Adds support for OS/2 version 2 bitmaps (#813)

* Added support for OS/2 version 2 bitmaps

* throw NotSupportedException, if the file header type is not BM

* renamed Os2v2 to Os2v2Size

* Added BmpThrowHelper similar to the JpegThrowHelper

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -168,7 +168,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                         break;
 
                     default:
-                        throw new NotSupportedException("Does not support this kind of bitmap files.");
+                        BmpThrowHelper.ThrowNotSupportedException("Does not support this kind of bitmap files.");
+
+                        break;
                 }
 
                 return image;
@@ -319,7 +321,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             {
                 if (this.stream.Read(cmd, 0, cmd.Length) != 2)
                 {
-                    throw new Exception("Failed to read 2 bytes from the stream");
+                    BmpThrowHelper.ThrowImageFormatException("Failed to read 2 bytes from the stream while uncompressing RLE4 bitmap.");
                 }
 
                 if (cmd[0] == RleCommand)
@@ -429,7 +431,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             {
                 if (this.stream.Read(cmd, 0, cmd.Length) != 2)
                 {
-                    throw new Exception("Failed to read 2 bytes from stream");
+                    BmpThrowHelper.ThrowImageFormatException("Failed to read 2 bytes from stream while uncompressing RLE8 bitmap.");
                 }
 
                 if (cmd[0] == RleCommand)
@@ -913,7 +915,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             int headerSize = BinaryPrimitives.ReadInt32LittleEndian(buffer);
             if (headerSize < BmpInfoHeader.CoreSize)
             {
-                throw new NotSupportedException($"ImageSharp does not support this BMP file. HeaderSize: {headerSize}.");
+                BmpThrowHelper.ThrowNotSupportedException($"ImageSharp does not support this BMP file. HeaderSize is '{headerSize}'.");
             }
 
             int skipAmount = 0;
@@ -926,23 +928,23 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             // read the rest of the header
             this.stream.Read(buffer, BmpInfoHeader.HeaderSizeSize, headerSize - BmpInfoHeader.HeaderSizeSize);
 
-            BmpInfoHeaderType inofHeaderType = BmpInfoHeaderType.WinVersion2;
+            BmpInfoHeaderType infoHeaderType = BmpInfoHeaderType.WinVersion2;
             if (headerSize == BmpInfoHeader.CoreSize)
             {
                 // 12 bytes
-                inofHeaderType = BmpInfoHeaderType.WinVersion2;
+                infoHeaderType = BmpInfoHeaderType.WinVersion2;
                 this.infoHeader = BmpInfoHeader.ParseCore(buffer);
             }
             else if (headerSize == BmpInfoHeader.Os22ShortSize)
             {
                 // 16 bytes
-                inofHeaderType = BmpInfoHeaderType.Os2Version2Short;
+                infoHeaderType = BmpInfoHeaderType.Os2Version2Short;
                 this.infoHeader = BmpInfoHeader.ParseOs22Short(buffer);
             }
             else if (headerSize == BmpInfoHeader.SizeV3)
             {
                 // == 40 bytes
-                inofHeaderType = BmpInfoHeaderType.WinVersion3;
+                infoHeaderType = BmpInfoHeaderType.WinVersion3;
                 this.infoHeader = BmpInfoHeader.ParseV3(buffer);
 
                 // if the info header is BMP version 3 and the compression type is BITFIELDS,
@@ -960,24 +962,30 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             else if (headerSize == BmpInfoHeader.AdobeV3Size)
             {
                 // == 52 bytes
-                inofHeaderType = BmpInfoHeaderType.AdobeVersion3;
+                infoHeaderType = BmpInfoHeaderType.AdobeVersion3;
                 this.infoHeader = BmpInfoHeader.ParseAdobeV3(buffer, withAlpha: false);
             }
             else if (headerSize == BmpInfoHeader.AdobeV3WithAlphaSize)
             {
                 // == 56 bytes
-                inofHeaderType = BmpInfoHeaderType.AdobeVersion3WithAlpha;
+                infoHeaderType = BmpInfoHeaderType.AdobeVersion3WithAlpha;
                 this.infoHeader = BmpInfoHeader.ParseAdobeV3(buffer, withAlpha: true);
             }
+            else if (headerSize == BmpInfoHeader.Os2v2Size)
+            {
+                // == 64 bytes
+                infoHeaderType = BmpInfoHeaderType.Os2Version2;
+                this.infoHeader = BmpInfoHeader.ParseOs2Version2(buffer);
+            }
             else if (headerSize >= BmpInfoHeader.SizeV4)
             {
                 // >= 108 bytes
-                inofHeaderType = headerSize == BmpInfoHeader.SizeV4 ? BmpInfoHeaderType.WinVersion4 : BmpInfoHeaderType.WinVersion5;
+                infoHeaderType = headerSize == BmpInfoHeader.SizeV4 ? BmpInfoHeaderType.WinVersion4 : BmpInfoHeaderType.WinVersion5;
                 this.infoHeader = BmpInfoHeader.ParseV4(buffer);
             }
             else
             {
-                throw new NotSupportedException($"ImageSharp does not support this BMP file. HeaderSize: {headerSize}.");
+                BmpThrowHelper.ThrowNotSupportedException($"ImageSharp does not support this BMP file. HeaderSize '{headerSize}'.");
             }
 
             // Resolution is stored in PPM.
@@ -1001,7 +1009,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             short bitsPerPixel = this.infoHeader.BitsPerPixel;
             this.bmpMetaData = this.metaData.GetFormatMetaData(BmpFormat.Instance);
-            this.bmpMetaData.InfoHeaderType = inofHeaderType;
+            this.bmpMetaData.InfoHeaderType = infoHeaderType;
 
             // We can only encode at these bit rates so far.
             if (bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel24)
@@ -1027,6 +1035,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             this.stream.Read(buffer, 0, BmpFileHeader.Size);
 
             this.fileHeader = BmpFileHeader.Parse(buffer);
+
+            if (this.fileHeader.Type != BmpConstants.TypeMarkers.Bitmap)
+            {
+                BmpThrowHelper.ThrowNotSupportedException($"ImageSharp does not support this BMP file. File header bitmap type marker '{this.fileHeader.Type}'.");
+            }
         }
 
         /// <summary>
@@ -1080,7 +1093,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 // 256 * 4
                 if (colorMapSize > 1024)
                 {
-                    throw new ImageFormatException($"Invalid bmp colormap size '{colorMapSize}'");
+                    BmpThrowHelper.ThrowImageFormatException($"Invalid bmp colormap size '{colorMapSize}'");
                 }
 
                 palette = new byte[colorMapSize];

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -41,6 +41,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         public const int AdobeV3WithAlphaSize = 56;
 
+        /// <summary>
+        /// Size of a IBM OS/2 2.x bitmap header.
+        /// </summary>
+        public const int Os2v2Size = 64;
+
         /// <summary>
         /// Defines the size of the BITMAPINFOHEADER (BMP Version 4) data structure in the bitmap file.
         /// </summary>
@@ -117,7 +122,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         }
 
         /// <summary>
-        /// Gets or sets the size of this header
+        /// Gets or sets the size of this header.
         /// </summary>
         public int HeaderSize { get; set; }
 
@@ -346,6 +351,53 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 alphaMask: withAlpha ? BinaryPrimitives.ReadInt32LittleEndian(data.Slice(52, 4)) : 0);
         }
 
+        /// <summary>
+        /// Parses a OS/2 version 2 bitmap header (64 bytes). Only the first 40 bytes are parsed which are
+        /// very similar to the Bitmap v3 header. The other 24 bytes are ignored, but they do not hold any
+        /// useful information for decoding the image.
+        /// </summary>
+        /// <param name="data">The data to parse.</param>
+        /// <returns>The parsed header.</returns>
+        /// <seealso href="https://www.fileformat.info/format/os2bmp/egff.htm"/>
+        public static BmpInfoHeader ParseOs2Version2(ReadOnlySpan<byte> data)
+        {
+            var infoHeader = new BmpInfoHeader(
+                headerSize: BinaryPrimitives.ReadInt32LittleEndian(data.Slice(0, 4)),
+                width: BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4)),
+                height: BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4)),
+                planes: BinaryPrimitives.ReadInt16LittleEndian(data.Slice(12, 2)),
+                bitsPerPixel: BinaryPrimitives.ReadInt16LittleEndian(data.Slice(14, 2)));
+
+            int compression = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(16, 4));
+
+            // The compression value in OS/2 bitmap has a different meaning than in windows bitmaps.
+            // Map the OS/2 value to the windows values.
+            switch (compression)
+            {
+                case 0:
+                    infoHeader.Compression = BmpCompression.RGB;
+                    break;
+                case 1:
+                    infoHeader.Compression = BmpCompression.RLE8;
+                    break;
+                case 2:
+                    infoHeader.Compression = BmpCompression.RLE4;
+                    break;
+                default:
+                    BmpThrowHelper.ThrowImageFormatException($"Compression type is not supported. ImageSharp only supports uncompressed, RLE4 and RLE8.");
+                    break;
+            }
+
+            infoHeader.ImageSize = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(20, 4));
+            infoHeader.XPelsPerMeter = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(24, 4));
+            infoHeader.YPelsPerMeter = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(28, 4));
+            infoHeader.ClrUsed = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(32, 4));
+            infoHeader.ClrImportant = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(36, 4));
+
+            // The following 24 bytes of the header are omitted.
+            return infoHeader;
+        }
+
         /// <summary>
         /// Parses the full BMP Version 4 BITMAPINFOHEADER header (108 bytes).
         /// </summary>

src/ImageSharp/Formats/Bmp/BmpThrowHelper.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Formats.Bmp
+{
+    internal static class BmpThrowHelper
+    {
+        /// <summary>
+        /// Cold path optimization for throwing <see cref="ImageFormatException"/>-s
+        /// </summary>
+        /// <param name="errorMessage">The error message for the exception.</param>
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public static void ThrowImageFormatException(string errorMessage)
+        {
+            throw new ImageFormatException(errorMessage);
+        }
+
+        /// <summary>
+        /// Cold path optimization for throwing <see cref="NotSupportedException"/>-s
+        /// </summary>
+        /// <param name="errorMessage">The error message for the exception.</param>
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public static void ThrowNotSupportedException(string errorMessage)
+        {
+            throw new NotSupportedException(errorMessage);
+        }
+    }
+}

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

@@ -214,12 +214,28 @@ namespace SixLabors.ImageSharp.Tests
         {
             using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
             {
-                image.DebugSave(provider, "png");
+                image.DebugSave(provider);
 
                 // TODO: Neither System.Drawing not MagickReferenceDecoder 
                 // can correctly decode this file.
                 // image.CompareToOriginal(provider);
             }
         }
+
+        [Theory]
+        [WithFile(Os2v2, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecode_Os2v2Header<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider);
+
+                // TODO: System.Drawing can not decode this image. MagickReferenceDecoder can decode it,
+                // but i think incorrectly. I have loaded the image with GIMP and exported as PNG.
+                // The results are the same as the image sharp implementation.
+                // image.CompareToOriginal(provider, new MagickReferenceDecoder());
+            }
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -216,6 +216,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string WinBmpv5 = "Bmp/pal8v5.bmp";
             public const string Bit8Palette4 = "Bmp/pal8-0.bmp";
             public const string Os2v2Short = "Bmp/pal8os2v2-16.bmp";
+            public const string Os2v2 = "Bmp/pal8os2v2.bmp";
 
             // Bitmap images with compression type BITFIELDS
             public const string Rgb32bfdef = "Bmp/rgb32bfdef.bmp";