introduce WebPLossyDecoder

introduce YUVPixel-Plane for the decoding process,
start reading header of the bitstream:
- Version,
- flag if frame is shown (only relevant for animations or VP8 videos, but the bit is present)
- size of the first partition.

src/ImageSharp/Formats/WebP/WebPDecoderCore.cs

@@ -328,8 +328,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
         private void ReadSimpleLossy<TPixel>(Buffer2D<TPixel> pixels, int width, int height, int imageDataSize)
             where TPixel : struct, IPixel<TPixel>
         {
-            // TODO: implement decoding. For simulating the decoding: skipping the chunk size bytes.
-            this.currentStream.Skip(imageDataSize - 10); // TODO: Not sure why we need to skip 10 bytes less here
+            var lossyDecoder = new WebPLossyDecoder(this.configuration, this.currentStream);
+            lossyDecoder.Decode(pixels, width, height, imageDataSize);
         }
 
         private void ReadSimpleLossless<TPixel>(Buffer2D<TPixel> pixels, int width, int height, int imageDataSize)

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -0,0 +1,70 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Memory;
+
+namespace SixLabors.ImageSharp.Formats.WebP
+{
+    class WebPLossyDecoder
+    {
+        private readonly Configuration configuration;
+
+        private readonly Stream currentStream;
+
+        private MemoryAllocator memoryAllocator;
+
+        public WebPLossyDecoder(
+            Configuration configuration,
+            Stream currentStream)
+        {
+            this.configuration = configuration;
+            this.currentStream = currentStream;
+            this.memoryAllocator = configuration.MemoryAllocator;
+        }
+
+        public void Decode<TPixel>(Buffer2D<TPixel> pixels, int width, int height, int imageDataSize)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            // TODO: implement decoding. For simulating the decoding: skipping the chunk size bytes.
+            this.currentStream.Skip(imageDataSize - 10); // TODO: Not sure why we need to skip 10 bytes less here
+
+            // we need buffers for Y U and V in size of the image
+            // TODO: increase size to enable using all prediction blocks? (see https://tools.ietf.org/html/rfc6386#page-9 )
+            Buffer2D<YUVPixel> yuvBufferCurrentFrame =
+                this.memoryAllocator
+                    .Allocate2D<YUVPixel>(width, height);
+
+            // TODO: var predictionBuffer - macro-block-sized with approximation of the portion of the image being reconstructed.
+            //  those prediction values are the base, the values from DCT processing are added to that
+
+            // TODO residue signal from DCT: 4x4 blocks of DCT transforms, 16Y, 4U, 4V
+            // TODO weiter bei  S.11
+
+            // bit STREAM: See https://tools.ietf.org/html/rfc6386#page-29 ("Frame Header")
+            Vp8LBitReader bitReader = new Vp8LBitReader(this.currentStream);
+            bool isInterframe = bitReader.ReadBit();
+            if (isInterframe)
+            {
+                throw new NotImplementedException("only key frames supported yet");
+            }
+
+            byte version = (byte)((bitReader.ReadBit() ? 2 : 0) | (bitReader.ReadBit() ? 1 : 0));
+            bool isShowFrame = bitReader.ReadBit();
+
+            uint firstPartitionSize = (bitReader.Read(16) << 3) | bitReader.Read(3);
+        }
+    }
+
+    struct YUVPixel
+    {
+        public byte Y { get; }
+
+        public byte U { get; }
+
+        public byte V { get; }
+    }
+}

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

@@ -30,5 +30,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.WebP
                 Assert.Equal(expectedBitsPerPixel, imageInfo.PixelType.BitsPerPixel);
             }
         }
+
+        [Theory]
+        [InlineData(Lossy.Alpha.LossyAlpha1, 1000, 307, 24)]
+        public void DecodeLossyImage_Tmp(string imagePath, int expectedWidth, int expectedHeight, int expectedBitsPerPixel)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using (var stream = new MemoryStream(testFile.Bytes, false))
+            {
+                var image = Image.Load(stream);
+                Assert.Equal(expectedWidth, image.Width);
+                Assert.Equal(expectedHeight, image.Height);
+            }
+        }
     }
 }