Write ALPH chunk (only uncompressed for now)

src/ImageSharp/Formats/Webp/AlphaEncoder.cs

@@ -0,0 +1,42 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Buffers;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Webp
+{
+    /// <summary>
+    /// Encodes the alpha channel data.
+    /// Data is either compressed as lossless webp image or uncompressed.
+    /// </summary>
+    internal static class AlphaEncoder
+    {
+        public static byte[] EncodeAlpha<TPixel>(Image<TPixel> image, Configuration configuration, MemoryAllocator memoryAllocator)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            Buffer2D<TPixel> imageBuffer = image.Frames.RootFrame.PixelBuffer;
+            int height = image.Height;
+            int width = image.Width;
+            byte[] alphaData = new byte[width * height];
+
+            using IMemoryOwner<Rgba32> rowBuffer = memoryAllocator.Allocate<Rgba32>(width);
+            Span<Rgba32> rgbaRow = rowBuffer.GetSpan();
+
+            for (int y = 0; y < height; y++)
+            {
+                Span<TPixel> rowSpan = imageBuffer.DangerousGetRowSpan(y);
+                PixelOperations<TPixel>.Instance.ToRgba32(configuration, rowSpan, rgbaRow);
+                int offset = y * width;
+                for (int x = 0; x < width; x++)
+                {
+                    alphaData[offset + x] = rgbaRow[x].A;
+                }
+            }
+
+            return alphaData;
+        }
+    }
+}

src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs

@@ -94,7 +94,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
         /// Calculates the chunk size of EXIF or XMP metadata.
         /// </summary>
         /// <param name="metadataBytes">The metadata profile bytes.</param>
-        /// <returns>The exif chunk size in bytes.</returns>
+        /// <returns>The metadata chunk size in bytes.</returns>
         protected uint MetadataChunkSize(byte[] metadataBytes)
         {
             uint metaSize = (uint)metadataBytes.Length;
@@ -103,6 +103,19 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             return metaChunkSize;
         }
 
+        /// <summary>
+        /// Calculates the chunk size of a alpha chunk.
+        /// </summary>
+        /// <param name="alphaBytes">The alpha chunk bytes.</param>
+        /// <returns>The alpha data chunk size in bytes.</returns>
+        protected uint AlphaChunkSize(byte[] alphaBytes)
+        {
+            uint alphaSize = (uint)alphaBytes.Length + 1;
+            uint alphaChunkSize = WebpConstants.ChunkHeaderSize + alphaSize + (alphaSize & 1);
+
+            return alphaChunkSize;
+        }
+
         /// <summary>
         /// Writes a metadata profile (EXIF or XMP) to the stream.
         /// </summary>
@@ -128,6 +141,34 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             }
         }
 
+        /// <summary>
+        /// Writes the alpha chunk to the stream.
+        /// </summary>
+        /// <param name="stream">The stream to write to.</param>
+        /// <param name="dataBytes">The alpha channel data bytes.</param>
+        protected void WriteAlphaChunk(Stream stream, byte[] dataBytes)
+        {
+            DebugGuard.NotNull(dataBytes, nameof(dataBytes));
+
+            uint size = (uint)dataBytes.Length + 1;
+            Span<byte> buf = this.scratchBuffer.AsSpan(0, 4);
+            BinaryPrimitives.WriteUInt32BigEndian(buf, (uint)WebpChunkType.Alpha);
+            stream.Write(buf);
+            BinaryPrimitives.WriteUInt32LittleEndian(buf, size);
+            stream.Write(buf);
+
+            // Write flags, all zero for now.
+            stream.WriteByte(0);
+
+            stream.Write(dataBytes);
+
+            // Add padding byte if needed.
+            if ((size & 1) == 1)
+            {
+                stream.WriteByte(0);
+            }
+        }
+
         /// <summary>
         /// Writes a VP8X header to the stream.
         /// </summary>

src/ImageSharp/Formats/Webp/BitWriter/Vp8BitWriter.cs

@@ -409,7 +409,8 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
         /// <param name="width">The width of the image.</param>
         /// <param name="height">The height of the image.</param>
         /// <param name="hasAlpha">Flag indicating, if a alpha channel is present.</param>
-        public void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, XmpProfile xmpProfile, uint width, uint height, bool hasAlpha)
+        /// <param name="alphaData">The alpha channel data.</param>
+        public void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, XmpProfile xmpProfile, uint width, uint height, bool hasAlpha, byte[] alphaData)
         {
             bool isVp8X = false;
             byte[] exifBytes = null;
@@ -418,7 +419,6 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             if (exifProfile != null)
             {
                 isVp8X = true;
-                riffSize += ExtendedFileChunkSize;
                 exifBytes = exifProfile.ToByteArray();
                 riffSize += this.MetadataChunkSize(exifBytes);
             }
@@ -426,11 +426,21 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             if (xmpProfile != null)
             {
                 isVp8X = true;
-                riffSize += ExtendedFileChunkSize;
                 xmpBytes = xmpProfile.Data;
                 riffSize += this.MetadataChunkSize(xmpBytes);
             }
 
+            if (hasAlpha)
+            {
+                isVp8X = true;
+                riffSize += this.AlphaChunkSize(alphaData);
+            }
+
+            if (isVp8X)
+            {
+                riffSize += ExtendedFileChunkSize;
+            }
+
             this.Finish();
             uint numBytes = (uint)this.NumBytes();
             int mbSize = this.enc.Mbw * this.enc.Mbh;
@@ -451,7 +461,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             riffSize += WebpConstants.TagSize + WebpConstants.ChunkHeaderSize + vp8Size;
 
             // Emit headers and partition #0
-            this.WriteWebpHeaders(stream, size0, vp8Size, riffSize, isVp8X, width, height, exifProfile, xmpProfile, hasAlpha);
+            this.WriteWebpHeaders(stream, size0, vp8Size, riffSize, isVp8X, width, height, exifProfile, xmpProfile, hasAlpha, alphaData);
             bitWriterPartZero.WriteToStream(stream);
 
             // Write the encoded image to the stream.
@@ -639,7 +649,18 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             while (it.Next());
         }
 
-        private void WriteWebpHeaders(Stream stream, uint size0, uint vp8Size, uint riffSize, bool isVp8X, uint width, uint height, ExifProfile exifProfile, XmpProfile xmpProfile, bool hasAlpha)
+        private void WriteWebpHeaders(
+            Stream stream,
+            uint size0,
+            uint vp8Size,
+            uint riffSize,
+            bool isVp8X,
+            uint width,
+            uint height,
+            ExifProfile exifProfile,
+            XmpProfile xmpProfile,
+            bool hasAlpha,
+            byte[] alphaData)
         {
             this.WriteRiffHeader(stream, riffSize);
 
@@ -647,6 +668,10 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             if (isVp8X)
             {
                 this.WriteVp8XHeader(stream, exifProfile, xmpProfile, width, height, hasAlpha);
+                if (hasAlpha)
+                {
+                    this.WriteAlphaChunk(stream, alphaData);
+                }
             }
 
             this.WriteVp8Header(stream, vp8Size);

src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs

@@ -300,7 +300,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             Span<byte> y = this.Y.GetSpan();
             Span<byte> u = this.U.GetSpan();
             Span<byte> v = this.V.GetSpan();
-            YuvConversion.ConvertRgbToYuv(image, this.configuration, this.memoryAllocator, y, u, v);
+            bool hasAlpha = YuvConversion.ConvertRgbToYuv(image, this.configuration, this.memoryAllocator, y, u, v);
 
             int yStride = width;
             int uvStride = (yStride + 1) >> 1;
@@ -322,8 +322,13 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             int expectedSize = this.Mbw * this.Mbh * averageBytesPerMacroBlock;
             this.bitWriter = new Vp8BitWriter(expectedSize, this);
 
-            // TODO: EncodeAlpha();
+            // Extract and encode alpha data, if present.
-            bool hasAlpha = false;
+            byte[] alphaData = null;
+            if (hasAlpha)
+            {
+                // TODO: This can potentially run in an separate task.
+                alphaData = AlphaEncoder.EncodeAlpha(image, this.configuration, this.memoryAllocator);
+            }
 
             // Stats-collection loop.
             this.StatLoop(width, height, yStride, uvStride);
@@ -358,7 +363,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             // Write bytes from the bitwriter buffer to the stream.
             ImageMetadata metadata = image.Metadata;
             metadata.SyncProfiles();
-            this.bitWriter.WriteEncodedImageToStream(stream, metadata.ExifProfile, metadata.XmpProfile, (uint)width, (uint)height, hasAlpha);
+            this.bitWriter.WriteEncodedImageToStream(stream, metadata.ExifProfile, metadata.XmpProfile, (uint)width, (uint)height, hasAlpha, alphaData);
         }
 
         /// <inheritdoc/>

src/ImageSharp/Formats/Webp/Lossy/YuvConversion.cs

@@ -318,7 +318,8 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         /// <param name="y">Span to store the luma component of the image.</param>
         /// <param name="u">Span to store the u component of the image.</param>
         /// <param name="v">Span to store the v component of the image.</param>
-        public static void ConvertRgbToYuv<TPixel>(Image<TPixel> image, Configuration configuration, MemoryAllocator memoryAllocator, Span<byte> y, Span<byte> u, Span<byte> v)
+        /// <returns>true, if the image contains alpha data.</returns>
+        public static bool ConvertRgbToYuv<TPixel>(Image<TPixel> image, Configuration configuration, MemoryAllocator memoryAllocator, Span<byte> y, Span<byte> u, Span<byte> v)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             Buffer2D<TPixel> imageBuffer = image.Frames.RootFrame.PixelBuffer;
@@ -335,6 +336,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             Span<Bgra32> bgraRow1 = bgraRow1Buffer.GetSpan();
             int uvRowIndex = 0;
             int rowIndex;
+            bool hasAlpha = false;
             for (rowIndex = 0; rowIndex < height - 1; rowIndex += 2)
             {
                 Span<TPixel> rowSpan = imageBuffer.DangerousGetRowSpan(rowIndex);
@@ -343,6 +345,10 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 PixelOperations<TPixel>.Instance.ToBgra32(configuration, nextRowSpan, bgraRow1);
 
                 bool rowsHaveAlpha = WebpCommonUtils.CheckNonOpaque(bgraRow0) && WebpCommonUtils.CheckNonOpaque(bgraRow1);
+                if (rowsHaveAlpha)
+                {
+                    hasAlpha = true;
+                }
 
                 // Downsample U/V planes, two rows at a time.
                 if (!rowsHaveAlpha)
@@ -375,10 +381,13 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 else
                 {
                     AccumulateRgba(bgraRow0, bgraRow0, tmpRgbSpan, width);
+                    hasAlpha = true;
                 }
 
                 ConvertRgbaToUv(tmpRgbSpan, u.Slice(uvRowIndex * uvWidth), v.Slice(uvRowIndex * uvWidth), uvWidth);
             }
+
+            return hasAlpha;
         }
 
         /// <summary>