Add support for decoding 6 bit per pixel tiff's

src/ImageSharp/Formats/Tiff/Constants/TiffConstants.cs

@@ -96,15 +96,20 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Constants
         public static readonly ushort[] BitsPerSample8Bit = { 8 };
 
         /// <summary>
-        /// The bits per sample for color images with 8 bits for each color channel.
+        /// The bits per sample for color images with 2 bits for each color channel.
         /// </summary>
-        public static readonly ushort[] BitsPerSampleRgb8Bit = { 8, 8, 8 };
+        public static readonly ushort[] BitsPerSampleRgb2Bit = { 2, 2, 2 };
 
         /// <summary>
         /// The bits per sample for color images with 4 bits for each color channel.
         /// </summary>
         public static readonly ushort[] BitsPerSampleRgb4Bit = { 4, 4, 4 };
 
+        /// <summary>
+        /// The bits per sample for color images with 8 bits for each color channel.
+        /// </summary>
+        public static readonly ushort[] BitsPerSampleRgb8Bit = { 8, 8, 8 };
+
         /// <summary>
         /// The list of mimetypes that equate to a tiff.
         /// </summary>

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/TiffColorDecoderFactory{TPixel}.cs

@@ -57,6 +57,16 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
                     DebugGuard.IsTrue(colorMap == null, "colorMap");
                     return new RgbTiffColor<TPixel>(bitsPerSample);
 
+                case TiffColorType.Rgb222:
+                    DebugGuard.IsTrue(
+                        bitsPerSample.Length == 3
+                        && bitsPerSample[0] == 2
+                        && bitsPerSample[1] == 2
+                        && bitsPerSample[2] == 2,
+                        "bitsPerSample");
+                    DebugGuard.IsTrue(colorMap == null, "colorMap");
+                    return new RgbTiffColor<TPixel>(bitsPerSample);
+
                 case TiffColorType.Rgb444:
                     DebugGuard.IsTrue(
                         bitsPerSample.Length == 3

src/ImageSharp/Formats/Tiff/PhotometricInterpretation/TiffColorType.cs

@@ -59,15 +59,20 @@ namespace SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation
         Rgb,
 
         /// <summary>
-        /// RGB Full Color. Optimized implementation for 8-bit images.
+        /// RGB color image with 2 bits for each channel.
         /// </summary>
-        Rgb888,
+        Rgb222,
 
         /// <summary>
         /// RGB color image with 4 bits for each channel.
         /// </summary>
         Rgb444,
 
+        /// <summary>
+        /// RGB Full Color. Optimized implementation for 8-bit images.
+        /// </summary>
+        Rgb888,
+
         /// <summary>
         /// RGB Full Color. Planar configuration of data.
         /// </summary>

src/ImageSharp/Formats/Tiff/TiffBitsPerPixel.cs

@@ -18,6 +18,13 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         /// </summary>
         Bit4 = 4,
 
+        /// <summary>
+        /// 6 bits per pixel. 2 bit for each color channel.
+        ///
+        /// Note: The TiffEncoder does not yet support 2 bits per color channel and will default to 24 bits per pixel instead.
+        /// </summary>
+        Bit6 = 6,
+
         /// <summary>
         /// 8 bits per pixel, grayscale or color palette images.
         /// </summary>
@@ -26,7 +33,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         /// <summary>
         /// 12 bits per pixel. 4 bit for each color channel.
         ///
-        /// Note: The TiffEncoder does not yet support 4 bits per color channel and will default to 24 bits per pixel.
+        /// Note: The TiffEncoder does not yet support 4 bits per color channel and will default to 24 bits per pixel instead.
         /// </summary>
         Bit12 = 12,
 

src/ImageSharp/Formats/Tiff/TiffBitsPerSample.cs

@@ -28,6 +28,11 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         /// </summary>
         Bit8 = 8,
 
+        /// <summary>
+        /// Six bits per sample, each channel has 2 bits.
+        /// </summary>
+        Bit6 = 6,
+
         /// <summary>
         /// Twelve bits per sample, each channel has 4 bits.
         /// </summary>

src/ImageSharp/Formats/Tiff/TiffBitsPerSampleExtensions.cs

@@ -21,6 +21,8 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                     return TiffConstants.BitsPerSample1Bit;
                 case TiffBitsPerSample.Bit4:
                     return TiffConstants.BitsPerSample4Bit;
+                case TiffBitsPerSample.Bit6:
+                    return TiffConstants.BitsPerSampleRgb2Bit;
                 case TiffBitsPerSample.Bit8:
                     return TiffConstants.BitsPerSample8Bit;
                 case TiffBitsPerSample.Bit12:
@@ -57,6 +59,13 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                         return TiffBitsPerSample.Bit12;
                     }
 
+                    if (bitsPerSample[2] == TiffConstants.BitsPerSampleRgb2Bit[2] &&
+                        bitsPerSample[1] == TiffConstants.BitsPerSampleRgb2Bit[1] &&
+                        bitsPerSample[0] == TiffConstants.BitsPerSampleRgb2Bit[0])
+                    {
+                        return TiffBitsPerSample.Bit6;
+                    }
+
                     break;
 
                 case 1:

src/ImageSharp/Formats/Tiff/TiffDecoderOptionsParser.cs

@@ -188,6 +188,9 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                             case TiffBitsPerSample.Bit12:
                                 options.ColorType = TiffColorType.Rgb444;
                                 break;
+                            case TiffBitsPerSample.Bit6:
+                                options.ColorType = TiffColorType.Rgb222;
+                                break;
                             default:
                                 TiffThrowHelper.ThrowNotSupported("Bits per sample is not supported.");
                                 break;
@@ -285,6 +288,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         {
             TiffBitsPerPixel.Bit1 => TiffBitsPerSample.Bit1,
             TiffBitsPerPixel.Bit4 => TiffBitsPerSample.Bit4,
+            TiffBitsPerPixel.Bit6 => TiffBitsPerSample.Bit6,
             TiffBitsPerPixel.Bit8 => TiffBitsPerSample.Bit8,
             TiffBitsPerPixel.Bit12 => TiffBitsPerSample.Bit12,
             TiffBitsPerPixel.Bit24 => TiffBitsPerSample.Bit24,

src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

@@ -319,8 +319,9 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                     case TiffBitsPerPixel.Bit8:
                         this.SetEncoderOptions(bitsPerPixel, photometricInterpretation ?? TiffPhotometricInterpretation.BlackIsZero, compression, predictor);
                         break;
+                    case TiffBitsPerPixel.Bit6:
                     case TiffBitsPerPixel.Bit12:
-                        // Encoding 12 bits per pixel is not yet supported. Default to 24 bits.
+                        // Encoding 12 and 6 bits per pixel is not yet supported. Default to 24 bits.
                         this.SetEncoderOptions(TiffBitsPerPixel.Bit24, TiffPhotometricInterpretation.Rgb, compression, TiffPredictor.None);
                         break;
                     default:

tests/ImageSharp.Tests/Formats/Tiff/TiffDecoderTests.cs

@@ -99,6 +99,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         public void TiffDecoder_CanDecode_4Bit_WithPalette<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider, ReferenceDecoder, useExactComparer: false, 0.01f);
 
+        [Theory]
+        [WithFile(FlowerRgb222Contiguous, PixelTypes.Rgba32)]
+        [WithFile(FlowerRgb222Planar, PixelTypes.Rgba32)]
+        public void TiffDecoder_CanDecode_6Bit<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
+
         [Theory]
         [WithFile(FlowerRgb444Contiguous, PixelTypes.Rgba32)]
         [WithFile(FlowerRgb444Planar, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/Formats/Tiff/TiffEncoderTests.cs

@@ -79,6 +79,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
 
         [Theory]
         [InlineData(TiffBitsPerPixel.Bit12)]
+        [InlineData(TiffBitsPerPixel.Bit6)]
         public void EncoderOptions_UnsupportedBitPerPixel_DefaultTo24Bits(TiffBitsPerPixel bitsPerPixel)
         {
             // arrange

tests/ImageSharp.Tests/TestImages.cs

@@ -562,6 +562,8 @@ namespace SixLabors.ImageSharp.Tests
             public const string Flower4BitPaletteGray = "Tiff/flower-minisblack-04.tiff";
             public const string FlowerRgb444Contiguous = "Tiff/flower-rgb-contig-04.tiff";
             public const string FlowerRgb444Planar = "Tiff/flower-rgb-planar-04.tiff";
+            public const string FlowerRgb222Contiguous = "Tiff/flower-rgb-contig-02.tiff";
+            public const string FlowerRgb222Planar = "Tiff/flower-rgb-planar-02.tiff";
 
             public const string SmallRgbDeflate = "Tiff/rgb_small_deflate.tiff";
             public const string SmallRgbLzw = "Tiff/rgb_small_lzw.tiff";

tests/Images/Input/Tiff/flower-rgb-contig-02.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:fbcd225c0db343f0cc984c35609b81f6413ebc1ba2ce2494d3607db375e969ff
+size 2685

tests/Images/Input/Tiff/flower-rgb-planar-02.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:21c4ede6382d8c72cb8e6f7939203d5111b362646a9727d95a2f63310ec8e5b3
+size 2795