Start implementing alpha decoding: uncompressed alpha without filtering works so far

src/ImageSharp/Formats/WebP/AlphaDecoder.cs

@@ -4,24 +4,27 @@
 using System;
 
 using SixLabors.ImageSharp.Formats.WebP.Filters;
+using SixLabors.ImageSharp.Processing;
 
 namespace SixLabors.ImageSharp.Formats.WebP
 {
-    internal ref struct AlphaDecoder
+    internal class AlphaDecoder
     {
         public int Width { get; set; }
 
         public int Height { get; set; }
 
-        public int Method { get; set; }
-
         public WebPFilterBase Filter { get; set; }
 
-        public int PreProcessing { get; set; }
+        private WebPFilterType FilterType { get; }
+
+        private int PreProcessing { get; }
+
+        private bool Compressed { get; }
 
-        public Vp8LDecoder Vp8LDec { get; set; }
+        private byte[] Data { get; }
 
-        public Vp8Io Io { get; set; }
+        private Vp8LDecoder Vp8LDec { get; set; }
 
         /// <summary>
         /// Although Alpha Channel requires only 1 byte per pixel,
@@ -30,22 +33,57 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// </summary>
         public bool Use8BDecode { get; set; }
 
-        // last output row (or null)
-        private Span<byte> PrevLine { get; set; }
+        public AlphaDecoder(int width, int height, byte[] data)
+        {
+            this.Width = width;
+            this.Height = height;
+            this.Data = data;
+
+            // Compression method: Either 0 (no compression) or 1 (Compressed using the WebP lossless format)
+            int method = data[0] & 0x03;
+            if (method < 0 || method > 1)
+            {
+                WebPThrowHelper.ThrowImageFormatException($"unexpected alpha compression method {method} found");
+            }
+
+            this.Compressed = !(method is 0);
+
+            // The filtering method used. Only  values between 0 and 3 are valid.
+            int filter = (data[0] >> 2) & 0x03;
+            if (filter < 0 || filter > 3)
+            {
+                WebPThrowHelper.ThrowImageFormatException($"unexpected alpha filter method {filter} found");
+            }
+
+            this.FilterType = (WebPFilterType)filter;
+
+            // These INFORMATIVE bits are used to signal the pre-processing that has been performed during compression.
+            // The decoder can use this information to e.g. dither the values or smooth the gradients prior to display.
+            // 0: no pre-processing, 1: level reduction
+            this.PreProcessing = (data[0] >> 4) & 0x03;
+        }
 
         private int PrevLineOffset { get; set; }
 
+        public void Decode(Vp8Decoder dec, Span<byte> dst)
+        {
+            if (this.Compressed is false)
+            {
+                this.Data.AsSpan(1, this.Width * this.Height).CopyTo(dst);
+            }
+        }
+
         // Taken from vp8l_dec.c AlphaApplyFilter
         public void AlphaApplyFilter(
             int firstRow,
             int lastRow,
+            Span<byte> prevLine,
             Span<byte> output,
             int outputOffset,
             int stride)
         {
             if (!(this.Filter is WebPFilterNone))
             {
-                Span<byte> prevLine = this.PrevLine;
                 int prevLineOffset = this.PrevLineOffset;
 
                 for (int y = firstRow; y < lastRow; y++)
@@ -62,8 +100,6 @@ namespace SixLabors.ImageSharp.Formats.WebP
                     prevLineOffset = outputOffset;
                     outputOffset += stride;
                 }
-
-                this.PrevLine = prevLine;
             }
         }
     }

src/ImageSharp/Formats/WebP/Filters/WebPFilterType.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Formats.WebP.Filters
+{
+    internal enum WebPFilterType
+    {
+        None = 0,
+
+        Horizontal = 1,
+
+        Vertical = 2,
+
+        Gradient = 3,
+    }
+}

src/ImageSharp/Formats/WebP/LoopFilter.cs

@@ -6,7 +6,9 @@ namespace SixLabors.ImageSharp.Formats.WebP
     internal enum LoopFilter
     {
         None = 0,
+
         Simple = 1,
+
         Complex = 2,
     }
 }

src/ImageSharp/Formats/WebP/WebPDecoderCore.cs

@@ -168,6 +168,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// - A 'VP8X' chunk with information about features used in the file.
         /// - An optional 'ICCP' chunk with color profile.
         /// - An optional 'ANIM' chunk with animation control data.
+        /// - An optional 'ALPH' chunk with alpha channel data.
         /// After the image header, image data will follow. After that optional image metadata chunks (EXIF and XMP) can follow.
         /// </summary>
         /// <returns>Information about this webp image.</returns>
@@ -240,19 +241,25 @@ namespace SixLabors.ImageSharp.Formats.WebP
                        };
             }
 
+            byte[] alphaData = null;
             if (isAlphaPresent)
             {
                 chunkType = this.ReadChunkType();
-                uint alphaChunkSize = this.ReadChunkSize();
+                if (chunkType != WebPChunkType.Alpha)
+                {
+                    WebPThrowHelper.ThrowImageFormatException($"unexpected chunk type {chunkType}, expected ALPH chunk is missing");
+                }
 
-                // ALPH chunks will be skipped for now.
-                this.currentStream.Skip((int)alphaChunkSize);
+                uint alphaChunkSize = this.ReadChunkSize();
+                alphaData = new byte[alphaChunkSize];
+                this.currentStream.Read(alphaData, 0, (int)alphaChunkSize);
             }
 
             var features = new WebPFeatures()
                                     {
                                         Animation = isAnimationPresent,
                                         Alpha = isAlphaPresent,
+                                        AlphaData = alphaData,
                                         ExifProfile = isExifPresent,
                                         IccProfile = isIccPresent,
                                         XmpMetaData = isXmpPresent

src/ImageSharp/Formats/WebP/WebPFeatures.cs

@@ -6,7 +6,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
     /// <summary>
     /// Image features of a VP8X image.
     /// </summary>
-    public class WebPFeatures
+    internal class WebPFeatures
     {
         /// <summary>
         /// Gets or sets a value indicating whether this image has a ICC Profile.
@@ -18,6 +18,11 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// </summary>
         public bool Alpha { get; set; }
 
+        /// <summary>
+        /// Gets or sets the alpha data, if an ALPH chunk is present.
+        /// </summary>
+        public byte[] AlphaData { get; set; }
+
         /// <summary>
         /// Gets or sets a value indicating whether this image has a EXIF Profile.
         /// </summary>

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -66,25 +66,46 @@ namespace SixLabors.ImageSharp.Formats.WebP
             // Decode image data.
             this.ParseFrame(decoder, io);
 
-            this.DecodePixelValues(width, height, decoder.Bgr, pixels);
+            byte[] decodedAlpha = null;
+            if (info.Features?.Alpha is true)
+            {
+                var alphaDecoder = new AlphaDecoder(width, height, info.Features.AlphaData);
+
+                // TODO: use memory allocator.
+                decodedAlpha = new byte[width * height];
+                alphaDecoder.Decode(decoder, decodedAlpha.AsSpan());
+            }
+
+            this.DecodePixelValues(width, height, decoder.Bgr, decodedAlpha, pixels);
         }
 
-        private void DecodePixelValues<TPixel>(int width, int height, Span<byte> pixelData, Buffer2D<TPixel> pixels)
+        private void DecodePixelValues<TPixel>(int width, int height, Span<byte> pixelData, byte[] alpha, Buffer2D<TPixel> pixels)
             where TPixel : struct, IPixel<TPixel>
         {
             TPixel color = default;
+            bool hasAlpha = alpha != null;
             for (int y = 0; y < height; y++)
             {
                 Span<TPixel> pixelRow = pixels.GetRowSpan(y);
                 for (int x = 0; x < width; x++)
                 {
-                    int idx = ((y * width) + x) * 3;
-                    byte b = pixelData[idx];
-                    byte g = pixelData[idx + 1];
-                    byte r = pixelData[idx + 2];
+                    int offset = (y * width) + x;
+                    int idxBgr = offset * 3;
+                    byte b = pixelData[idxBgr];
+                    byte g = pixelData[idxBgr + 1];
+                    byte r = pixelData[idxBgr + 2];
 
                     // TODO: use bulk conversion here.
-                    color.FromBgr24(new Bgr24(r, g, b));
+                    if (hasAlpha)
+                    {
+                        byte a = alpha[offset];
+                        color.FromBgra32(new Bgra32(r, g, b, a));
+                    }
+                    else
+                    {
+                        color.FromBgr24(new Bgr24(r, g, b));
+                    }
+
                     pixelRow[x] = color;
                 }
             }