Add support for horizontal predictor with 16 bit color tiff's

src/ImageSharp/Formats/Tiff/Compression/HorizontalPredictor.cs

@@ -43,6 +43,9 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Compression
                 case TiffColorType.Rgb888:
                     UndoRgb24Bit(pixelBytes, width);
                     break;
+                case TiffColorType.Rgb161616:
+                    UndoRgb48Bit(pixelBytes, width, isBigEndian);
+                    break;
             }
         }
 
@@ -236,5 +239,81 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Compression
                 }
             }
         }
+
+        private static void UndoRgb48Bit(Span<byte> pixelBytes, int width, bool isBigEndian)
+        {
+            int rowBytesCount = width * 6;
+            int height = pixelBytes.Length / rowBytesCount;
+            if (isBigEndian)
+            {
+                for (int y = 0; y < height; y++)
+                {
+                    int offset = 0;
+                    Span<byte> rowBytes = pixelBytes.Slice(y * rowBytesCount, rowBytesCount);
+                    ushort r = TiffUtils.ConvertToUShortBigEndian(rowBytes.Slice(offset, 2));
+                    offset += 2;
+                    ushort g = TiffUtils.ConvertToUShortBigEndian(rowBytes.Slice(offset, 2));
+                    offset += 2;
+                    ushort b = TiffUtils.ConvertToUShortBigEndian(rowBytes.Slice(offset, 2));
+                    offset += 2;
+
+                    for (int x = 1; x < width; x++)
+                    {
+                        Span<byte> rowSpan = rowBytes.Slice(offset, 2);
+                        ushort deltaR = TiffUtils.ConvertToUShortBigEndian(rowSpan);
+                        r += deltaR;
+                        BinaryPrimitives.WriteUInt16BigEndian(rowSpan, r);
+                        offset += 2;
+
+                        rowSpan = rowBytes.Slice(offset, 2);
+                        ushort deltaG = TiffUtils.ConvertToUShortBigEndian(rowSpan);
+                        g += deltaG;
+                        BinaryPrimitives.WriteUInt16BigEndian(rowSpan, g);
+                        offset += 2;
+
+                        rowSpan = rowBytes.Slice(offset, 2);
+                        ushort deltaB = TiffUtils.ConvertToUShortBigEndian(rowSpan);
+                        b += deltaB;
+                        BinaryPrimitives.WriteUInt16BigEndian(rowSpan, b);
+                        offset += 2;
+                    }
+                }
+            }
+            else
+            {
+                for (int y = 0; y < height; y++)
+                {
+                    int offset = 0;
+                    Span<byte> rowBytes = pixelBytes.Slice(y * rowBytesCount, rowBytesCount);
+                    ushort r = TiffUtils.ConvertToUShortLittleEndian(rowBytes.Slice(offset, 2));
+                    offset += 2;
+                    ushort g = TiffUtils.ConvertToUShortLittleEndian(rowBytes.Slice(offset, 2));
+                    offset += 2;
+                    ushort b = TiffUtils.ConvertToUShortLittleEndian(rowBytes.Slice(offset, 2));
+                    offset += 2;
+
+                    for (int x = 1; x < width; x++)
+                    {
+                        Span<byte> rowSpan = rowBytes.Slice(offset, 2);
+                        ushort deltaR = TiffUtils.ConvertToUShortLittleEndian(rowSpan);
+                        r += deltaR;
+                        BinaryPrimitives.WriteUInt16LittleEndian(rowSpan, r);
+                        offset += 2;
+
+                        rowSpan = rowBytes.Slice(offset, 2);
+                        ushort deltaG = TiffUtils.ConvertToUShortLittleEndian(rowSpan);
+                        g += deltaG;
+                        BinaryPrimitives.WriteUInt16LittleEndian(rowSpan, g);
+                        offset += 2;
+
+                        rowSpan = rowBytes.Slice(offset, 2);
+                        ushort deltaB = TiffUtils.ConvertToUShortLittleEndian(rowSpan);
+                        b += deltaB;
+                        BinaryPrimitives.WriteUInt16LittleEndian(rowSpan, b);
+                        offset += 2;
+                    }
+                }
+            }
+        }
     }
 }

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

@@ -228,6 +228,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         public void TiffDecoder_CanDecode_48Bit<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 
+        [Theory]
+        [WithFile(FlowerRgb161616PredictorBigEndian, PixelTypes.Rgba32)]
+        [WithFile(FlowerRgb161616PredictorLittleEndian, PixelTypes.Rgba32)]
+        public void TiffDecoder_CanDecode_48Bit_WithPredictor<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
+
         [Theory]
         [WithFile(FlowerRgb242424Contiguous, PixelTypes.Rgba32)]
         [WithFile(FlowerRgb242424ContiguousLittleEndian, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -573,6 +573,8 @@ namespace SixLabors.ImageSharp.Tests
             public const string FlowerRgb242424ContiguousLittleEndian = "Tiff/flower-rgb-contig-24_lsb.tiff";
             public const string FlowerRgb161616Contiguous = "Tiff/flower-rgb-contig-16.tiff";
             public const string FlowerRgb161616ContiguousLittleEndian = "Tiff/flower-rgb-contig-16_lsb.tiff";
+            public const string FlowerRgb161616PredictorBigEndian = "Tiff/flower-rgb-contig-16_msb_zip_predictor.tiff";
+            public const string FlowerRgb161616PredictorLittleEndian = "Tiff/flower-rgb-contig-16_lsb_zip_predictor.tiff";
             public const string FlowerRgb161616Planar = "Tiff/flower-rgb-planar-16.tiff";
             public const string FlowerRgb161616PlanarLittleEndian = "Tiff/flower-rgb-planar-16_lsb.tiff";
             public const string FlowerRgb141414Contiguous = "Tiff/flower-rgb-contig-14.tiff";

tests/Images/Input/Tiff/flower-rgb-contig-16_lsb_zip_predictor.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:eb465c21009def345d192319ba13ba2e1e537310eec3ab2cce680f0d111a4f18
+size 18256

tests/Images/Input/Tiff/flower-rgb-contig-16_msb_zip_predictor.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c098beb9c8f250e9d4f6eb66a3a42f3852ad3ca86aabbdbacfa897d93ec8bc0d
+size 18254