Fix issue decoding lossy webp with compressed alpha data, fixes #2257

src/ImageSharp/Formats/Webp/AlphaDecoder.cs

@@ -61,7 +61,13 @@ internal class AlphaDecoder : IDisposable
             var bitReader = new Vp8LBitReader(data);
             this.LosslessDecoder = new WebpLosslessDecoder(bitReader, memoryAllocator, configuration);
             this.LosslessDecoder.DecodeImageStream(this.Vp8LDec, width, height, true);
-            this.Use8BDecode = this.Vp8LDec.Transforms.Count > 0 && Is8BOptimizable(this.Vp8LDec.Metadata);
+
+            // Special case: if alpha data uses only the color indexing transform and
+            // doesn't use color cache (a frequent case), we will use DecodeAlphaData()
+            // method that only needs allocation of 1 byte per pixel (alpha channel).
+            this.Use8BDecode = this.Vp8LDec.Transforms.Count is 1
+                               && this.Vp8LDec.Transforms[0].TransformType == Vp8LTransformType.ColorIndexingTransform
+                               && Is8BOptimizable(this.Vp8LDec.Metadata);
         }
     }
 

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

@@ -396,6 +396,17 @@ public class WebpDecoderTests
         image.CompareToOriginal(provider, ReferenceDecoder);
     }
 
+    // https://github.com/SixLabors/ImageSharp/issues/2257
+    [Theory]
+    [WithFile(Lossy.Issue2257, PixelTypes.Rgba32)]
+    public void WebpDecoder_CanDecode_Issue2257<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(WebpDecoder);
+        image.DebugSave(provider);
+        image.CompareToOriginal(provider, ReferenceDecoder);
+    }
+
     [Theory]
     [WithFile(Lossless.LossLessCorruptImage3, PixelTypes.Rgba32)]
     public void WebpDecoder_ThrowImageFormatException_OnInvalidImages<TPixel>(TestImageProvider<TPixel> provider)

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

@@ -288,7 +288,7 @@ public class WebpEncoderTests
             referenceDecoder: new MagickReferenceDecoder());
 
         int encodedBytes = File.ReadAllBytes(encodedFile).Length;
-        Assert.True(encodedBytes <= expectedFileSize);
+        Assert.True(encodedBytes <= expectedFileSize, $"encoded bytes are {encodedBytes} and should be smaller then expected file size of {expectedFileSize}");
     }
 
     [Theory]

tests/ImageSharp.Tests/TestImages.cs

@@ -737,6 +737,7 @@ public static class TestImages
             // Issues
             public const string Issue1594 = "Webp/issues/Issue1594.webp";
             public const string Issue2243 = "Webp/issues/Issue2243.webp";
+            public const string Issue2257 = "Webp/issues/Issue2257.webp";
         }
     }
 

tests/Images/Input/Webp/issues/Issue2257.webp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:55f87aee4283615bad9705ac459867facba5b6be114d7e1ece51db7c1ef87916
+size 117810