Add progressive JPEG encoder

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

@@ -237,6 +237,82 @@ internal class HuffmanScanEncoder
         this.FlushRemainingBytes();
     }
 
+    /// <summary>
+    /// Encodes the DC coefficients for a given component's blocks in a scan.
+    /// </summary>
+    /// <param name="component">The component whose DC coefficients need to be encoded.</param>
+    /// <param name="cancellationToken">The token to request cancellation.</param>
+    public void EncodeDcScan(Component component, CancellationToken cancellationToken)
+    {
+        int h = component.HeightInBlocks;
+        int w = component.WidthInBlocks;
+
+        ref HuffmanLut dcHuffmanTable = ref this.dcHuffmanTables[component.DcTableId];
+
+        for (int i = 0; i < h; i++)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            Span<Block8x8> blockSpan = component.SpectralBlocks.DangerousGetRowSpan(y: i);
+            ref Block8x8 blockRef = ref MemoryMarshal.GetReference(blockSpan);
+
+            for (nuint k = 0; k < (uint)w; k++)
+            {
+                this.WriteDc(
+                    component,
+                    ref Unsafe.Add(ref blockRef, k),
+                    ref dcHuffmanTable);
+
+
+                if (this.IsStreamFlushNeeded)
+                {
+                    this.FlushToStream();
+                }
+            }
+        }
+
+        this.FlushRemainingBytes();
+    }
+
+    /// <summary>
+    /// Encodes the AC coefficients for a specified range of blocks in a component's scan.
+    /// </summary>
+    /// <param name="component">The component whose AC coefficients need to be encoded.</param>
+    /// <param name="start">The starting index of the AC coefficient range to encode.</param>
+    /// <param name="end">The ending index of the AC coefficient range to encode.</param>
+    /// <param name="cancellationToken">The token to request cancellation.</param>
+    public void EncodeAcScan(Component component, nint start, nint end, CancellationToken cancellationToken)
+    {
+        int h = component.HeightInBlocks;
+        int w = component.WidthInBlocks;
+
+        ref HuffmanLut acHuffmanTable = ref this.acHuffmanTables[component.AcTableId];
+
+        for (int i = 0; i < h; i++)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            Span<Block8x8> blockSpan = component.SpectralBlocks.DangerousGetRowSpan(y: i);
+            ref Block8x8 blockRef = ref MemoryMarshal.GetReference(blockSpan);
+
+            for (nuint k = 0; k < (uint)w; k++)
+            {
+                this.WriteAcBlock(
+                    ref Unsafe.Add(ref blockRef, k),
+                    start,
+                    end,
+                    ref acHuffmanTable);
+
+                if (this.IsStreamFlushNeeded)
+                {
+                    this.FlushToStream();
+                }
+            }
+        }
+
+        this.FlushRemainingBytes();
+    }
+
     /// <summary>
     /// Encodes scan in baseline interleaved mode for any amount of component with arbitrary sampling factors.
     /// </summary>
@@ -371,16 +447,64 @@ internal class HuffmanScanEncoder
         this.FlushRemainingBytes();
     }
 
-    private void WriteBlock(
+    private void WriteDc(
         Component component,
         ref Block8x8 block,
-        ref HuffmanLut dcTable,
-        ref HuffmanLut acTable)
+        ref HuffmanLut dcTable)
     {
         // Emit the DC delta.
         int dc = block[0];
         this.EmitHuffRLE(dcTable.Values, 0, dc - component.DcPredictor);
         component.DcPredictor = dc;
+    }
+
+    private void WriteAcBlock(
+        ref Block8x8 block,
+        nint start,
+        nint end,
+        ref HuffmanLut acTable)
+    {
+        // Emit the AC components.
+        int[] acHuffTable = acTable.Values;
+
+        int runLength = 0;
+        ref short blockRef = ref Unsafe.As<Block8x8, short>(ref block);
+        for (nint zig = start; zig < end; zig++)
+        {
+            const int zeroRun1 = 1 << 4;
+            const int zeroRun16 = 16 << 4;
+
+            int ac = Unsafe.Add(ref blockRef, zig);
+            if (ac == 0)
+            {
+                runLength += zeroRun1;
+            }
+            else
+            {
+                while (runLength >= zeroRun16)
+                {
+                    this.EmitHuff(acHuffTable, 0xf0);
+                    runLength -= zeroRun16;
+                }
+
+                this.EmitHuffRLE(acHuffTable, runLength, ac);
+                runLength = 0;
+            }
+        }
+
+        if (runLength > 0)
+        {
+            this.EmitHuff(acHuffTable, 0x00);
+        }
+    }
+
+    private void WriteBlock(
+        Component component,
+        ref Block8x8 block,
+        ref HuffmanLut dcTable,
+        ref HuffmanLut acTable)
+    {
+        this.WriteDc(component, ref block, ref dcTable);
 
         // Emit the AC components.
         int[] acHuffTable = acTable.Values;

src/ImageSharp/Formats/Jpeg/JpegEncoder.cs

@@ -13,6 +13,11 @@ public sealed class JpegEncoder : ImageEncoder
     /// </summary>
     private int? quality;
 
+    /// <summary>
+    /// Backing field for <see cref="ProgressiveScans"/>
+    /// </summary>
+    private int progressiveScans = 4;
+
     /// <summary>
     /// Gets the quality, that will be used to encode the image. Quality
     /// index must be between 1 and 100 (compression from max to min).
@@ -33,6 +38,34 @@ public sealed class JpegEncoder : ImageEncoder
         }
     }
 
+    /// <summary>
+    /// Gets a value indicating whether progressive encoding is used.
+    /// </summary>
+    public bool Progressive { get; init; }
+
+    /// <summary>
+    /// Gets number of scans per component for progressive encoding.
+    /// Defaults to <value>4</value>.
+    /// </summary>
+    /// <remarks>
+    /// Number of scans must be between 2 and 64.
+    /// There is at least one scan for the DC coefficients and one for the remaining 63 AC coefficients.
+    /// </remarks>
+    /// <exception cref="ArgumentException">Progressive scans must be in [2..64] range.</exception>
+    public int ProgressiveScans
+    {
+        get => this.progressiveScans;
+        init
+        {
+            if (value is < 2 or > 64)
+            {
+                throw new ArgumentException("Progressive scans must be in [2..64] range.");
+            }
+
+            this.progressiveScans = value;
+        }
+    }
+
     /// <summary>
     /// Gets the component encoding mode.
     /// </summary>

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -108,7 +108,14 @@ internal sealed unsafe partial class JpegEncoderCore : IImageEncoderInternals
 
         // Write scans with actual pixel data
         using SpectralConverter<TPixel> spectralConverter = new(frame, image, this.QuantizationTables);
-        this.WriteHuffmanScans(frame, frameConfig, spectralConverter, scanEncoder, buffer, cancellationToken);
+        if (this.encoder.Progressive)
+        {
+            this.WriteProgressiveScans(frame, frameConfig, spectralConverter, scanEncoder, buffer, cancellationToken);
+        }
+        else
+        {
+            this.WriteHuffmanScans(frame, frameConfig, spectralConverter, scanEncoder, buffer, cancellationToken);
+        }
 
         // Write the End Of Image marker.
         this.WriteEndOfImageMarker(buffer);
@@ -569,7 +576,8 @@ internal sealed unsafe partial class JpegEncoderCore : IImageEncoderInternals
 
         // Length (high byte, low byte), 8 + components * 3.
         int markerlen = 8 + (3 * components.Length);
-        this.WriteMarkerHeader(JpegConstants.Markers.SOF0, markerlen, buffer);
+        byte marker = this.encoder.Progressive ? JpegConstants.Markers.SOF2 : JpegConstants.Markers.SOF0;
+        this.WriteMarkerHeader(marker, markerlen, buffer);
         buffer[5] = (byte)components.Length;
         buffer[0] = 8; // Data Precision. 8 for now, 12 and 16 bit jpegs not supported
         buffer[1] = (byte)(height >> 8);
@@ -603,7 +611,17 @@ internal sealed unsafe partial class JpegEncoderCore : IImageEncoderInternals
     /// </summary>
     /// <param name="components">The collecction of component configuration items.</param>
     /// <param name="buffer">Temporary buffer.</param>
-    private void WriteStartOfScan(Span<JpegComponentConfig> components, Span<byte> buffer)
+    private void WriteStartOfScan(Span<JpegComponentConfig> components, Span<byte> buffer) =>
+        this.WriteStartOfScan(components, buffer, 0x00, 0x3f);
+
+    /// <summary>
+    /// Writes the StartOfScan marker.
+    /// </summary>
+    /// <param name="components">The collecction of component configuration items.</param>
+    /// <param name="buffer">Temporary buffer.</param>
+    /// <param name="spectralStart">Start of spectral selection</param>
+    /// <param name="spectralEnd">End of spectral selection</param>
+    private void WriteStartOfScan(Span<JpegComponentConfig> components, Span<byte> buffer, byte spectralStart, byte spectralEnd)
     {
         // Write the SOS (Start Of Scan) marker "\xff\xda" followed by 12 bytes:
         // - the marker length "\x00\x0c",
@@ -636,8 +654,8 @@ internal sealed unsafe partial class JpegEncoderCore : IImageEncoderInternals
             buffer[i2 + 6] = (byte)tableSelectors;
         }
 
-        buffer[sosSize - 1] = 0x00; // Ss - Start of spectral selection.
-        buffer[sosSize] = 0x3f; // Se - End of spectral selection.
+        buffer[sosSize - 1] = spectralStart; // Ss - Start of spectral selection.
+        buffer[sosSize] = spectralEnd; // Se - End of spectral selection.
         buffer[sosSize + 1] = 0x00; // Ah + Ah (Successive approximation bit position high + low)
         this.outputStream.Write(buffer, 0, sosSize + 2);
     }
@@ -700,6 +718,55 @@ internal sealed unsafe partial class JpegEncoderCore : IImageEncoderInternals
         }
     }
 
+    /// <summary>
+    /// Writes the progressive scans
+    /// </summary>
+    /// <typeparam name="TPixel">The type of pixel format.</typeparam>
+    /// <param name="frame">The current frame.</param>
+    /// <param name="frameConfig">The frame configuration.</param>
+    /// <param name="spectralConverter">The spectral converter.</param>
+    /// <param name="encoder">The scan encoder.</param>
+    /// <param name="buffer">Temporary buffer.</param>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    private void WriteProgressiveScans<TPixel>(
+        JpegFrame frame,
+        JpegFrameConfig frameConfig,
+        SpectralConverter<TPixel> spectralConverter,
+        HuffmanScanEncoder encoder,
+        Span<byte> buffer,
+        CancellationToken cancellationToken)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        frame.AllocateComponents(fullScan: true);
+        spectralConverter.ConvertFull();
+
+        Span<JpegComponentConfig> components = frameConfig.Components;
+
+        // Phase 1: DC scan
+        for (int i = 0; i < frame.Components.Length; i++)
+        {
+            this.WriteStartOfScan(components.Slice(i, 1), buffer, 0x00, 0x00);
+
+            encoder.EncodeDcScan(frame.Components[i], cancellationToken);
+        }
+
+        // Phase 2: AC scans
+        int acScans = this.encoder.ProgressiveScans - 1;
+        int valuesPerScan = 64 / acScans;
+        for (int scan = 0; scan < acScans; scan++)
+        {
+            int start = Math.Max(1, scan * valuesPerScan);
+            int end = scan == acScans - 1 ? 64 : (scan + 1) * valuesPerScan;
+
+            for (int i = 0; i < components.Length; i++)
+            {
+                this.WriteStartOfScan(components.Slice(i, 1), buffer, (byte)start, (byte)(end - 1));
+
+                encoder.EncodeAcScan(frame.Components[i], start, end, cancellationToken);
+            }
+        }
+    }
+
     /// <summary>
     /// Writes the header for a marker with the given length.
     /// </summary>