Preserve BmpBitsPerPixel

src/ImageSharp/Formats/Bmp/BmpBitsPerPixel.cs

@@ -6,16 +6,16 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// <summary>
     /// Enumerates the available bits per pixel for bitmap.
     /// </summary>
-    public enum BmpBitsPerPixel
+    public enum BmpBitsPerPixel : short
     {
         /// <summary>
         /// 24 bits per pixel. Each pixel consists of 3 bytes.
         /// </summary>
-        Pixel24 = 3,
+        Pixel24 = 24,
 
         /// <summary>
         /// 32 bits per pixel. Each pixel consists of 4 bytes.
         /// </summary>
-        Pixel32 = 4
+        Pixel32 = 32
     }
 }

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -536,6 +536,17 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             this.metaData = meta;
 
+            short bitsPerPixel = this.infoHeader.BitsPerPixel;
+            var bmpMetaData = new BmpMetaData();
+            this.metaData.AddOrUpdateFormatMetaData(BmpFormat.Instance, bmpMetaData);
+
+            // We can only encode at these bit rates so far.
+            if (bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel24)
+                || bitsPerPixel.Equals((short)BmpBitsPerPixel.Pixel32))
+            {
+                bmpMetaData.BitsPerPixel = (BmpBitsPerPixel)bitsPerPixel;
+            }
+
             // skip the remaining header because we can't read those parts
             this.stream.Skip(skipAmount);
         }
@@ -581,9 +592,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             if (this.infoHeader.ClrUsed == 0)
             {
-                if (this.infoHeader.BitsPerPixel == 1 ||
-                    this.infoHeader.BitsPerPixel == 4 ||
-                    this.infoHeader.BitsPerPixel == 8)
+                if (this.infoHeader.BitsPerPixel == 1
+                    || this.infoHeader.BitsPerPixel == 4
+                    || this.infoHeader.BitsPerPixel == 8)
                 {
                     colorMapSize = (int)Math.Pow(2, this.infoHeader.BitsPerPixel) * 4;
                 }

src/ImageSharp/Formats/Bmp/BmpEncoder.cs

@@ -16,7 +16,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// <summary>
         /// Gets or sets the number of bits per pixel.
         /// </summary>
-        public BmpBitsPerPixel BitsPerPixel { get; set; } = BmpBitsPerPixel.Pixel24;
+        public BmpBitsPerPixel? BitsPerPixel { get; set; }
 
         /// <inheritdoc/>
         public void Encode<TPixel>(Image<TPixel> image, Stream stream)

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -21,10 +21,10 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// </summary>
         private int padding;
 
-        private readonly BmpBitsPerPixel bitsPerPixel;
-
         private readonly MemoryAllocator memoryAllocator;
 
+        private BmpBitsPerPixel? bitsPerPixel;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="BmpEncoderCore"/> class.
         /// </summary>
@@ -48,10 +48,12 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             Guard.NotNull(image, nameof(image));
             Guard.NotNull(stream, nameof(stream));
 
-            // Cast to int will get the bytes per pixel
-            short bpp = (short)(8 * (int)this.bitsPerPixel);
+            BmpMetaData bmpMetaData = image.MetaData.GetOrAddFormatMetaData(BmpFormat.Instance);
+            this.bitsPerPixel = this.bitsPerPixel ?? bmpMetaData.BitsPerPixel;
+
+            short bpp = (short)this.bitsPerPixel;
             int bytesPerLine = 4 * (((image.Width * bpp) + 31) / 32);
-            this.padding = bytesPerLine - (image.Width * (int)this.bitsPerPixel);
+            this.padding = bytesPerLine - (int)(image.Width * (bpp / 8F));
 
             // Set Resolution.
             ImageMetaData meta = image.MetaData;
@@ -145,10 +147,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             }
         }
 
-        private IManagedByteBuffer AllocateRow(int width, int bytesPerPixel)
-        {
-            return this.memoryAllocator.AllocatePaddedPixelRowBuffer(width, bytesPerPixel, this.padding);
-        }
+        private IManagedByteBuffer AllocateRow(int width, int bytesPerPixel) => this.memoryAllocator.AllocatePaddedPixelRowBuffer(width, bytesPerPixel, this.padding);
 
         /// <summary>
         /// Writes the 32bit color palette to the stream.

src/ImageSharp/Formats/Bmp/BmpMetaData.cs

@@ -8,6 +8,11 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// </summary>
     public class BmpMetaData
     {
-        // TODO: Analyse what properties we would like to preserve.
+        /// <summary>
+        /// Gets or sets the number of bits per pixel.
+        /// </summary>
+        public BmpBitsPerPixel BitsPerPixel { get; set; } = BmpBitsPerPixel.Pixel24;
+
+        // TODO: Colors used once we support encoding palette bmps.
     }
 }

src/ImageSharp/Formats/Bmp/IBmpEncoderOptions.cs

@@ -12,6 +12,6 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// <summary>
         /// Gets the number of bits per pixel.
         /// </summary>
-        BmpBitsPerPixel BitsPerPixel { get; }
+        BmpBitsPerPixel? BitsPerPixel { get; }
     }
 }

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

@@ -28,10 +28,14 @@ namespace SixLabors.ImageSharp.Tests
             { TestImages.Bmp.RLE, 2835, 2835, PixelResolutionUnit.PixelsPerMeter }
         };
 
-        public BmpEncoderTests(ITestOutputHelper output)
+        public static readonly TheoryData<string, BmpBitsPerPixel> BmpBitsPerPixelFiles =
+        new TheoryData<string, BmpBitsPerPixel>
         {
-            this.Output = output;
-        }
+            { TestImages.Bmp.Car, BmpBitsPerPixel.Pixel24 },
+            { TestImages.Bmp.Bit32Rgb, BmpBitsPerPixel.Pixel32 }
+        };
+
+        public BmpEncoderTests(ITestOutputHelper output) => this.Output = output;
 
         private ITestOutputHelper Output { get; }
 
@@ -61,13 +65,35 @@ namespace SixLabors.ImageSharp.Tests
         }
 
         [Theory]
-        [WithTestPatternImages(nameof(BitsPerPixel), 24, 24, PixelTypes.Rgba32 | PixelTypes.Bgra32 | PixelTypes.Rgb24)]
-        public void Encode_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
-            where TPixel : struct, IPixel<TPixel>
+        [MemberData(nameof(BmpBitsPerPixelFiles))]
+        public void Encode_PreserveBitsPerPixel(string imagePath, BmpBitsPerPixel bmpBitsPerPixel)
         {
-            TestBmpEncoderCore(provider, bitsPerPixel);
+            var options = new BmpEncoder();
+
+            var testFile = TestFile.Create(imagePath);
+            using (Image<Rgba32> input = testFile.CreateImage())
+            {
+                using (var memStream = new MemoryStream())
+                {
+                    input.Save(memStream, options);
+
+                    memStream.Position = 0;
+                    using (var output = Image.Load<Rgba32>(memStream))
+                    {
+                        BmpMetaData meta = output.MetaData.GetOrAddFormatMetaData(BmpFormat.Instance);
+
+                        Assert.Equal(bmpBitsPerPixel, meta.BitsPerPixel);
+                    }
+                }
+            }
         }
 
+
+        [Theory]
+        [WithTestPatternImages(nameof(BitsPerPixel), 24, 24, PixelTypes.Rgba32 | PixelTypes.Bgra32 | PixelTypes.Rgb24)]
+        public void Encode_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
+            where TPixel : struct, IPixel<TPixel> => TestBmpEncoderCore(provider, bitsPerPixel);
+
         [Theory]
         [WithTestPatternImages(nameof(BitsPerPixel), 48, 24, PixelTypes.Rgba32)]
         [WithTestPatternImages(nameof(BitsPerPixel), 47, 8, PixelTypes.Rgba32)]
@@ -75,10 +101,7 @@ namespace SixLabors.ImageSharp.Tests
         [WithSolidFilledImages(nameof(BitsPerPixel), 1, 1, 255, 100, 50, 255, PixelTypes.Rgba32)]
         [WithTestPatternImages(nameof(BitsPerPixel), 7, 5, PixelTypes.Rgba32)]
         public void Encode_WorksWithDifferentSizes<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
-            where TPixel : struct, IPixel<TPixel>
-        {
-            TestBmpEncoderCore(provider, bitsPerPixel);
-        }
+            where TPixel : struct, IPixel<TPixel> => TestBmpEncoderCore(provider, bitsPerPixel);
 
         private static void TestBmpEncoderCore<TPixel>(TestImageProvider<TPixel> provider, BmpBitsPerPixel bitsPerPixel)
             where TPixel : struct, IPixel<TPixel>

tests/ImageSharp.Tests/TestImages.cs

@@ -178,6 +178,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Bit8Inverted = "Bmp/test8-inverted.bmp";
             public const string Bit16 = "Bmp/test16.bmp";
             public const string Bit16Inverted = "Bmp/test16-inverted.bmp";
+            public const string Bit32Rgb = "Bmp/rgb32.bmp";
 
             public static readonly string[] All
             = {