Merge pull request #2173 from SixLabors/bp/Issue2171

Fix issue when writing lossy webp with alpha data

Author: JimBobSquarePants

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

@@ -382,7 +382,7 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream)
                 (uint)width,
                 (uint)height,
                 hasAlpha,
-                alphaData,
+                alphaData.Slice(0, alphaDataSize),
                 this.alphaCompression && alphaCompressionSucceeded);
         }
 

tests/ImageSharp.Tests/Formats/WebP/Vp8LHistogramTests.cs

@@ -7,8 +7,9 @@
 using SixLabors.ImageSharp.Tests.TestUtilities;
 using Xunit;
 
-namespace SixLabors.ImageSharp.Tests.Formats.WebP
+namespace SixLabors.ImageSharp.Tests.Formats.Webp
 {
+    [Trait("Format", "Webp")]
     public class Vp8LHistogramTests
     {
         private static void RunAddVectorTest()

tests/ImageSharp.Tests/Formats/WebP/Vp8ResidualTests.cs

@@ -5,7 +5,7 @@
 using SixLabors.ImageSharp.Tests.TestUtilities;
 using Xunit;
 
-namespace SixLabors.ImageSharp.Tests.Formats.WebP
+namespace SixLabors.ImageSharp.Tests.Formats.Webp
 {
     [Trait("Format", "Webp")]
     public class Vp8ResidualTests

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

@@ -7,6 +7,7 @@
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.TestUtilities;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
+using SixLabors.ImageSharp.Tests.TestUtilities.ReferenceCodecs;
 using Xunit;
 using static SixLabors.ImageSharp.Tests.TestImages.Webp;
 
@@ -268,9 +269,9 @@ public void Encode_Lossy_WithDifferentMethodsAndQuality_Works<TPixel>(TestImageP
         }
 
         [Theory]
-        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32, false)]
-        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32, true)]
-        public void Encode_Lossy_WithAlpha_Works<TPixel>(TestImageProvider<TPixel> provider, bool compressed)
+        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32, false, 64020)]
+        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32, true, 16200)]
+        public void Encode_Lossy_WithAlpha_Works<TPixel>(TestImageProvider<TPixel> provider, bool compressed, int expectedFileSize)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             var encoder = new WebpEncoder()
@@ -280,7 +281,16 @@ public void Encode_Lossy_WithAlpha_Works<TPixel>(TestImageProvider<TPixel> provi
             };
 
             using Image<TPixel> image = provider.GetImage();
-            image.VerifyEncoder(provider, "webp", $"with_alpha_compressed_{compressed}", encoder, ImageComparer.Tolerant(0.04f));
+            string encodedFile = image.VerifyEncoder(
+                provider,
+                "webp",
+                $"with_alpha_compressed_{compressed}",
+                encoder,
+                ImageComparer.Tolerant(0.04f),
+                referenceDecoder: new MagickReferenceDecoder());
+
+            int encodedBytes = File.ReadAllBytes(encodedFile).Length;
+            Assert.True(encodedBytes <= expectedFileSize);
         }
 
         [Theory]

tests/ImageSharp.Tests/TestUtilities/ReferenceCodecs/MagickReferenceDecoder.cs

@@ -26,7 +26,7 @@ public MagickReferenceDecoder()
 
         public MagickReferenceDecoder(bool validate) => this.validate = validate;
 
-        public static MagickReferenceDecoder Instance { get; } = new MagickReferenceDecoder();
+        public static MagickReferenceDecoder Instance { get; } = new();
 
         private static void FromRgba32Bytes<TPixel>(Configuration configuration, Span<byte> rgbaBytes, IMemoryGroup<TPixel> destinationGroup)
             where TPixel : unmanaged, ImageSharp.PixelFormats.IPixel<TPixel>

tests/ImageSharp.Tests/TestUtilities/TestImageExtensions.cs

@@ -661,7 +661,8 @@ internal static void VerifyOperation<TPixel>(
         /// Loads the expected image with a reference decoder + compares it to <paramref name="image"/>.
         /// Also performs a debug save using <see cref="ImagingTestCaseUtility.SaveTestOutputFile{TPixel}"/>.
         /// </summary>
-        internal static void VerifyEncoder<TPixel>(
+        /// <returns>The path to the encoded output file.</returns>
+        internal static string VerifyEncoder<TPixel>(
             this Image<TPixel> image,
             ITestImageProvider provider,
             string extension,
@@ -687,6 +688,8 @@ internal static void VerifyEncoder<TPixel>(
                 ImageComparer comparer = customComparer ?? ImageComparer.Exact;
                 comparer.VerifySimilarity(encodedImage, image);
             }
+
+            return actualOutputFile;
         }
 
         internal static AllocatorBufferCapacityConfigurator LimitAllocatorBufferCapacity<TPixel>(