added parsing of the complete bitmap V4 header to get the color masks…

… infos for the BITFIELDS compression

src/ImageSharp/Formats/Bmp/BmpConstants.cs

@@ -19,5 +19,41 @@ internal static class BmpConstants
         /// The list of file extensions that equate to a bmp.
         /// </summary>
         public static readonly IEnumerable<string> FileExtensions = new[] { "bm", "bmp", "dip" };
+
+        /// <summary>
+        /// Valid magic bytes markers identifying a Bitmap file.
+        /// </summary>
+        internal static class TypeMarkers
+        {
+            /// <summary>
+            /// Single-image BMP file that may have been created under Windows or OS/2.
+            /// </summary>
+            public const int Bitmap = 19778;
+
+            /// <summary>
+            /// OS/2 Bitmap Array.
+            /// </summary>
+            public const int BitmapArray = 16706;
+
+            /// <summary>
+            /// OS/2 Color Icon.
+            /// </summary>
+            public const int ColorIcon = 18755;
+
+            /// <summary>
+            /// OS/2 Color Pointer.
+            /// </summary>
+            public const int ColorPointer = 20547;
+
+            /// <summary>
+            /// OS/2 Icon.
+            /// </summary>
+            public const int Icon = 17225;
+
+            /// <summary>
+            /// OS/2 Pointer.
+            /// </summary>
+            public const int Pointer = 21584;
+        }
     }
 }

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -214,22 +214,16 @@ private static int CalculatePadding(int width, int componentCount)
         private void ReadBitFields<TPixel>(Buffer2D<TPixel> pixels, bool inverted)
             where TPixel : struct, IPixel<TPixel>
         {
-            byte[] buffer = new byte[12];
-            this.stream.Read(buffer, 0, 12);
-            Span<byte> data = buffer.AsSpan<byte>();
-            int redMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(0, 4));
-            int greenMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4));
-            int blueMask = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4));
             if (this.infoHeader.BitsPerPixel == 16)
             {
                 this.ReadRgb16(
                     pixels,
                     this.infoHeader.Width,
                     this.infoHeader.Height,
                     inverted,
-                    redMask,
-                    greenMask,
-                    blueMask);
+                    this.infoHeader.RedMask,
+                    this.infoHeader.GreenMask,
+                    this.infoHeader.BlueMask);
             }
             else
             {
@@ -238,9 +232,9 @@ private void ReadBitFields<TPixel>(Buffer2D<TPixel> pixels, bool inverted)
                     this.infoHeader.Width,
                     this.infoHeader.Height,
                     inverted,
-                    redMask,
-                    greenMask,
-                    blueMask);
+                    this.infoHeader.RedMask,
+                    this.infoHeader.GreenMask,
+                    this.infoHeader.BlueMask);
             }
         }
 
@@ -601,6 +595,11 @@ private static int CalculateRightShift(uint n)
             return count;
         }
 
+        /// <summary>
+        /// Counts none zero bits.
+        /// </summary>
+        /// <param name="n">A color mask.</param>
+        /// <returns>The none zero bits.</returns>
         private static int CountBits(uint n)
         {
             int count = 0;
@@ -651,10 +650,27 @@ private void ReadInfoHeader()
                 // 16 bytes
                 this.infoHeader = BmpInfoHeader.ParseOs22Short(buffer);
             }
-            else if (headerSize >= BmpInfoHeader.Size)
+            else if (headerSize == BmpInfoHeader.SizeV3)
+            {
+                // == 40 bytes
+                this.infoHeader = BmpInfoHeader.ParseV3(buffer);
+
+                // 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)
+                {
+                    byte[] bitfieldsBuffer = new byte[12];
+                    this.stream.Read(bitfieldsBuffer, 0, 12);
+                    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));
+                }
+            }
+            else if (headerSize >= BmpInfoHeader.SizeV3)
             {
                 // >= 40 bytes
-                this.infoHeader = BmpInfoHeader.Parse(buffer);
+                this.infoHeader = BmpInfoHeader.ParseV4(buffer);
             }
             else
             {