Bitmap Decoder can now decode BitmapArray

src/ImageSharp/Formats/Bmp/BmpArrayFileHeader.cs

@@ -0,0 +1,53 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Formats.Bmp
+{
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    internal readonly struct BmpArrayFileHeader
+    {
+        public BmpArrayFileHeader(short type, int size, int offsetToNext, short width, short height)
+        {
+            this.Type = type;
+            this.Size = size;
+            this.OffsetToNext = offsetToNext;
+            this.ScreenWidth = width;
+            this.ScreenHeight = height;
+        }
+
+        /// <summary>
+        /// Gets the Bitmap identifier.
+        /// The field used to identify the bitmap file: 0x42 0x41 (Hex code points for B and A).
+        /// </summary>
+        public short Type { get; }
+
+        /// <summary>
+        /// Gets the size of this header.
+        /// </summary>
+        public int Size { get; }
+
+        /// <summary>
+        /// Gets the offset to next OS2BMPARRAYFILEHEADER.
+        /// This offset is calculated from the starting byte of the file. A value of zero indicates that this header is for the last image in the array list.
+        /// </summary>
+        public int OffsetToNext { get; }
+
+        /// <summary>
+        /// Gets the width of the image display in pixels.
+        /// </summary>
+        public short ScreenWidth { get; }
+
+        /// <summary>
+        /// Gets the height of the image display in pixels.
+        /// </summary>
+        public short ScreenHeight { get; }
+
+        public static BmpArrayFileHeader Parse(Span<byte> data)
+        {
+            return MemoryMarshal.Cast<byte, BmpArrayFileHeader>(data)[0];
+        }
+    }
+}

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -1186,11 +1186,25 @@ private void ReadFileHeader()
 #endif
             this.stream.Read(buffer, 0, BmpFileHeader.Size);
 
-            this.fileHeader = BmpFileHeader.Parse(buffer);
-
-            if (this.fileHeader.Type != BmpConstants.TypeMarkers.Bitmap)
+            short fileTypeMarker = BinaryPrimitives.ReadInt16LittleEndian(buffer);
+            switch (fileTypeMarker)
             {
-                BmpThrowHelper.ThrowNotSupportedException($"ImageSharp does not support this BMP file. File header bitmap type marker '{this.fileHeader.Type}'.");
+                case BmpConstants.TypeMarkers.Bitmap:
+                    this.fileHeader = BmpFileHeader.Parse(buffer);
+                    break;
+                case BmpConstants.TypeMarkers.BitmapArray:
+                    var arrayHeader = BmpArrayFileHeader.Parse(buffer);
+                    this.stream.Read(buffer, 0, BmpFileHeader.Size);
+                    this.fileHeader = BmpFileHeader.Parse(buffer);
+                    if (this.fileHeader.Type != BmpConstants.TypeMarkers.Bitmap)
+                    {
+                        BmpThrowHelper.ThrowNotSupportedException($"Unsupported bitmap file inside a BitmapArray file. File header bitmap type marker '{this.fileHeader.Type}'.");
+                    }
+
+                    break;
+                default:
+                    BmpThrowHelper.ThrowNotSupportedException($"ImageSharp does not support this BMP file. File header bitmap type marker '{this.fileHeader.Type}'.");
+                    break;
             }
         }
 

src/ImageSharp/Formats/Bmp/BmpImageFormatDetector.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -22,7 +22,9 @@ public IImageFormat DetectFormat(ReadOnlySpan<byte> header)
 
         private bool IsSupportedFileFormat(ReadOnlySpan<byte> header)
         {
-            return header.Length >= this.HeaderSize && BinaryPrimitives.ReadInt16LittleEndian(header) == BmpConstants.TypeMarkers.Bitmap;
+            short fileTypeMarker = BinaryPrimitives.ReadInt16LittleEndian(header);
+            return header.Length >= this.HeaderSize &&
+                   (fileTypeMarker == BmpConstants.TypeMarkers.Bitmap || fileTypeMarker == BmpConstants.TypeMarkers.BitmapArray);
         }
     }
-}
+}

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 using System;
 using System.Buffers.Binary;
@@ -272,7 +272,7 @@ public BmpInfoHeader(
         /// Parses the BITMAPCOREHEADER (BMP Version 2) consisting of the headerSize, width, height, planes, and bitsPerPixel fields (12 bytes).
         /// </summary>
         /// <param name="data">The data to parse.</param>
-        /// <returns>Parsed header</returns>
+        /// <returns>The parsed header.</returns>
         /// <seealso href="https://msdn.microsoft.com/en-us/library/windows/desktop/dd183372.aspx"/>
         public static BmpInfoHeader ParseCore(ReadOnlySpan<byte> data)
         {
@@ -289,7 +289,7 @@ public static BmpInfoHeader ParseCore(ReadOnlySpan<byte> data)
         /// are 4 bytes instead of 2, resulting in 16 bytes total.
         /// </summary>
         /// <param name="data">The data to parse.</param>
-        /// <returns>Parsed header</returns>
+        /// <returns>The parsed header.</returns>
         /// <seealso href="https://www.fileformat.info/format/os2bmp/egff.htm"/>
         public static BmpInfoHeader ParseOs22Short(ReadOnlySpan<byte> data)
         {
@@ -457,4 +457,4 @@ internal void VerifyDimensions()
             }
         }
     }
-}
+}

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

@@ -465,8 +465,7 @@ public void BmpDecoder_CanDecode_Os2v2XShortHeader<TPixel>(TestImageProvider<TPi
             {
                 image.DebugSave(provider);
 
-                // TODO: Neither System.Drawing not MagickReferenceDecoder 
-                // can correctly decode this file.
+                // TODO: Neither System.Drawing or MagickReferenceDecoder can correctly decode this file.
                 // image.CompareToOriginal(provider);
             }
         }
@@ -486,5 +485,19 @@ public void BmpDecoder_CanDecode_Os2v2Header<TPixel>(TestImageProvider<TPixel> p
                 // image.CompareToOriginal(provider, new MagickReferenceDecoder());
             }
         }
+
+        [Theory]
+        [WithFile(Os2BitmapArray, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecode_Os2BitmapArray<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider);
+
+                // TODO: Neither System.Drawing or MagickReferenceDecoder can correctly decode this file.
+                // image.CompareToOriginal(provider);
+            }
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -261,6 +261,7 @@ 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 Os2BitmapArray = "Bmp/9S.BMP";
             public const string LessThanFullSizedPalette = "Bmp/pal8os2sp.bmp";
             public const string Pal8Offset = "Bmp/pal8offs.bmp";
             public const string OversizedPalette = "Bmp/pal8oversizepal.bmp";