Grayscale Jpeg encoding

src/ImageSharp/Formats/Jpeg/Components/Encoder/L8ToYConverter.cs

@@ -0,0 +1,49 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Runtime.CompilerServices;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
+{
+    /// <summary>
+    /// Provides 8-bit lookup tables for converting from L8 to Y colorspace.
+    /// </summary>
+    internal unsafe struct L8ToYConverter
+    {
+        /// <summary>
+        /// Initializes
+        /// </summary>
+        /// <returns>The initialized <see cref="L8ToYConverter"/></returns>
+        public static L8ToYConverter Create()
+        {
+            L8ToYConverter converter = default;
+            return converter;
+        }
+
+        /// <summary>
+        /// Optimized method to allocates the correct y, cb, and cr values to the DCT blocks from the given r, g, b values.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void ConvertPixelInto(
+            int l,
+            ref Block8x8F yResult,
+            int i) => yResult[i] = l;
+
+        public void Convert(Span<L8> l8Span, ref Block8x8F yBlock)
+        {
+            ref L8 l8Start = ref l8Span[0];
+
+            for (int i = 0; i < 64; i++)
+            {
+                ref L8 c = ref Unsafe.Add(ref l8Start, i);
+
+                this.ConvertPixelInto(
+                    c.PackedValue,
+                    ref yBlock,
+                    i);
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Jpeg/Components/Encoder/LuminanceForwardConverter{TPixel}.cs

@@ -0,0 +1,59 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
+{
+    /// <summary>
+    /// On-stack worker struct to efficiently encapsulate the TPixel -> L8 -> Y conversion chain of 8x8 pixel blocks.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel type to work on</typeparam>
+    internal ref struct LuminanceForwardConverter<TPixel>
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        /// <summary>
+        /// The Y component
+        /// </summary>
+        public Block8x8F Y;
+
+        /// <summary>
+        /// The converter
+        /// </summary>
+        private L8ToYConverter converter;
+
+        /// <summary>
+        /// Temporal 8x8 block to hold TPixel data
+        /// </summary>
+        private GenericBlock8x8<TPixel> pixelBlock;
+
+        /// <summary>
+        /// Temporal RGB block
+        /// </summary>
+        private GenericBlock8x8<L8> l8Block;
+
+        public static LuminanceForwardConverter<TPixel> Create()
+        {
+            var result = default(LuminanceForwardConverter<TPixel>);
+            result.converter = L8ToYConverter.Create();
+            return result;
+        }
+
+        /// <summary>
+        /// Converts a 8x8 image area inside 'pixels' at position (x,y) placing the result members of the structure (<see cref="Y"/>)
+        /// </summary>
+        public void Convert(ImageFrame<TPixel> frame, int x, int y, ref RowOctet<TPixel> currentRows)
+        {
+            this.pixelBlock.LoadAndStretchEdges(frame.PixelBuffer, x, y, ref currentRows);
+
+            Span<L8> l8Span = this.l8Block.AsSpanUnsafe();
+            PixelOperations<TPixel>.Instance.ToL8(frame.GetConfiguration(), this.pixelBlock.AsSpanUnsafe(), l8Span);
+
+            ref Block8x8F yBlock = ref this.Y;
+
+            this.converter.Convert(l8Span, ref yBlock);
+        }
+    }
+}

src/ImageSharp/Formats/Jpeg/IJpegEncoderOptions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Formats.Jpeg
@@ -20,5 +20,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// </summary>
         /// <value>The subsample ratio of the jpg image.</value>
         JpegSubsample? Subsample { get; }
+
+        /// <summary>
+        /// Gets the color type.
+        /// </summary>
+        JpegColorType? ColorType { get; }
     }
-}
+}

src/ImageSharp/Formats/Jpeg/JpegColorType.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Formats.Jpeg
+{
+    /// <summary>
+    /// Provides enumeration of available JPEG color types.
+    /// </summary>
+    public enum JpegColorType : byte
+    {
+        /// <summary>
+        /// YCbCr (luminance, blue chroma, red chroma) color as defined in the ITU-T T.871 specification.
+        /// </summary>
+        YCbCr = 0,
+
+        /// <summary>
+        /// Single channel, luminance.
+        /// </summary>
+        Luminance = 1
+    }
+}

src/ImageSharp/Formats/Jpeg/JpegEncoder.cs

@@ -25,6 +25,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// </summary>
         public JpegSubsample? Subsample { get; set; }
 
+        /// <summary>
+        /// Gets or sets the color type, that will be used to encode the image.
+        /// </summary>
+        public JpegColorType? ColorType { get; set; }
+
         /// <summary>
         /// Encodes the image to the specified stream from the <see cref="Image{TPixel}"/>.
         /// </summary>
@@ -35,6 +40,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         where TPixel : unmanaged, IPixel<TPixel>
         {
             var encoder = new JpegEncoderCore(this);
+            this.EnrichColorType<TPixel>();
             encoder.Encode(image, stream);
         }
 
@@ -50,7 +56,21 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             where TPixel : unmanaged, IPixel<TPixel>
         {
             var encoder = new JpegEncoderCore(this);
+            this.EnrichColorType<TPixel>();
             return encoder.EncodeAsync(image, stream, cancellationToken);
         }
+
+        /// <summary>
+        /// If ColorType was not set, set it based on the given <typeparamref name="TPixel"/>.
+        /// </summary>
+        private void EnrichColorType<TPixel>()
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            if (this.ColorType == null)
+            {
+                bool isGrayscale = typeof(TPixel) == typeof(L8) || typeof(TPixel) == typeof(L16);
+                this.ColorType = isGrayscale ? JpegColorType.Luminance : JpegColorType.YCbCr;
+            }
+        }
     }
 }

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -57,6 +57,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// </summary>
         private readonly int? quality;
 
+        /// <summary>
+        /// Gets or sets the subsampling method to use.
+        /// </summary>
+        private readonly JpegColorType? colorType;
+
         /// <summary>
         /// The accumulated bits to write to the stream.
         /// </summary>
@@ -90,6 +95,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         {
             this.quality = options.Quality;
             this.subsample = options.Subsample;
+            this.colorType = options.ColorType;
         }
 
         /// <summary>
@@ -115,42 +121,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                 8, 8, 8,
             };
 
-        /// <summary>
-        /// Gets the SOS (Start Of Scan) marker "\xff\xda" followed by 12 bytes:
-        /// - the marker length "\x00\x0c",
-        /// - the number of components "\x03",
-        /// - component 1 uses DC table 0 and AC table 0 "\x01\x00",
-        /// - component 2 uses DC table 1 and AC table 1 "\x02\x11",
-        /// - component 3 uses DC table 1 and AC table 1 "\x03\x11",
-        /// - the bytes "\x00\x3f\x00". Section B.2.3 of the spec says that for
-        /// sequential DCTs, those bytes (8-bit Ss, 8-bit Se, 4-bit Ah, 4-bit Al)
-        /// should be 0x00, 0x3f, 0x00&lt;&lt;4 | 0x00.
-        /// </summary>
-        // The C# compiler emits this as a compile-time constant embedded in the PE file.
-        // This is effectively compiled down to: return new ReadOnlySpan<byte>(&data, length)
-        // More details can be found: https://github.com/dotnet/roslyn/pull/24621
-        private static ReadOnlySpan<byte> SosHeaderYCbCr => new byte[]
-            {
-                JpegConstants.Markers.XFF, JpegConstants.Markers.SOS,
-
-                // Marker
-                0x00, 0x0c,
-
-                // Length (high byte, low byte), must be 6 + 2 * (number of components in scan)
-                0x03, // Number of components in a scan, 3
-                0x01, // Component Id Y
-                0x00, // DC/AC Huffman table
-                0x02, // Component Id Cb
-                0x11, // DC/AC Huffman table
-                0x03, // Component Id Cr
-                0x11, // DC/AC Huffman table
-                0x00, // Ss - Start of spectral selection.
-                0x3f, // Se - End of spectral selection.
-                0x00
-
-                // Ah + Ah (Successive approximation bit position high + low)
-            };
-
         /// <summary>
         /// Gets the unscaled quantization tables in zig-zag order. Each
         /// encoder copies and scales the tables according to its quality parameter.
@@ -212,10 +182,16 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.outputStream = stream;
             ImageMetadata metadata = image.Metadata;
 
+            // Compute number of components based on color type in options.
+            int componentCount = (this.colorType == JpegColorType.Luminance) ? 1 : 3;
+
             // System.Drawing produces identical output for jpegs with a quality parameter of 0 and 1.
             int qlty = Numerics.Clamp(this.quality ?? metadata.GetJpegMetadata().Quality, 1, 100);
             this.subsample ??= qlty >= 91 ? JpegSubsample.Ratio444 : JpegSubsample.Ratio420;
 
+            // Force SubSample into Grayscale for single component ColorType.
+            this.subsample = (componentCount == 1) ? JpegSubsample.Grayscale : this.subsample;
+
             // Convert from a quality rating to a scaling factor.
             int scale;
             if (qlty < 50)
@@ -229,10 +205,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
 
             // Initialize the quantization tables.
             InitQuantizationTable(0, scale, ref this.luminanceQuantTable);
-            InitQuantizationTable(1, scale, ref this.chrominanceQuantTable);
-
-            // Compute number of components based on input image type.
-            const int componentCount = 3;
+            if (componentCount > 1)
+            {
+                InitQuantizationTable(1, scale, ref this.chrominanceQuantTable);
+            }
 
             // Write the Start Of Image marker.
             this.WriteApplicationHeader(metadata);
@@ -250,7 +226,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.WriteDefineHuffmanTables(componentCount);
 
             // Write the image data.
-            this.WriteStartOfScan(image, cancellationToken);
+            this.WriteStartOfScan(image, componentCount, cancellationToken);
 
             // Write the End Of Image marker.
             this.buffer[0] = JpegConstants.Markers.XFF;
@@ -468,6 +444,55 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             }
         }
 
+        /// <summary>
+        /// Encodes the image with no chroma, just luminance.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="pixels">The pixel accessor providing access to the image pixels.</param>
+        /// <param name="cancellationToken">The token to monitor for cancellation.</param>
+        /// <param name="emitBufferBase">The reference to the emit buffer.</param>
+        private void EncodeGrayscale<TPixel>(Image<TPixel> pixels, CancellationToken cancellationToken, ref byte emitBufferBase)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            // TODO: Need a JpegScanEncoder<TPixel> class or struct that encapsulates the scan-encoding implementation. (Similar to JpegScanDecoder.)
+            // (Partially done with YCbCrForwardConverter<TPixel>)
+            Block8x8F temp1 = default;
+            Block8x8F temp2 = default;
+
+            Block8x8F onStackLuminanceQuantTable = this.luminanceQuantTable;
+
+            var unzig = ZigZag.CreateUnzigTable();
+
+            // ReSharper disable once InconsistentNaming
+            int prevDCY = 0;
+
+            var pixelConverter = LuminanceForwardConverter<TPixel>.Create();
+            ImageFrame<TPixel> frame = pixels.Frames.RootFrame;
+            Buffer2D<TPixel> pixelBuffer = frame.PixelBuffer;
+            RowOctet<TPixel> currentRows = default;
+
+            for (int y = 0; y < pixels.Height; y += 8)
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+                currentRows.Update(pixelBuffer, y);
+
+                for (int x = 0; x < pixels.Width; x += 8)
+                {
+                    pixelConverter.Convert(frame, x, y, ref currentRows);
+
+                    prevDCY = this.WriteBlock(
+                        QuantIndex.Luminance,
+                        prevDCY,
+                        ref pixelConverter.Y,
+                        ref temp1,
+                        ref temp2,
+                        ref onStackLuminanceQuantTable,
+                        ref unzig,
+                        ref emitBufferBase);
+                }
+            }
+        }
+
         /// <summary>
         /// Writes the application header containing the JFIF identifier plus extra data.
         /// </summary>
@@ -898,6 +923,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
 
             switch (this.subsample)
             {
+                case JpegSubsample.Grayscale:
+                    subsamples = stackalloc byte[]
+                    {
+                        0x11,
+                        0x00,
+                        0x00
+                    };
+                    break;
                 case JpegSubsample.Ratio444:
                     subsamples = stackalloc byte[]
                     {
@@ -926,26 +959,13 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.buffer[4] = (byte)(width & 0xff); // (2 bytes, Hi-Lo), must be > 0 if DNL not supported
             this.buffer[5] = (byte)componentCount;
 
-            // Number of components (1 byte), usually 1 = Gray scaled, 3 = color YCbCr or YIQ, 4 = color CMYK)
-            if (componentCount == 1)
+            for (int i = 0; i < componentCount; i++)
             {
-                this.buffer[6] = 1;
+                int i3 = 3 * i;
+                this.buffer[i3 + 6] = (byte)(i + 1);
 
-                // No subsampling for grayscale images.
-                this.buffer[7] = 0x11;
-                this.buffer[8] = 0x00;
-            }
-            else
-            {
-                for (int i = 0; i < componentCount; i++)
-                {
-                    int i3 = 3 * i;
-                    this.buffer[i3 + 6] = (byte)(i + 1);
-
-                    // We use 4:2:0 chroma subsampling by default.
-                    this.buffer[i3 + 7] = subsamples[i];
-                    this.buffer[i3 + 8] = chroma[i];
-                }
+                this.buffer[i3 + 7] = subsamples[i];
+                this.buffer[i3 + 8] = chroma[i];
             }
 
             this.outputStream.Write(this.buffer, 0, (3 * (componentCount - 1)) + 9);
@@ -956,16 +976,60 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="image">The pixel accessor providing access to the image pixels.</param>
+        /// <param name="componentCount">The number of components in a pixel.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation.</param>
-        private void WriteStartOfScan<TPixel>(Image<TPixel> image, CancellationToken cancellationToken)
+        private void WriteStartOfScan<TPixel>(Image<TPixel> image, int componentCount, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             // TODO: Need a JpegScanEncoder<TPixel> class or struct that encapsulates the scan-encoding implementation. (Similar to JpegScanDecoder.)
-            // TODO: We should allow grayscale writing.
-            this.outputStream.Write(SosHeaderYCbCr);
+            Span<byte> componentId = stackalloc byte[]
+            {
+                0x01,
+                0x02,
+                0x03
+            };
+            Span<byte> huffmanId = stackalloc byte[]
+            {
+                0x00,
+                0x11,
+                0x11
+            };
+
+            // Write the SOS (Start Of Scan) marker "\xff\xda" followed by 12 bytes:
+            // - the marker length "\x00\x0c",
+            // - the number of components "\x03",
+            // - component 1 uses DC table 0 and AC table 0 "\x01\x00",
+            // - component 2 uses DC table 1 and AC table 1 "\x02\x11",
+            // - component 3 uses DC table 1 and AC table 1 "\x03\x11",
+            // - the bytes "\x00\x3f\x00". Section B.2.3 of the spec says that for
+            // sequential DCTs, those bytes (8-bit Ss, 8-bit Se, 4-bit Ah, 4-bit Al)
+            // should be 0x00, 0x3f, 0x00&lt;&lt;4 | 0x00.
+            this.buffer[0] = JpegConstants.Markers.XFF;
+            this.buffer[1] = JpegConstants.Markers.SOS;
+
+            // Length (high byte, low byte), must be 6 + 2 * (number of components in scan)
+            int sosSize = 6 + (2 * componentCount);
+            this.buffer[2] = 0x00;
+            this.buffer[3] = (byte)sosSize;
+            this.buffer[4] = (byte)componentCount; // Number of components in a scan
+            for (int i = 0; i < componentCount; i++)
+            {
+                int i2 = 2 * i;
+                this.buffer[i2 + 5] = componentId[i]; // Component Id
+                this.buffer[i2 + 6] = huffmanId[i]; // DC/AC Huffman table
+            }
+
+            this.buffer[sosSize - 1] = 0x00; // Ss - Start of spectral selection.
+            this.buffer[sosSize] = 0x3f; // Se - End of spectral selection.
+            this.buffer[sosSize + 1] = 0x00; // Ah + Ah (Successive approximation bit position high + low)
+            this.outputStream.Write(this.buffer, 0, sosSize + 2);
+
             ref byte emitBufferBase = ref MemoryMarshal.GetReference<byte>(this.emitBuffer);
             switch (this.subsample)
             {
+                case JpegSubsample.Grayscale:
+                    this.EncodeGrayscale(image, cancellationToken, ref emitBufferBase);
+                    break;
                 case JpegSubsample.Ratio444:
                     this.Encode444(image, cancellationToken, ref emitBufferBase);
                     break;

src/ImageSharp/Formats/Jpeg/JpegSubsample.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Formats.Jpeg
@@ -8,6 +8,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
     /// </summary>
     public enum JpegSubsample
     {
+        /// <summary>
+        /// Only luminance - No chrome channels.
+        /// </summary>
+        Grayscale,
+
         /// <summary>
         /// High Quality - Each of the three Y'CbCr components have the same sample rate,
         /// thus there is no chroma subsampling.

tests/ImageSharp.Tests/Formats/Jpg/JpegEncoderTests.cs

@@ -83,6 +83,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
         public void EncodeBaseline_WorksWithDifferentSizes<TPixel>(TestImageProvider<TPixel> provider, JpegSubsample subsample, int quality)
             where TPixel : unmanaged, IPixel<TPixel> => TestJpegEncoderCore(provider, subsample, quality);
 
+        [Theory]
+        [WithFile(TestImages.Png.BikeGrayscale, nameof(BitsPerPixel_Quality), PixelTypes.L8)]
+        [WithSolidFilledImages(nameof(BitsPerPixel_Quality), 1, 1, 100, 100, 100, 255, PixelTypes.L8)]
+        public void EncodeBaseline_GrayscaleWorksWithDifferentSizes<TPixel>(TestImageProvider<TPixel> provider, JpegSubsample subsample, int quality)
+            where TPixel : unmanaged, IPixel<TPixel> => TestJpegEncoderCore(provider, subsample, quality, JpegColorType.Luminance);
+
         [Theory]
         [WithTestPatternImages(nameof(BitsPerPixel_Quality), 48, 48, PixelTypes.Rgba32 | PixelTypes.Bgra32)]
         public void EncodeBaseline_IsNotBoundToSinglePixelType<TPixel>(TestImageProvider<TPixel> provider, JpegSubsample subsample, int quality)
@@ -101,7 +107,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 : ImageComparer.TolerantPercentage(5f);
 
             provider.LimitAllocatorBufferCapacity().InBytesSqrt(200);
-            TestJpegEncoderCore(provider, subsample, 100, comparer);
+            TestJpegEncoderCore(provider, subsample, 100, JpegColorType.YCbCr, comparer);
         }
 
         /// <summary>
@@ -131,6 +137,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             TestImageProvider<TPixel> provider,
             JpegSubsample subsample,
             int quality = 100,
+            JpegColorType colorType = JpegColorType.YCbCr,
             ImageComparer comparer = null)
             where TPixel : unmanaged, IPixel<TPixel>
         {
@@ -142,7 +149,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             var encoder = new JpegEncoder
             {
                 Subsample = subsample,
-                Quality = quality
+                Quality = quality,
+                ColorType = colorType
             };
             string info = $"{subsample}-Q{quality}";