Add test case for tiled gray tiff, deflate compressed with predictor

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

@@ -67,6 +67,11 @@ internal static class HorizontalPredictor
         // TODO: Implement missing colortypes, see above.
         switch (colorType)
         {
+            case TiffColorType.BlackIsZero8:
+            case TiffColorType.WhiteIsZero8:
+            case TiffColorType.PaletteColor:
+                UndoGray8BitRow(pixelBytes, width, y);
+                break;
             case TiffColorType.Rgb888:
             case TiffColorType.CieLab:
                 UndoRgb24BitRow(pixelBytes, width, y);
@@ -168,20 +173,26 @@ internal static class HorizontalPredictor
         }
     }
 
+    private static void UndoGray8BitRow(Span<byte> pixelBytes, int width, int y)
+    {
+        int rowBytesCount = width;
+        int height = pixelBytes.Length / rowBytesCount;
+        Span<byte> rowBytes = pixelBytes.Slice(y * rowBytesCount, rowBytesCount);
+        byte pixelValue = rowBytes[0];
+        for (int x = 1; x < width; x++)
+        {
+            pixelValue += rowBytes[x];
+            rowBytes[x] = pixelValue;
+        }
+    }
+
     private static void UndoGray8Bit(Span<byte> pixelBytes, int width)
     {
         int rowBytesCount = width;
         int height = pixelBytes.Length / rowBytesCount;
         for (int y = 0; y < height; y++)
         {
-            Span<byte> rowBytes = pixelBytes.Slice(y * rowBytesCount, rowBytesCount);
-
-            byte pixelValue = rowBytes[0];
-            for (int x = 1; x < width; x++)
-            {
-                pixelValue += rowBytes[x];
-                rowBytes[x] = pixelValue;
-            }
+            UndoGray8BitRow(pixelBytes, width, y);
         }
     }
 

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

@@ -89,6 +89,7 @@ public class TiffDecoderTests : TiffDecoderBaseTester
     [WithFile(TiledPlanar, PixelTypes.Rgba32)]
     [WithFile(TiledRgbaDeflateCompressedWithPredictor, PixelTypes.Rgba32)]
     [WithFile(TiledRgbDeflateCompressedWithPredictor, PixelTypes.Rgba32)]
+    [WithFile(TiledGrayDeflateCompressedWithPredictor, PixelTypes.Rgba32)]
     public void TiffDecoder_CanDecode_Tiled<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 

tests/ImageSharp.Tests/TestImages.cs

@@ -988,6 +988,7 @@ public static class TestImages
         public const string TiledPlanar = "Tiff/rgb_uncompressed_tiled_planar.tiff";
         public const string TiledRgbaDeflateCompressedWithPredictor = "Tiff/tiled_rgba_deflate_compressed_predictor.tiff";
         public const string TiledRgbDeflateCompressedWithPredictor = "Tiff/tiled_rgb_deflate_compressed_predictor.tiff";
+        public const string TiledGrayDeflateCompressedWithPredictor = "Tiff/tiled_gray_deflate_compressed_predictor.tiff";
 
         // Images with alpha channel.
         public const string Rgba2BitUnassociatedAlpha = "Tiff/RgbaUnassociatedAlpha2bit.tiff";

tests/Images/Input/Tiff/tiled_gray_deflate_compressed_predictor.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:b5133a684a86dbc9ef48f1f988ec505765f086861c96033d350b7565ebd28271
+size 52663