Add support for decoding RLE24 Bitmaps (#939)

* Add support for decoding RLE24

* Simplified determining colorMapSize, OS/2 always has 3 bytes for each palette entry

* Enum value for RLE24 is remapped to a different value, to be clearly separate from valid windows values.

Author: brianpopow

src/ImageSharp/Formats/Bmp/BmpCompression.cs

@@ -1,10 +1,10 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Formats.Bmp
 {
     /// <summary>
-    /// Defines how the compression type of the image data
+    /// Defines the compression type of the image data
     /// in the bitmap file.
     /// </summary>
     internal enum BmpCompression : int
@@ -21,7 +21,7 @@ internal enum BmpCompression : int
 
         /// <summary>
         /// Two bytes are one data record. If the first byte is not zero, the
-        /// next two half bytes will be repeated as much as the value of the first byte.
+        /// next byte will be repeated as much as the value of the first byte.
         /// 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.
@@ -30,7 +30,7 @@ internal enum BmpCompression : int
 
         /// <summary>
         /// Two bytes are one data record. If the first byte is not zero, the
-        /// next byte will be repeated as much as the value of the first byte.
+        /// next two half bytes will be repeated as much as the value of the first byte.
         /// 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.
@@ -60,6 +60,17 @@ internal enum BmpCompression : int
         /// 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
+        BI_ALPHABITFIELDS = 6,
+
+        /// <summary>
+        /// OS/2 specific compression type.
+        /// Similar to run length encoding of 4 and 8 bit.
+        /// The only difference is that run values encoded are three bytes in size (one byte per RGB color component),
+        /// rather than four or eight bits in size.
+        ///
+        /// Note: Because compression value of 4 is ambiguous for BI_RGB for windows and RLE24 for OS/2, the enum value is remapped
+        /// to a different value.
+        /// </summary>
+        RLE24 = 100,
     }
-}
+}

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Formats.Bmp
+{
+    /// <summary>
+    /// Indicates which bitmap file marker was read.
+    /// </summary>
+    public enum BmpFileMarkerType
+    {
+        /// <summary>
+        /// Single-image BMP file that may have been created under Windows or OS/2.
+        /// </summary>
+        Bitmap,
+
+        /// <summary>
+        /// OS/2 Bitmap Array.
+        /// </summary>
+        BitmapArray,
+
+        /// <summary>
+        /// OS/2 Color Icon.
+        /// </summary>
+        ColorIcon,
+
+        /// <summary>
+        /// OS/2 Color Pointer.
+        /// </summary>
+        ColorPointer,
+
+        /// <summary>
+        /// OS/2 Icon.
+        /// </summary>
+        Icon,
+
+        /// <summary>
+        /// OS/2 Pointer.
+        /// </summary>
+        Pointer
+    }
+}

src/ImageSharp/Formats/Bmp/BmpFileMarkerType.cs

@@ -388,8 +388,12 @@ public static BmpInfoHeader ParseOs2Version2(ReadOnlySpan<byte> data)
                 case 2:
                     infoHeader.Compression = BmpCompression.RLE4;
                     break;
+                case 4:
+                    infoHeader.Compression = BmpCompression.RLE24;
+                    break;
                 default:
-                    BmpThrowHelper.ThrowImageFormatException($"Compression type is not supported. ImageSharp only supports uncompressed, RLE4 and RLE8.");
+                    // Compression type 3 (1DHuffman) is not supported.
+                    BmpThrowHelper.ThrowImageFormatException("Compression type is not supported. ImageSharp only supports uncompressed, RLE4, RLE8 and RLE24.");
                     break;
             }
 

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -228,6 +228,22 @@ public void BmpDecoder_CanDecode_RunLengthEncoded_8Bit<TPixel>(TestImageProvider
             }
         }
 
+        [Theory]
+        [WithFile(RLE24, PixelTypes.Rgba32)]
+        [WithFile(RLE24Cut, PixelTypes.Rgba32)]
+        [WithFile(RLE24Delta, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecode_RunLengthEncoded_24Bit<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder() { RleSkippedPixelHandling = RleSkippedPixelHandling.Black }))
+            {
+                image.DebugSave(provider);
+
+                // TODO: Neither System.Drawing nor MagickReferenceDecoder decode this file.
+                // image.CompareToOriginal(provider);
+            }
+        }
+
         [Theory]
         [WithFile(RgbaAlphaBitfields, PixelTypes.Rgba32)]
         public void BmpDecoder_CanDecodeAlphaBitfields<TPixel>(TestImageProvider<TPixel> provider)
@@ -521,6 +537,7 @@ public void BmpDecoder_CanDecode_Os2v2Header<TPixel>(TestImageProvider<TPixel> p
         }
 
         [Theory]
+        [WithFile(Os2BitmapArray, PixelTypes.Rgba32)]
         [WithFile(Os2BitmapArray9s, PixelTypes.Rgba32)]
         [WithFile(Os2BitmapArrayDiamond, PixelTypes.Rgba32)]
         [WithFile(Os2BitmapArraySkater, PixelTypes.Rgba32)]

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

@@ -231,6 +231,9 @@ public static class Bmp
             public const string NegHeight = "Bmp/neg_height.bmp";
             public const string CoreHeader = "Bmp/BitmapCoreHeaderQR.bmp";
             public const string V5Header = "Bmp/BITMAPV5HEADER.bmp";
+            public const string RLE24 = "Bmp/rgb24rle24.bmp";
+            public const string RLE24Cut = "Bmp/rle24rlecut.bmp";
+            public const string RLE24Delta = "Bmp/rle24rlecut.bmp";
             public const string RLE8 = "Bmp/RunLengthEncoded.bmp";
             public const string RLE8Cut = "Bmp/pal8rlecut.bmp";
             public const string RLE8Delta = "Bmp/pal8rletrns.bmp";
@@ -262,6 +265,7 @@ 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/ba-bm.bmp";
             public const string Os2BitmapArray9s = "Bmp/9S.BMP";
             public const string Os2BitmapArrayDiamond = "Bmp/DIAMOND.BMP";
             public const string Os2BitmapArrayMarble = "Bmp/GMARBLE.BMP";