#244 Add support for interlaced PNG encoding

src/ImageSharp/Formats/Png/Adam7.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -31,6 +31,34 @@ internal static class Adam7
         /// </summary>
         public static readonly int[] RowIncrement = { 8, 8, 8, 4, 4, 2, 2 };
 
+        /// <summary>
+        /// Gets the width of the block.
+        /// </summary>
+        /// <param name="width">The width.</param>
+        /// <param name="pass">The pass.</param>
+        /// <returns>
+        /// The <see cref="int" />
+        /// </returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int ComputeBlockWidth(int width, int pass)
+        {
+            return (width + ColumnIncrement[pass] - 1 - FirstColumn[pass]) / ColumnIncrement[pass];
+        }
+
+        /// <summary>
+        /// Gets the height of the block.
+        /// </summary>
+        /// <param name="height">The height.</param>
+        /// <param name="pass">The pass.</param>
+        /// <returns>
+        /// The <see cref="int" />
+        /// </returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int ComputeBlockHeight(int height, int pass)
+        {
+            return (height + RowIncrement[pass] - 1 - FirstRow[pass]) / RowIncrement[pass];
+        }
+
         /// <summary>
         /// Returns the correct number of columns for each interlaced pass.
         /// </summary>
@@ -53,4 +81,4 @@ public static int ComputeColumns(int width, int passIndex)
             }
         }
     }
-}
+}

src/ImageSharp/Formats/Png/IPngEncoderOptions.cs

@@ -35,7 +35,7 @@ internal interface IPngEncoderOptions
         /// <summary>
         /// Gets the threshold of characters in text metadata, when compression should be used.
         /// </summary>
-        int CompressTextThreshold { get; }
+        int TextCompressionThreshold { get; }
 
         /// <summary>
         /// Gets the gamma value, that will be written the image.
@@ -52,5 +52,10 @@ internal interface IPngEncoderOptions
         /// Gets the transparency threshold.
         /// </summary>
         byte Threshold { get; }
+
+        /// <summary>
+        /// Gets a value indicating whether this instance should write an Adam7 interlaced image.
+        /// </summary>
+        PngInterlaceMode? InterlaceMethod { get; }
     }
 }

src/ImageSharp/Formats/Png/PngConstants.cs

@@ -52,7 +52,7 @@ internal static class PngConstants
         };
 
         /// <summary>
-        /// The header bytes as a big endian coded ulong.
+        /// The header bytes as a big-endian coded ulong.
         /// </summary>
         public const ulong HeaderValue = 0x89504E470D0A1A0AUL;
 

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -31,7 +31,7 @@ internal sealed class PngDecoderCore
         private readonly byte[] buffer = new byte[4];
 
         /// <summary>
-        /// Reusable crc for validating chunks.
+        /// Reusable CRC for validating chunks.
         /// </summary>
         private readonly Crc32 crc = new Crc32();
 
@@ -106,12 +106,7 @@ internal sealed class PngDecoderCore
         private int currentRow = Adam7.FirstRow[0];
 
         /// <summary>
-        /// The current pass for an interlaced PNG.
-        /// </summary>
-        private int pass;
-
-        /// <summary>
-        /// The current number of bytes read in the current scanline.
+        /// The current number of bytes read in the current scanline
         /// </summary>
         private int currentRowBytesRead;
 
@@ -551,13 +546,15 @@ private void DecodePixelData<TPixel>(Stream compressedStream, ImageFrame<TPixel>
         private void DecodeInterlacedPixelData<TPixel>(Stream compressedStream, ImageFrame<TPixel> image, PngMetadata pngMetadata)
             where TPixel : struct, IPixel<TPixel>
         {
+            int pass = 0;
+            int width = this.header.Width;
             while (true)
             {
-                int numColumns = Adam7.ComputeColumns(this.header.Width, this.pass);
+                int numColumns = Adam7.ComputeColumns(width, pass);
 
                 if (numColumns == 0)
                 {
-                    this.pass++;
+                    pass++;
 
                     // This pass contains no data; skip to next pass
                     continue;
@@ -605,23 +602,23 @@ private void DecodeInterlacedPixelData<TPixel>(Stream compressedStream, ImageFra
                     }
 
                     Span<TPixel> rowSpan = image.GetPixelRowSpan(this.currentRow);
-                    this.ProcessInterlacedDefilteredScanline(this.scanline.GetSpan(), rowSpan, pngMetadata, Adam7.FirstColumn[this.pass], Adam7.ColumnIncrement[this.pass]);
+                    this.ProcessInterlacedDefilteredScanline(this.scanline.GetSpan(), rowSpan, pngMetadata, Adam7.FirstColumn[pass], Adam7.ColumnIncrement[pass]);
 
                     this.SwapBuffers();
 
-                    this.currentRow += Adam7.RowIncrement[this.pass];
+                    this.currentRow += Adam7.RowIncrement[pass];
                 }
 
-                this.pass++;
+                pass++;
                 this.previousScanline.Clear();
 
-                if (this.pass < 7)
+                if (pass < 7)
                 {
-                    this.currentRow = Adam7.FirstRow[this.pass];
+                    this.currentRow = Adam7.FirstRow[pass];
                 }
                 else
                 {
-                    this.pass = 0;
+                    pass = 0;
                     break;
                 }
             }
@@ -859,6 +856,7 @@ private void ReadHeaderChunk(PngMetadata pngMetadata, ReadOnlySpan<byte> data)
 
             pngMetadata.BitDepth = (PngBitDepth)this.header.BitDepth;
             pngMetadata.ColorType = this.header.ColorType;
+            pngMetadata.InterlaceMethod = this.header.InterlaceMethod;
 
             this.pngColorType = this.header.ColorType;
         }
@@ -1202,7 +1200,6 @@ private bool TryReadChunkLength(out int result)
             }
 
             result = default;
-
             return false;
         }
 

src/ImageSharp/Formats/Png/PngEncoder.cs

@@ -36,9 +36,10 @@ public sealed class PngEncoder : IImageEncoder, IPngEncoderOptions
         public int CompressionLevel { get; set; } = 6;
 
         /// <summary>
-        /// Gets or sets the threshold of characters in text metadata, when compression should be used. Defaults to 1024.
+        /// Gets or sets the threshold of characters in text metadata, when compression should be used.
+        /// Defaults to 1024.
         /// </summary>
-        public int CompressTextThreshold { get; set; } = 1024;
+        public int TextCompressionThreshold { get; set; } = 1024;
 
         /// <summary>
         /// Gets or sets the gamma value, that will be written the image.
@@ -47,14 +48,19 @@ public sealed class PngEncoder : IImageEncoder, IPngEncoderOptions
 
         /// <summary>
         /// Gets or sets quantizer for reducing the color count.
-        /// Defaults to the <see cref="WuQuantizer"/>
+        /// Defaults to the <see cref="WuQuantizer"/>.
         /// </summary>
         public IQuantizer Quantizer { get; set; }
 
         /// <summary>
         /// Gets or sets the transparency threshold.
         /// </summary>
-        public byte Threshold { get; set; } = 255;
+        public byte Threshold { get; set; } = byte.MaxValue;
+
+        /// <summary>
+        /// Gets or sets a value indicating whether this instance should write an Adam7 interlaced image.
+        /// </summary>
+        public PngInterlaceMode? InterlaceMethod { get; set; }
 
         /// <summary>
         /// Encodes the image to the specified stream from the <see cref="Image{TPixel}"/>.
@@ -65,7 +71,7 @@ public sealed class PngEncoder : IImageEncoder, IPngEncoderOptions
         public void Encode<TPixel>(Image<TPixel> image, Stream stream)
             where TPixel : struct, IPixel<TPixel>
         {
-            using (var encoder = new PngEncoderCore(image.GetMemoryAllocator(), this))
+            using (var encoder = new PngEncoderCore(image.GetMemoryAllocator(), image.GetConfiguration(), new PngEncoderOptions(this)))
             {
                 encoder.Encode(image, stream);
             }