Allow deflate compression for paletted tiff's

src/ImageSharp/Formats/Tiff/README.md

@@ -40,7 +40,7 @@
 
 |                           |Encoder|Decoder|Comments                  |
 |---------------------------|:-----:|:-----:|--------------------------|
-|None                       |   Y   |   Y   | encoding only rgb so far |
+|None                       |   Y   |   Y   |                          |
 |Ccitt1D                    |       |   Y   |                          |
 |PackBits                   |       |   Y   |                          |
 |CcittGroup3Fax             |       |   Y   |                          |
@@ -48,7 +48,7 @@
 |Lzw                        |       |   Y   | Based on ImageSharp GIF LZW implementation - this code could be modified to be (i) shared, or (ii) optimised for each case |
 |Old Jpeg                   |       |       | We should not even try to support this |
 |Jpeg (Technote 2)          |       |       |                          |
-|Deflate (Technote 2)       |  (Y)  |   Y   | Based on PNG Deflate. Deflate encoding only for RGB now gray, should we allow this for palette too? |
+|Deflate (Technote 2)       |  (Y)  |   Y   | Based on PNG Deflate. |
 |Old Deflate (Technote 2)   |       |   Y   |                          |
 
 ### Photometric Interpretation Formats
@@ -75,7 +75,7 @@
 |ImageWidth                 |   Y   |   Y   |                          |
 |ImageLength                |   Y   |   Y   |                          |
 |BitsPerSample              |   Y   |   Y   |                          |
-|Compression                |       |   Y   |                          |
+|Compression                |   Y   |   Y   |                          |
 |PhotometricInterpretation  |   Y   |   Y   |                          |
 |Thresholding               |       |       |                          |
 |CellWidth                  |       |       |                          |

src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

@@ -153,7 +153,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff
             switch (this.Mode)
             {
                 case TiffEncodingMode.ColorPalette:
-                    imageDataBytes = writer.WritePalettedRgb(image, this.quantizer, this.padding, out colorMap);
+                    imageDataBytes = writer.WritePalettedRgb(image, this.quantizer, this.padding, this.CompressionType, out colorMap);
                     break;
                 case TiffEncodingMode.Gray:
                     imageDataBytes = writer.WriteGray(image, this.padding, this.CompressionType);
@@ -389,6 +389,12 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                 return (ushort)TiffCompression.Deflate;
             }
 
+            if (this.CompressionType == TiffEncoderCompression.Deflate &&
+                this.Mode == TiffEncodingMode.ColorPalette)
+            {
+                return (ushort)TiffCompression.Deflate;
+            }
+
             return (ushort)TiffCompression.None;
         }
     }

src/ImageSharp/Formats/Tiff/Utils/TiffWriter.cs

@@ -201,10 +201,12 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         /// <param name="padding">The padding bytes for each row.</param>
         /// <param name="colorMap">The color map.</param>
         /// <returns>The number of bytes written.</returns>
-        public int WritePalettedRgb<TPixel>(Image<TPixel> image, IQuantizer quantizer, int padding, out IExifValue colorMap)
+        public int WritePalettedRgb<TPixel>(Image<TPixel> image, IQuantizer quantizer, int padding, TiffEncoderCompression compression, out IExifValue colorMap)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             int colorPaletteSize = 256 * 3 * 2;
+            using var memoryStream = new MemoryStream();
+            using var deflateStream = new ZlibDeflateStream(this.memoryAllocator, memoryStream, PngCompressionLevel.Level6); // TODO: make compression level configurable
             using IManagedByteBuffer row = this.AllocateRow(image.Width, 1, padding);
             using IQuantizer<TPixel> frameQuantizer = quantizer.CreatePixelSpecificQuantizer<TPixel>(this.configuration);
             using IndexedImageFrame<TPixel> quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(image.Frames.RootFrame, image.Bounds());
@@ -253,16 +255,41 @@ namespace SixLabors.ImageSharp.Formats.Tiff
             for (int y = 0; y < image.Height; y++)
             {
                 ReadOnlySpan<byte> pixelSpan = quantized.GetPixelRowSpan(y);
-                this.output.Write(pixelSpan);
-                bytesWritten += pixelSpan.Length;
+
+                if (compression == TiffEncoderCompression.Deflate)
+                {
+                    deflateStream.Write(pixelSpan);
+                }
+                else
+                {
+                    // No compression.
+                    this.output.Write(pixelSpan);
+                    bytesWritten += pixelSpan.Length;
+                }
 
                 for (int i = 0; i < padding; i++)
                 {
-                    this.output.WriteByte(0);
-                    bytesWritten++;
+                    if (compression == TiffEncoderCompression.Deflate)
+                    {
+                        deflateStream.WriteByte(0);
+                    }
+                    else
+                    {
+                        // no compression.
+                        this.output.WriteByte(0);
+                        bytesWritten++;
+                    }
                 }
             }
 
+            if (compression == TiffEncoderCompression.Deflate)
+            {
+                deflateStream.Flush();
+                byte[] buffer = memoryStream.ToArray();
+                this.output.Write(buffer);
+                bytesWritten += buffer.Length;
+            }
+
             return bytesWritten;
         }
 

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

@@ -71,6 +71,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         public void TiffEncoder_EncodeColorPalette_Works<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffEncoderCore(provider, TiffBitsPerPixel.Pixel24, TiffEncodingMode.ColorPalette);
 
+        // TODO: this test fails, but the output looks correct. I thinks its due to the fact that a quantizer is used to create the palette.
+        [Theory]
+        [WithFile(TestImages.Tiff.Calliphora_PaletteUncompressed, PixelTypes.Rgba32)]
+        public void TiffEncoder_EncodeColorPalette_WithDeflateCompression_Works<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel> => TestTiffEncoderCore(provider, TiffBitsPerPixel.Pixel24, TiffEncodingMode.ColorPalette, TiffEncoderCompression.Deflate);
+
         private static void TestTiffEncoderCore<TPixel>(
             TestImageProvider<TPixel> provider,
             TiffBitsPerPixel bitsPerPixel,