BmpFileHeader.Size - this.infoHeader.HeaderSize;
                     int colorCountForBitDepth = ImageMaths.GetColorCountForBitDepth(this.infoHeader.BitsPerPixel);
                     bytesPerColorMapEntry = colorMapSizeBytes / colorCountForBitDepth;
+
+                    // Edge case for less-than-full-sized palette: bytesPerColorMapEntry should be at least 3.
+                    bytesPerColorMapEntry = Math.Max(bytesPerColorMapEntry, 3);
                     colorMapSize = colorMapSizeBytes;
                 }
             }

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

@@ -60,6 +60,20 @@ public void BmpDecoder_CanDecodeBitfields<TPixel>(TestImageProvider<TPixel> prov
             }
         }
 
+        [Theory]
+        [WithFile(RgbaAlphaBitfields, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecodeAlphaBitfields<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider);
+
+                // TODO: Neither System.Drawing nor MagickReferenceDecoder decode this file.
+                // image.CompareToOriginal(provider);
+            }
+        }
+
         [Theory]
         [WithFile(Bit32Rgba, PixelTypes.Rgba32)]
         public void BmpDecoder_CanDecodeBitmap_WithAlphaChannel<TPixel>(TestImageProvider<TPixel> provider)
@@ -114,6 +128,18 @@ public void BmpDecoder_CanDecodeBmpv3<TPixel>(TestImageProvider<TPixel> provider
             }
         }
 
+        [Theory]
+        [WithFile(LessThanFullSizedPalette, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecodeLessThanFullPalete<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, new MagickReferenceDecoder());
+            }
+        }
+
         [Theory]
         [WithFile(Rgba32bf56, PixelTypes.Rgba32)]
         public void BmpDecoder_CanDecodeAdobeBmpv3<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -221,6 +221,7 @@ public static class Bmp
             public const string Bit8Palette4 = "Bmp/pal8-0.bmp";
             public const string Os2v2Short = "Bmp/pal8os2v2-16.bmp";
             public const string Os2v2 = "Bmp/pal8os2v2.bmp";
+            public const string LessThanFullSizedPalette = "Bmp/pal8os2sp.bmp";
             public const string Pal8Offset = "Bmp/pal8offs.bmp";
 
             // Bitmap images with compression type BITFIELDS
@@ -232,6 +233,7 @@ public static class Bmp
             public const string Issue735 = "Bmp/issue735.bmp";
             public const string Rgba32bf56 = "Bmp/rgba32h56.bmp";
             public const string Rgba321010102 = "Bmp/rgba32-1010102.bmp";
+            public const string RgbaAlphaBitfields = "Bmp/rgba32abf.bmp";
 
             public static readonly string[] BitFields 
             = {