Merge pull request #791 from brianpopow/feature/BmpOS22ShortSupport

WIP: Adds support for Decoding Bitmaps with 16 bytes headers

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -510,6 +510,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 // 12 bytes
                 this.infoHeader = BmpInfoHeader.ParseCore(buffer);
             }
+            else if (headerSize == BmpInfoHeader.Os22ShortSize)
+            {
+                // 16 bytes
+                this.infoHeader = BmpInfoHeader.ParseOs22Short(buffer);
+            }
             else if (headerSize >= BmpInfoHeader.Size)
             {
                 // >= 40 bytes

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -26,6 +26,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         public const int CoreSize = 12;
 
+        /// <summary>
+        /// Defines the size of the short variant of the OS22XBITMAPHEADER data structure in the bitmap file.
+        /// </summary>
+        public const int Os22ShortSize = 16;
+
         /// <summary>
         /// Defines the size of the biggest supported header data structure in the bitmap file.
         /// </summary>
@@ -143,7 +148,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// <summary>
         /// Parses the BITMAPCOREHEADER consisting of the headerSize, width, height, planes, and bitsPerPixel fields (12 bytes).
         /// </summary>
-        /// <param name="data">The data to parse,</param>
+        /// <param name="data">The data to parse.</param>
         /// <returns>Parsed header</returns>
         /// <seealso href="https://msdn.microsoft.com/en-us/library/windows/desktop/dd183372.aspx"/>
         public static BmpInfoHeader ParseCore(ReadOnlySpan<byte> data)
@@ -156,6 +161,23 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 bitsPerPixel: BinaryPrimitives.ReadInt16LittleEndian(data.Slice(10, 2)));
         }
 
+        /// <summary>
+        /// Parses a short variant of the OS22XBITMAPHEADER. It is identical to the BITMAPCOREHEADER, except that the width and height
+        /// are 4 bytes instead of 2.
+        /// </summary>
+        /// <param name="data">The data to parse.</param>
+        /// <returns>Parsed header</returns>
+        /// <seealso href="https://www.fileformat.info/format/os2bmp/egff.htm"/>
+        public static BmpInfoHeader ParseOs22Short(ReadOnlySpan<byte> data)
+        {
+            return 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)));
+        }
+
         public unsafe void WriteTo(Span<byte> buffer)
         {
             ref BmpInfoHeader dest = ref Unsafe.As<byte, BmpInfoHeader>(ref MemoryMarshal.GetReference(buffer));

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

@@ -89,18 +89,17 @@ namespace SixLabors.ImageSharp.Tests
         }
 
         [Theory]
-        [MemberData(nameof(RatioFiles))]
-        public void Identify_VerifyRatio(string imagePath, int xResolution, int yResolution, PixelResolutionUnit resolutionUnit)
+        [WithFile(Os2v2Short, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecode_Os2v2XShortHeader<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
         {
-            var testFile = TestFile.Create(imagePath);
-            using (var stream = new MemoryStream(testFile.Bytes, false))
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
             {
-                var decoder = new BmpDecoder();
-                IImageInfo image = decoder.Identify(Configuration.Default, stream);
-                ImageMetaData meta = image.MetaData;
-                Assert.Equal(xResolution, meta.HorizontalResolution);
-                Assert.Equal(yResolution, meta.VerticalResolution);
-                Assert.Equal(resolutionUnit, meta.ResolutionUnits);
+                image.DebugSave(provider, "png");
+
+                // TODO: Neither System.Drawing not MagickReferenceDecoder 
+                // can correctly decode this file.
+                // image.CompareToOriginal(provider);
             }
         }
     }

tests/ImageSharp.Tests/TestImages.cs

@@ -201,6 +201,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Bit16 = "Bmp/test16.bmp";
             public const string Bit16Inverted = "Bmp/test16-inverted.bmp";
             public const string Bit32Rgb = "Bmp/rgb32.bmp";
+            public const string Os2v2Short = "Bmp/pal8os2v2-16.bmp";
 
             public static readonly string[] All
             = {