Allow deflate compression for gray tiff

src/ImageSharp/Formats/Tiff/README.md

@@ -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, should we allow this for the gray and palette too? |
+|Deflate (Technote 2)       |  (Y)  |   Y   | Based on PNG Deflate. Deflate encoding only for RGB now gray, should we allow this for palette too? |
 |Old Deflate (Technote 2)   |       |   Y   |                          |
 
 ### Photometric Interpretation Formats

src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

@@ -156,7 +156,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                     imageDataBytes = writer.WritePalettedRgb(image, this.quantizer, this.padding, out colorMap);
                     break;
                 case TiffEncodingMode.Gray:
-                    imageDataBytes = writer.WriteGray(image, this.padding);
+                    imageDataBytes = writer.WriteGray(image, this.padding, this.CompressionType);
                     break;
                 default:
                     imageDataBytes = writer.WriteRgbImageData(image, this.padding, this.CompressionType);
@@ -378,7 +378,13 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         private ushort GetCompressionType()
         {
             if (this.CompressionType == TiffEncoderCompression.Deflate &&
-                this.PhotometricInterpretation == TiffPhotometricInterpretation.Rgb)
+                this.Mode == TiffEncodingMode.Rgb)
+            {
+                return (ushort)TiffCompression.Deflate;
+            }
+
+            if (this.CompressionType == TiffEncoderCompression.Deflate &&
+                this.Mode == TiffEncodingMode.Gray)
             {
                 return (ushort)TiffCompression.Deflate;
             }

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

@@ -272,12 +272,19 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         /// <typeparam name="TPixel">The pixel data.</typeparam>
         /// <param name="image">The image to write to the stream.</param>
         /// <param name="padding">The padding bytes for each row.</param>
+        /// <param name="compression">The compression to use.</param>
         /// <returns>The number of bytes written.</returns>
-        public int WriteGray<TPixel>(Image<TPixel> image, int padding)
+        public int WriteGray<TPixel>(Image<TPixel> image, int padding, TiffEncoderCompression compression)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             using IManagedByteBuffer row = this.AllocateRow(image.Width, 1, padding);
             Span<byte> rowSpan = row.GetSpan();
+
+            if (compression == TiffEncoderCompression.Deflate)
+            {
+                return this.WriteGrayDeflateCompressed(image, rowSpan);
+            }
+
             int bytesWritten = 0;
             for (int y = 0; y < image.Height; y++)
             {
@@ -290,6 +297,37 @@ namespace SixLabors.ImageSharp.Formats.Tiff
             return bytesWritten;
         }
 
+        /// <summary>
+        /// Writes the image data as 8 bit gray with deflate compression to the stream.
+        /// </summary>
+        /// <param name="image">The image to write to the stream.</param>
+        /// <param name="rowSpan">A span of a pixel row.</param>
+        /// <returns>The number of bytes written.</returns>
+        private int WriteGrayDeflateCompressed<TPixel>(Image<TPixel> image, Span<byte> rowSpan)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            int bytesWritten = 0;
+            using var memoryStream = new MemoryStream();
+
+            // TODO: move zlib compression from png to a common place?
+            using var deflateStream =
+                new ZlibDeflateStream(this.memoryAllocator, memoryStream, PngCompressionLevel.Level6); // TODO: make compression level configurable
+
+            for (int y = 0; y < image.Height; y++)
+            {
+                Span<TPixel> pixelRow = image.GetPixelRowSpan(y);
+                PixelOperations<TPixel>.Instance.ToL8Bytes(this.configuration, pixelRow, rowSpan, pixelRow.Length);
+                deflateStream.Write(rowSpan);
+            }
+
+            deflateStream.Flush();
+
+            byte[] buffer = memoryStream.ToArray();
+            this.output.Write(buffer);
+            bytesWritten += buffer.Length;
+            return bytesWritten;
+        }
+
         private IManagedByteBuffer AllocateRow(int width, int bytesPerPixel, int padding) => this.memoryAllocator.AllocatePaddedPixelRowBuffer(width, bytesPerPixel, padding);
 
         /// <summary>

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

@@ -8,6 +8,7 @@ using SixLabors.ImageSharp.Formats.Tiff;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using SixLabors.ImageSharp.Tests.TestUtilities.ReferenceCodecs;
+
 using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Formats.Tiff
@@ -59,6 +60,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         public void TiffEncoder_EncodeGray_Works<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffEncoderCore(provider, TiffBitsPerPixel.Pixel8, TiffEncodingMode.Gray);
 
+        [Theory]
+        [WithFile(TestImages.Tiff.GrayscaleUncompressed, PixelTypes.Rgba32)]
+        public void TiffEncoder_EncodeGray_WithDeflateCompression_Works<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel> => TestTiffEncoderCore(provider, TiffBitsPerPixel.Pixel8, TiffEncodingMode.Gray, TiffEncoderCompression.Deflate);
+
+        // 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_Works<TPixel>(TestImageProvider<TPixel> provider)