Start implementing compressed alpha decoding

src/ImageSharp/Formats/WebP/AlphaDecoder.cs

@@ -4,19 +4,27 @@
 using System;
 
 using SixLabors.ImageSharp.Formats.WebP.Filters;
-using SixLabors.ImageSharp.Processing;
+using SixLabors.Memory;
 
 namespace SixLabors.ImageSharp.Formats.WebP
 {
     internal class AlphaDecoder
     {
-        public int Width { get; set; }
+        public int Width { get; }
 
-        public int Height { get; set; }
+        public int Height { get; }
 
-        public WebPFilterBase Filter { get; set; }
+        public WebPFilterBase Filter { get; }
 
-        private WebPFilterType FilterType { get; }
+        public WebPFilterType FilterType { get; }
+
+        public int CropTop { get; }
+
+        public int LastRow { get; set; }
+
+        public Vp8LDecoder Vp8LDec { get; }
+
+        public byte[] Alpha { get; }
 
         private int PreProcessing { get; }
 
@@ -24,7 +32,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         private byte[] Data { get; }
 
-        private Vp8LDecoder Vp8LDec { get; set; }
+        private WebPLosslessDecoder LosslessDecoder { get; }
 
         /// <summary>
         /// Although Alpha Channel requires only 1 byte per pixel,
@@ -33,14 +41,15 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// </summary>
         public bool Use8BDecode { get; set; }
 
-        public AlphaDecoder(int width, int height, byte[] data)
+        public AlphaDecoder(int width, int height, byte[] data, byte alphaChunkHeader, MemoryAllocator memoryAllocator)
         {
             this.Width = width;
             this.Height = height;
             this.Data = data;
+            this.LastRow = 0;
 
             // Compression method: Either 0 (no compression) or 1 (Compressed using the WebP lossless format)
-            int method = data[0] & 0x03;
+            int method = alphaChunkHeader & 0x03;
             if (method < 0 || method > 1)
             {
                 WebPThrowHelper.ThrowImageFormatException($"unexpected alpha compression method {method} found");
@@ -49,7 +58,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             this.Compressed = !(method is 0);
 
             // The filtering method used. Only  values between 0 and 3 are valid.
-            int filter = (data[0] >> 2) & 0x03;
+            int filter = (alphaChunkHeader >> 2) & 0x03;
             if (filter < 0 || filter > 3)
             {
                 WebPThrowHelper.ThrowImageFormatException($"unexpected alpha filter method {filter} found");
@@ -60,16 +69,49 @@ namespace SixLabors.ImageSharp.Formats.WebP
             // 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;
+            this.PreProcessing = (alphaChunkHeader >> 4) & 0x03;
+
+            this.Vp8LDec = new Vp8LDecoder(width, height, memoryAllocator);
+
+            // TODO: use memory allocator
+            this.Alpha = new byte[width * height];
+
+            if (this.Compressed)
+            {
+                var bitReader = new Vp8LBitReader(data);
+                this.LosslessDecoder = new WebPLosslessDecoder(bitReader, memoryAllocator);
+                this.LosslessDecoder.DecodeImageStream(this.Vp8LDec, width, height, true);
+            }
         }
 
         private int PrevLineOffset { get; set; }
 
-        public void Decode(Vp8Decoder dec, Span<byte> dst)
+        public void Decode()
         {
             if (this.Compressed is false)
             {
-                this.Data.AsSpan(1, this.Width * this.Height).CopyTo(dst);
+                if (this.Data.Length < (this.Width * this.Height))
+                {
+                    WebPThrowHelper.ThrowImageFormatException("not enough data in the ALPH chunk");
+                }
+
+                switch (this.FilterType)
+                {
+                    case WebPFilterType.None:
+                        this.Data.AsSpan(0, this.Width * this.Height).CopyTo(this.Alpha);
+                        break;
+                    case WebPFilterType.Horizontal:
+
+                        break;
+                    case WebPFilterType.Vertical:
+                        break;
+                    case WebPFilterType.Gradient:
+                        break;
+                }
+            }
+            else
+            {
+                this.LosslessDecoder.DecodeAlphaData(this);
             }
         }
 
@@ -82,24 +124,26 @@ namespace SixLabors.ImageSharp.Formats.WebP
             int outputOffset,
             int stride)
         {
-            if (!(this.Filter is WebPFilterNone))
+            if (this.Filter is WebPFilterNone)
             {
-                int prevLineOffset = this.PrevLineOffset;
+                return;
+            }
 
-                for (int y = firstRow; y < lastRow; y++)
-                {
-                    this.Filter
-                        .Unfilter(
-                            prevLine,
-                            prevLineOffset,
-                            output,
-                            outputOffset,
-                            output,
-                            outputOffset,
-                            stride);
-                    prevLineOffset = outputOffset;
-                    outputOffset += stride;
-                }
+            int prevLineOffset = this.PrevLineOffset;
+
+            for (int y = firstRow; y < lastRow; y++)
+            {
+                this.Filter
+                    .Unfilter(
+                        prevLine,
+                        prevLineOffset,
+                        output,
+                        outputOffset,
+                        output,
+                        outputOffset,
+                        stride);
+                prevLineOffset = outputOffset;
+                outputOffset += stride;
             }
         }
     }

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

@@ -18,7 +18,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Filters
     //     Fast
     // }
 
-    abstract class WebPFilterBase
+    internal abstract class WebPFilterBase
     {
         /// <summary>
         /// </summary>

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

@@ -5,7 +5,7 @@ using System;
 
 namespace SixLabors.ImageSharp.Formats.WebP.Filters
 {
-    class WebPFilterHorizontal : WebPFilterBase
+    internal class WebPFilterHorizontal : WebPFilterBase
     {
         public override void Unfilter(
             Span<byte> prevLine,
@@ -63,7 +63,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Filters
                 predsOffset = inputOffset;
             }
 
-            if (row == 0)
+            if (row is 0)
             {
                 // leftmost pixel is the same as Input for topmost scanline
                 output[0] = input[0];

src/ImageSharp/Formats/WebP/Vp8Decoder.cs

@@ -61,7 +61,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             this.TmpYBuffer = new byte[(width * height) + extraY]; // TODO: figure out min buffer length
             this.TmpUBuffer = new byte[(width * height) + extraUv]; // TODO: figure out min buffer length
             this.TmpVBuffer = new byte[(width * height) + extraUv]; // TODO: figure out min buffer length
-            this.Bgr = new byte[width * height * 4];
+            this.Pixels = new byte[width * height * 4];
 
             for (int i = 0; i < this.YuvBuffer.Length; i++)
             {
@@ -202,7 +202,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         public byte[] TmpVBuffer { get; }
 
-        public byte[] Bgr { get; }
+        public byte[] Pixels { get; }
 
         /// <summary>
         /// Gets or sets filter strength info.

src/ImageSharp/Formats/WebP/Vp8LBitReader.cs

@@ -61,7 +61,29 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// <summary>
         /// True if a bit was read past the end of buffer.
         /// </summary>
-        private bool eos;
+        public bool Eos;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Vp8LBitReader"/> class.
+        /// </summary>
+        /// <param name="data">Lossless compressed image data.</param>
+        public Vp8LBitReader(byte[] data)
+        {
+            this.Data = data;
+            this.len = data.Length;
+            this.value = 0;
+            this.bitPos = 0;
+            this.Eos = false;
+
+            ulong currentValue = 0;
+            for (int i = 0; i < 8; ++i)
+            {
+                currentValue |= (ulong)this.Data[i] << (8 * i);
+            }
+
+            this.value = currentValue;
+            this.pos = 8;
+        }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Vp8LBitReader"/> class.
@@ -78,7 +100,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             this.len = length;
             this.value = 0;
             this.bitPos = 0;
-            this.eos = false;
+            this.Eos = false;
 
             if (length > sizeof(long))
             {
@@ -104,7 +126,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
         {
             Guard.MustBeGreaterThan(nBits, 0, nameof(nBits));
 
-            if (!this.eos && nBits <= Vp8LMaxNumBitRead)
+            if (!this.Eos && nBits <= Vp8LMaxNumBitRead)
             {
                 ulong val = this.PrefetchBits() & this.bitMask[nBits];
                 int newBits = this.bitPos + nBits;
@@ -139,7 +161,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// <summary>
         /// Return the pre-fetched bits, so they can be looked up.
         /// </summary>
-        /// <returns>Prefetched bits.</returns>
+        /// <returns>The pre-fetched bits.</returns>
         public ulong PrefetchBits()
         {
             return this.value >> (this.bitPos & (Vp8LLbits - 1));
@@ -162,7 +184,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// <returns>True, if end of buffer was reached.</returns>
         public bool IsEndOfStream()
         {
-            return this.eos || ((this.pos == this.len) && (this.bitPos > Vp8LLbits));
+            return this.Eos || ((this.pos == this.len) && (this.bitPos > Vp8LLbits));
         }
 
         private void DoFillBitWindow()
@@ -191,7 +213,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         private void SetEndOfStream()
         {
-            this.eos = true;
+            this.Eos = true;
             this.bitPos = 0; // To avoid undefined behaviour with shifts.
         }
     }

src/ImageSharp/Formats/WebP/Vp8LDecoder.cs

@@ -1,25 +1,31 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
+using System.Buffers;
 using System.Collections.Generic;
 
+using SixLabors.Memory;
+
 namespace SixLabors.ImageSharp.Formats.WebP
 {
     /// <summary>
     /// Holds information for decoding a lossless webp image.
     /// </summary>
-    internal class Vp8LDecoder
+    internal class Vp8LDecoder : IDisposable
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="Vp8LDecoder"/> class.
         /// </summary>
         /// <param name="width">The width of the image.</param>
         /// <param name="height">The height of the image.</param>
-        public Vp8LDecoder(int width, int height)
+        /// <param name="memoryAllocator">Used for allocating memory for the pixel data output.</param>
+        public Vp8LDecoder(int width, int height, MemoryAllocator memoryAllocator)
         {
             this.Width = width;
             this.Height = height;
             this.Metadata = new Vp8LMetadata();
+            this.Pixels = memoryAllocator.Allocate<uint>(width * height, AllocationOptions.Clean);
         }
 
         /// <summary>
@@ -41,5 +47,22 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// Gets or sets the transformations which needs to be reversed.
         /// </summary>
         public List<Vp8LTransform> Transforms { get; set; }
+
+        /// <summary>
+        /// Gets the pixel data.
+        /// </summary>
+        public IMemoryOwner<uint> Pixels { get; }
+
+        ///<inheritdoc/>
+        public void Dispose()
+        {
+            this.Pixels.Dispose();
+            foreach (Vp8LTransform transform in this.Transforms)
+            {
+                transform.Data?.Dispose();
+            }
+
+            this.Metadata?.HuffmanImage?.Dispose();
+        }
     }
 }

src/ImageSharp/Formats/WebP/WebPDecoderCore.cs

@@ -242,6 +242,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             }
 
             byte[] alphaData = null;
+            byte alphaChunkHeader = 0;
             if (isAlphaPresent)
             {
                 chunkType = this.ReadChunkType();
@@ -251,8 +252,9 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 }
 
                 uint alphaChunkSize = this.ReadChunkSize();
-                alphaData = new byte[alphaChunkSize];
-                this.currentStream.Read(alphaData, 0, (int)alphaChunkSize);
+                alphaChunkHeader = (byte)this.currentStream.ReadByte();
+                alphaData = new byte[alphaChunkSize - 1];
+                this.currentStream.Read(alphaData, 0, alphaData.Length);
             }
 
             var features = new WebPFeatures()
@@ -260,6 +262,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                                         Animation = isAnimationPresent,
                                         Alpha = isAlphaPresent,
                                         AlphaData = alphaData,
+                                        AlphaChunkHeader = alphaChunkHeader,
                                         ExifProfile = isExifPresent,
                                         IccProfile = isIccPresent,
                                         XmpMetaData = isXmpPresent

src/ImageSharp/Formats/WebP/WebPFeatures.cs

@@ -23,6 +23,11 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// </summary>
         public byte[] AlphaData { get; set; }
 
+        /// <summary>
+        /// Gets or sets the alpha chunk header.
+        /// </summary>
+        public byte AlphaChunkHeader { get; set; }
+
         /// <summary>
         /// Gets or sets a value indicating whether this image has a EXIF Profile.
         /// </summary>

src/ImageSharp/Formats/WebP/WebPLosslessDecoder.cs

@@ -5,7 +5,9 @@ using System;
 using System.Buffers;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.InteropServices;
 
+using SixLabors.ImageSharp.Formats.WebP.Filters;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Memory;
@@ -25,6 +27,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         private static readonly int BitsSpecialMarker = 0x100;
 
+        private static readonly int NumArgbCacheRows = 16;
+
         private static readonly uint PackedNonLiteralCode = 0;
 
         private static readonly int CodeToPlaneCodes = WebPLookupTables.CodeToPlane.Length;
@@ -82,22 +86,18 @@ namespace SixLabors.ImageSharp.Formats.WebP
         public void Decode<TPixel>(Buffer2D<TPixel> pixels, int width, int height)
             where TPixel : struct, IPixel<TPixel>
         {
-            var decoder = new Vp8LDecoder(width, height);
-            IMemoryOwner<uint> pixelData = this.DecodeImageStream(decoder, width, height, true);
-            this.DecodePixelValues(decoder, pixelData.GetSpan(), pixels);
-
-            // Free up allocated memory.
-            pixelData.Dispose();
-            foreach (Vp8LTransform transform in decoder.Transforms)
+            using (var decoder = new Vp8LDecoder(width, height, this.memoryAllocator))
             {
-                transform.Data?.Dispose();
+                this.DecodeImageStream(decoder, width, height, true);
+                this.DecodeImageData(decoder, decoder.Pixels.Memory.Span);
+                this.DecodePixelValues(decoder, pixels);
             }
-
-            decoder.Metadata?.HuffmanImage?.Dispose();
         }
 
-        private IMemoryOwner<uint> DecodeImageStream(Vp8LDecoder decoder, int xSize, int ySize, bool isLevel0)
+        public IMemoryOwner<uint> DecodeImageStream(Vp8LDecoder decoder, int xSize, int ySize, bool isLevel0)
         {
+            int transformXSize = xSize;
+            int transformYSize = ySize;
             int numberOfTransformsPresent = 0;
             if (isLevel0)
             {
@@ -111,7 +111,12 @@ namespace SixLabors.ImageSharp.Formats.WebP
                         WebPThrowHelper.ThrowImageFormatException($"The maximum number of transforms of {WebPConstants.MaxNumberOfTransforms} was exceeded");
                     }
 
-                    this.ReadTransformation(xSize, ySize, decoder);
+                    this.ReadTransformation(transformXSize, transformYSize, decoder);
+                    if (decoder.Transforms[numberOfTransformsPresent].TransformType == Vp8LTransformType.ColorIndexingTransform)
+                    {
+                        transformXSize = LosslessUtils.SubSampleSize(transformXSize, decoder.Transforms[numberOfTransformsPresent].Bits);
+                    }
+
                     numberOfTransformsPresent++;
                 }
             }
@@ -135,14 +140,13 @@ namespace SixLabors.ImageSharp.Formats.WebP
             }
 
             // Read the Huffman codes (may recurse).
-            this.ReadHuffmanCodes(decoder, xSize, ySize, colorCacheBits, isLevel0);
+            this.ReadHuffmanCodes(decoder, transformXSize, transformYSize, colorCacheBits, isLevel0);
             decoder.Metadata.ColorCacheSize = colorCacheSize;
 
-            // Finish setting up the color-cache
-            ColorCache colorCache = null;
+            // Finish setting up the color-cache.
             if (colorCachePresent)
             {
-                colorCache = new ColorCache();
+                decoder.Metadata.ColorCache = new ColorCache();
                 colorCacheSize = 1 << colorCacheBits;
                 decoder.Metadata.ColorCacheSize = colorCacheSize;
                 if (!(colorCacheBits >= 1 && colorCacheBits <= WebPConstants.MaxColorCacheBits))
@@ -150,24 +154,32 @@ namespace SixLabors.ImageSharp.Formats.WebP
                     WebPThrowHelper.ThrowImageFormatException("Invalid color cache bits found");
                 }
 
-                colorCache.Init(colorCacheBits);
+                decoder.Metadata.ColorCache.Init(colorCacheBits);
             }
             else
             {
                 decoder.Metadata.ColorCacheSize = 0;
             }
 
-            this.UpdateDecoder(decoder, xSize, ySize);
+            this.UpdateDecoder(decoder, transformXSize, transformYSize);
+            if (isLevel0)
+            {
+                // level 0 complete.
+                return null;
+            }
 
+            // Use the Huffman trees to decode the LZ77 encoded data.
             IMemoryOwner<uint> pixelData = this.memoryAllocator.Allocate<uint>(decoder.Width * decoder.Height, AllocationOptions.Clean);
-            this.DecodeImageData(decoder, pixelData.GetSpan(), colorCacheSize, colorCache);
+            this.DecodeImageData(decoder, pixelData.GetSpan());
 
             return pixelData;
         }
 
-        private void DecodePixelValues<TPixel>(Vp8LDecoder decoder, Span<uint> pixelData, Buffer2D<TPixel> pixels)
+        private void DecodePixelValues<TPixel>(Vp8LDecoder decoder, Buffer2D<TPixel> pixels)
             where TPixel : struct, IPixel<TPixel>
         {
+            Span<uint> pixelData = decoder.Pixels.GetSpan();
+
             // Apply reverse transformations, if any are present.
             this.ApplyInverseTransforms(decoder, pixelData);
 
@@ -189,7 +201,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             }
         }
 
-        private void DecodeImageData(Vp8LDecoder decoder, Span<uint> pixelData, int colorCacheSize, ColorCache colorCache)
+        private void DecodeImageData(Vp8LDecoder decoder, Span<uint> pixelData)
         {
             int lastPixel = 0;
             int width = decoder.Width;
@@ -197,6 +209,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
             int row = lastPixel / width;
             int col = lastPixel % width;
             int lenCodeLimit = WebPConstants.NumLiteralCodes + WebPConstants.NumLengthCodes;
+            int colorCacheSize = decoder.Metadata.ColorCacheSize;
+            ColorCache colorCache = decoder.Metadata.ColorCache;
             int colorCacheLimit = lenCodeLimit + colorCacheSize;
             int mask = decoder.Metadata.HuffmanMask;
             HTreeGroup[] hTreeGroup = this.GetHTreeGroupForPos(decoder.Metadata, col, row);
@@ -207,7 +221,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             while (decodedPixels < totalPixels)
             {
                 int code;
-                if ((col & mask) == 0)
+                if ((col & mask) is 0)
                 {
                     hTreeGroup = this.GetHTreeGroupForPos(decoder.Metadata, col, row);
                 }
@@ -621,7 +635,6 @@ namespace SixLabors.ImageSharp.Formats.WebP
                                      : (numColors > 4) ? 1
                                      : (numColors > 2) ? 2
                                      : 3;
-                    transform.XSize = LosslessUtils.SubSampleSize(transform.XSize, bits);
                     transform.Bits = bits;
                     using (IMemoryOwner<uint> colorMap = this.DecodeImageStream(decoder, (int)numColors, 1, false))
                     {
@@ -683,6 +696,208 @@ namespace SixLabors.ImageSharp.Formats.WebP
             }
         }
 
+        public void DecodeAlphaData(AlphaDecoder dec)
+        {
+            Span<uint> pixelData = dec.Vp8LDec.Pixels.Memory.Span;
+            Span<byte> data = MemoryMarshal.Cast<uint, byte>(pixelData);
+            int row = 0;
+            int col = 0;
+            Vp8LDecoder vp8LDec = dec.Vp8LDec;
+            int width = vp8LDec.Width;
+            int height = vp8LDec.Height;
+            Vp8LMetadata hdr = vp8LDec.Metadata;
+            int pos = 0; // Current position.
+            int end = width * height; // End of data.
+            int last = end; // Last pixel to decode.
+            int lastRow = height;
+            int lenCodeLimit = WebPConstants.NumLiteralCodes + WebPConstants.NumLengthCodes;
+            int mask = hdr.HuffmanMask;
+            HTreeGroup[] htreeGroup = (pos < last) ? this.GetHTreeGroupForPos(hdr, col, row) : null;
+            while (!this.bitReader.Eos && pos < last)
+            {
+                // Only update when changing tile.
+                if ((col & mask) is 0)
+                {
+                    htreeGroup = this.GetHTreeGroupForPos(hdr, col, row);
+                }
+
+                this.bitReader.FillBitWindow();
+                int code = (int)this.ReadSymbol(htreeGroup[0].HTrees[HuffIndex.Green]);
+                if (code < WebPConstants.NumLiteralCodes)
+                {
+                    // Literal
+                    data[pos] = (byte)code;
+                    ++pos;
+                    ++col;
+
+                    if (col >= width)
+                    {
+                        col = 0;
+                        ++row;
+                        if (row <= lastRow && (row % NumArgbCacheRows is 0))
+                        {
+                            this.ExtractPalettedAlphaRows(dec, row);
+                        }
+                    }
+                }
+                else if (code < lenCodeLimit)
+                {
+                    // Backward reference
+                    int lengthSym = code - WebPConstants.NumLiteralCodes;
+                    int length = this.GetCopyLength(lengthSym);
+                    int distSymbol = (int)this.ReadSymbol(htreeGroup[0].HTrees[HuffIndex.Dist]);
+                    this.bitReader.FillBitWindow();
+                    int distCode = this.GetCopyDistance(distSymbol);
+                    int dist = this.PlaneCodeToDistance(width, distCode);
+                    if (pos >= dist && end - pos >= length)
+                    {
+                        //CopyBlock8b(data + pos, dist, length);
+                    }
+                    else
+                    {
+                        // TODO: error?
+                        break;
+                    }
+
+                    pos += length;
+                    col += length;
+                    while (col >= width)
+                    {
+                        col -= width;
+                        ++row;
+                        if (row <= lastRow && (row % NumArgbCacheRows is 0))
+                        {
+                            this.ExtractPalettedAlphaRows(dec, row);
+                        }
+                    }
+
+                    if (pos < last && (col & mask) > 0)
+                    {
+                        htreeGroup = this.GetHTreeGroupForPos(hdr, col, row);
+                    }
+                }
+                else
+                {
+                    WebPThrowHelper.ThrowImageFormatException("bitstream error while parsing alpha data");
+                }
+
+                this.bitReader.Eos = this.bitReader.IsEndOfStream();
+            }
+
+            // Process the remaining rows corresponding to last row-block.
+            this.ExtractPalettedAlphaRows(dec, row > lastRow ? lastRow : row);
+        }
+
+        private void ExtractPalettedAlphaRows(AlphaDecoder dec, int lastRow)
+        {
+            // For vertical and gradient filtering, we need to decode the part above the
+            // cropTop row, in order to have the correct spatial predictors.
+            int topRow = (dec.FilterType is WebPFilterType.None || dec.FilterType is WebPFilterType.Horizontal)
+                             ? dec.CropTop
+                             : dec.LastRow;
+            int firstRow = (dec.LastRow < topRow) ? topRow : dec.LastRow;
+            if (lastRow > firstRow)
+            {
+                // Special method for paletted alpha data. We only process the cropped area.
+                Span<byte> output = dec.Alpha.AsSpan();
+                Span<uint> pixelData = dec.Vp8LDec.Pixels.Memory.Span;
+                Span<byte> pixelDataAsBytes = MemoryMarshal.Cast<uint, byte>(pixelData);
+                Span<byte> dst = output.Slice(dec.Width * firstRow);
+                Span<byte> input = pixelDataAsBytes.Slice(dec.Vp8LDec.Width * firstRow);
+
+                // TODO: check if any and the correct transform is present
+                Vp8LTransform transform = dec.Vp8LDec.Transforms[0];
+                this.ColorIndexInverseTransformAlpha(transform, firstRow, lastRow, input, dst);
+                //dec.AlphaApplyFilter(firstRow, lastRow, dst, width);
+            }
+
+            dec.LastRow = lastRow;
+        }
+
+        private void ColorIndexInverseTransformAlpha(
+            Vp8LTransform transform,
+            int yStart,
+            int yEnd,
+            Span<byte> src,
+            Span<byte> dst)
+        {
+            int bitsPerPixel = 8 >> transform.Bits;
+            int width = transform.XSize;
+            Span<uint> colorMap = transform.Data.Memory.Span;
+            int srcOffset = 0;
+            int dstOffset = 0;
+            if (bitsPerPixel < 8)
+            {
+                int pixelsPerByte = 1 << transform.Bits;
+                int countMask = pixelsPerByte - 1;
+                int bitMask = (1 << bitsPerPixel) - 1;
+                for (int y = yStart; y < yEnd; ++y)
+                {
+                    int packedPixels = 0;
+                    for (int x = 0; x < width; ++x)
+                    {
+                        if ((x & countMask) is 0)
+                        {
+                            packedPixels = src[srcOffset];
+                            srcOffset++;
+                        }
+
+                        dst[dstOffset] = GetAlphaValue((int)colorMap[packedPixels & bitMask]);
+                        dstOffset++;
+                        packedPixels >>= bitsPerPixel;
+                    }
+                }
+            }
+            else
+            {
+                MapAlpha(src, colorMap, dst, yStart, yEnd, width);
+            }
+        }
+
+        private static void MapAlpha(Span<byte> src, Span<uint> colorMap, Span<byte> dst, int yStart, int yEnd, int width)
+        {
+            int offset = 0;
+            for (int y = yStart; y < yEnd; ++y)
+            {
+                for (int x = 0; x < width; ++x)
+                {
+                    dst[offset] = GetAlphaValue((int)colorMap[src[offset]]);
+                    offset++;
+                }
+            }
+        }
+
+        private static bool Is8bOptimizable(Vp8LMetadata hdr)
+        {
+            if (hdr.ColorCacheSize > 0)
+            {
+                return false;
+            }
+
+            // When the Huffman tree contains only one symbol, we can skip the
+            // call to ReadSymbol() for red/blue/alpha channels.
+            for (int i = 0; i < hdr.NumHTreeGroups; ++i)
+            {
+                List<HuffmanCode[]> htrees = hdr.HTreeGroups[i].HTrees;
+                if (htrees[HuffIndex.Red][0].Value > 0)
+                {
+                    return false;
+                }
+
+                if (htrees[HuffIndex.Blue][0].Value > 0)
+                {
+                    return false;
+                }
+
+                if (htrees[HuffIndex.Alpha][0].Value > 0)
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
         private void UpdateDecoder(Vp8LDecoder decoder, int width, int height)
         {
             int numBits = decoder.Metadata.HuffmanSubSampleBits;
@@ -752,12 +967,11 @@ namespace SixLabors.ImageSharp.Formats.WebP
         }
 
         /// <summary>
-        /// Decodes the next Huffman code from bit-stream.
+        /// Decodes the next Huffman code from the bit-stream.
         /// FillBitWindow(br) needs to be called at minimum every second call to ReadSymbol, in order to pre-fetch enough bits.
         /// </summary>
         private uint ReadSymbol(Span<HuffmanCode> table)
         {
-            // TODO: if the bitReader field is moved to this base class we could omit the parameter.
             uint val = (uint)this.bitReader.PrefetchBits();
             Span<HuffmanCode> tableSpan = table.Slice((int)(val & HuffmanUtils.HuffmanTableMask));
             int nBits = tableSpan[0].BitsUsed - HuffmanUtils.HuffmanTableBits;
@@ -832,5 +1046,10 @@ namespace SixLabors.ImageSharp.Formats.WebP
             huff.Value |= hCode.Value << shift;
             return hCode.BitsUsed;
         }
+
+        private static byte GetAlphaValue(int val)
+        {
+            return (byte)((val >> 8) & 0xff);
+        }
     }
 }

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
 
@@ -66,20 +65,19 @@ namespace SixLabors.ImageSharp.Formats.WebP
             // Decode image data.
             this.ParseFrame(decoder, io);
 
-            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());
+                var alphaDecoder = new AlphaDecoder(width, height, info.Features.AlphaData, info.Features.AlphaChunkHeader, this.memoryAllocator);
+                alphaDecoder.Decode();
+                this.DecodePixelValues(width, height, decoder.Pixels, pixels, alphaDecoder.Alpha);
+            }
+            else
+            {
+                this.DecodePixelValues(width, height, decoder.Pixels, pixels);
             }
-
-            this.DecodePixelValues(width, height, decoder.Bgr, decodedAlpha, pixels);
         }
 
-        private void DecodePixelValues<TPixel>(int width, int height, Span<byte> pixelData, byte[] alpha, Buffer2D<TPixel> pixels)
+        private void DecodePixelValues<TPixel>(int width, int height, Span<byte> pixelData, Buffer2D<TPixel> pixels, byte[] alpha = null)
             where TPixel : struct, IPixel<TPixel>
         {
             TPixel color = default;
@@ -628,7 +626,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         private int EmitRgb(Vp8Decoder dec, Vp8Io io)
         {
-            byte[] buf = dec.Bgr;
+            byte[] buf = dec.Pixels;
             int numLinesOut = io.MbH; // a priori guess.
             Span<byte> curY = io.Y;
             Span<byte> curU = io.U;
@@ -1359,37 +1357,6 @@ namespace SixLabors.ImageSharp.Formats.WebP
             return io;
         }
 
-        private static bool Is8bOptimizable(Vp8LMetadata hdr)
-        {
-            if (hdr.ColorCacheSize > 0)
-            {
-                return false;
-            }
-
-            // When the Huffman tree contains only one symbol, we can skip the
-            // call to ReadSymbol() for red/blue/alpha channels.
-            for (int i = 0; i < hdr.NumHTreeGroups; ++i)
-            {
-                List<HuffmanCode[]> htrees = hdr.HTreeGroups[i].HTrees;
-                if (htrees[HuffIndex.Red][0].Value > 0)
-                {
-                    return false;
-                }
-
-                if (htrees[HuffIndex.Blue][0].Value > 0)
-                {
-                    return false;
-                }
-
-                if (htrees[HuffIndex.Alpha][0].Value > 0)
-                {
-                    return false;
-                }
-            }
-
-            return true;
-        }
-
         private static uint NzCodeBits(uint nzCoeffs, int nz, int dcNz)
         {
             nzCoeffs <<= 2;

tests/ImageSharp.Tests/Formats/WebP/WebPDecoderTests.cs

@@ -169,11 +169,10 @@ namespace SixLabors.ImageSharp.Tests.Formats.WebP
         }
 
         [Theory]
-        [WithFile(Lossy.Alpha1, PixelTypes.Rgba32)]
-        [WithFile(Lossy.Alpha2, PixelTypes.Rgba32)]
-        [WithFile(Lossy.Alpha3, PixelTypes.Rgba32)]
-        [WithFile(Lossy.Alpha4, PixelTypes.Rgba32)]
         [WithFile(Lossy.AlphaNoCompression, PixelTypes.Rgba32)]
+        [WithFile(Lossy.AlphaNoCompressionNoFilter, PixelTypes.Rgba32)]
+        [WithFile(Lossy.AlphaCompressedNoFilter, PixelTypes.Rgba32)]
+        [WithFile(Lossy.AlphaNoCompressionHorizontalFilter, PixelTypes.Rgba32)]
         public void WebpDecoder_CanDecode_Lossy_WithAlpha<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : struct, IPixel<TPixel>
         {

tests/ImageSharp.Tests/TestImages.cs

@@ -491,10 +491,15 @@ namespace SixLabors.ImageSharp.Tests
                 // Lossy images with an alpha channel.
                 public const string Alpha1 = "WebP/lossy_alpha1.webp";
                 public const string Alpha2 = "WebP/lossy_alpha2.webp";
-                public const string Alpha3 = "WebP/lossy_alpha3.webp";
-                public const string Alpha4 = "WebP/lossy_alpha4.webp";
                 public const string AlphaNoCompression = "WebP/alpha_no_compression.webp";
-
+                public const string AlphaNoCompressionNoFilter = "WebP/alpha_filter_0_method_0.webp";
+                public const string AlphaCompressedNoFilter = "WebP/alpha_filter_0_method_1.webp";
+                public const string AlphaNoCompressionHorizontalFilter = "WebP/alpha_filter_1_method_0.webp";
+                public const string AlphaCompressedHorizontalFilter = "WebP/alpha_filter_1_method_1.webp";
+                public const string AlphaNoCompressionVerticalFilter = "WebP/alpha_filter_2_method_0.webp";
+                public const string AlphaCompressedVerticalFilter = "WebP/alpha_filter_2_method_1.webp";
+                public const string AlphaNoCompressionGradientFilter = "WebP/alpha_filter_3_method_0.webp";
+                public const string AlphaCompressedGradientFilter = "WebP/alpha_filter_3_method_1.webp";
             }
         }
     }