Add webp decoder option BackgroundColorHandling to decide howto handl…

…e the background color in the ANIM chunk

src/ImageSharp/Formats/Tiff/Compression/Decompressors/WebpTiffCompression.cs

@@ -32,7 +32,7 @@ public WebpTiffCompression(DecoderOptions options, MemoryAllocator memoryAllocat
     /// <inheritdoc/>
     protected override void Decompress(BufferedReadStream stream, int byteCount, int stripHeight, Span<byte> buffer, CancellationToken cancellationToken)
     {
-        using WebpDecoderCore decoder = new(this.options);
+        using WebpDecoderCore decoder = new(new WebpDecoderOptions());
         using Image<Rgb24> image = decoder.Decode<Rgb24>(stream, cancellationToken);
         CopyImageBytesToBuffer(buffer, image.Frames.RootFrame.PixelBuffer);
     }

src/ImageSharp/Formats/Tiff/Compression/TiffDecompressorsFactory.cs

@@ -4,6 +4,7 @@
 using SixLabors.ImageSharp.Formats.Tiff.Compression.Decompressors;
 using SixLabors.ImageSharp.Formats.Tiff.Constants;
 using SixLabors.ImageSharp.Formats.Tiff.PhotometricInterpretation;
+using SixLabors.ImageSharp.Formats.Webp;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Tiff.Compression;

src/ImageSharp/Formats/Webp/BackgroundColorHandling.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Webp;
+
+/// <summary>
+/// Enum to decide how to handle the background color of the Animation chunk during decoding.
+/// </summary>
+public enum BackgroundColorHandling
+{
+    /// <summary>
+    /// The background color of the ANIM chunk will be used to initialize the canvas to fill the unused space on the canvas around the frame.
+    /// Also, if AnimationDisposalMethod.Dispose is used, this color will be used to restore the canvas background.
+    /// </summary>
+    Standard = 0,
+
+    /// <summary>
+    /// The background color of the ANIM chunk is ignored and instead the canvas is initialized with black, BGRA(0, 0, 0, 0).
+    /// </summary>
+    Ignore = 1
+}

src/ImageSharp/Formats/Webp/WebpAnimationDecoder.cs

@@ -52,17 +52,24 @@ internal class WebpAnimationDecoder : IDisposable
     /// </summary>
     private IMemoryOwner<byte>? alphaData;
 
+    /// <summary>
+    /// The flag to decide how to handle the background color in the Animation Chunk.
+    /// </summary>
+    private readonly BackgroundColorHandling backgroundColorHandling;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="WebpAnimationDecoder"/> class.
     /// </summary>
     /// <param name="memoryAllocator">The memory allocator.</param>
     /// <param name="configuration">The global configuration.</param>
     /// <param name="maxFrames">The maximum number of frames to decode. Inclusive.</param>
-    public WebpAnimationDecoder(MemoryAllocator memoryAllocator, Configuration configuration, uint maxFrames)
+    /// <param name="backgroundColorHandling">The flag to decide how to handle the background color in the Animation Chunk.</param>
+    public WebpAnimationDecoder(MemoryAllocator memoryAllocator, Configuration configuration, uint maxFrames, BackgroundColorHandling backgroundColorHandling)
     {
         this.memoryAllocator = memoryAllocator;
         this.configuration = configuration;
         this.maxFrames = maxFrames;
+        this.backgroundColorHandling = backgroundColorHandling;
     }
 
     /// <summary>
@@ -94,7 +101,10 @@ public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, WebpFeatures feat
             switch (chunkType)
             {
                 case WebpChunkType.Animation:
-                    uint dataSize = this.ReadFrame(stream, ref image, ref previousFrame, width, height, features.AnimationBackgroundColor!.Value);
+                    Color backgroundColor = this.backgroundColorHandling == BackgroundColorHandling.Ignore
+                        ? new Color(new Bgra32(0, 0, 0, 0))
+                        : features.AnimationBackgroundColor!.Value;
+                    uint dataSize = this.ReadFrame(stream, ref image, ref previousFrame, width, height, backgroundColor);
                     remainingBytes -= (int)dataSize;
                     break;
                 case WebpChunkType.Xmp:

src/ImageSharp/Formats/Webp/WebpDecoder.cs

@@ -8,7 +8,7 @@ namespace SixLabors.ImageSharp.Formats.Webp;
 /// <summary>
 /// Image decoder for generating an image out of a webp stream.
 /// </summary>
-public sealed class WebpDecoder : ImageDecoder
+public sealed class WebpDecoder : SpecializedImageDecoder<WebpDecoderOptions>
 {
     private WebpDecoder()
     {
@@ -25,25 +25,33 @@ protected override ImageInfo Identify(DecoderOptions options, Stream stream, Can
         Guard.NotNull(options, nameof(options));
         Guard.NotNull(stream, nameof(stream));
 
-        using WebpDecoderCore decoder = new(options);
+        using WebpDecoderCore decoder = new(new WebpDecoderOptions() { GeneralOptions = options });
         return decoder.Identify(options.Configuration, stream, cancellationToken);
     }
 
     /// <inheritdoc/>
-    protected override Image<TPixel> Decode<TPixel>(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
+    protected override Image<TPixel> Decode<TPixel>(WebpDecoderOptions options, Stream stream, CancellationToken cancellationToken)
     {
         Guard.NotNull(options, nameof(options));
         Guard.NotNull(stream, nameof(stream));
 
         using WebpDecoderCore decoder = new(options);
-        Image<TPixel> image = decoder.Decode<TPixel>(options.Configuration, stream, cancellationToken);
+        Image<TPixel> image = decoder.Decode<TPixel>(options.GeneralOptions.Configuration, stream, cancellationToken);
 
-        ScaleToTargetSize(options, image);
+        ScaleToTargetSize(options.GeneralOptions, image);
 
         return image;
     }
 
+    /// <inheritdoc/>
+    protected override Image Decode(WebpDecoderOptions options, Stream stream, CancellationToken cancellationToken)
+        => this.Decode<Rgba32>(options, stream, cancellationToken);
+
     /// <inheritdoc/>
     protected override Image Decode(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
         => this.Decode<Rgba32>(options, stream, cancellationToken);
+
+    /// <inheritdoc/>
+    protected override WebpDecoderOptions CreateDefaultSpecializedOptions(DecoderOptions options)
+        => new() { GeneralOptions = options };
 }

src/ImageSharp/Formats/Webp/WebpDecoderCore.cs

@@ -48,16 +48,22 @@ internal sealed class WebpDecoderCore : IImageDecoderInternals, IDisposable
     /// </summary>
     private WebpImageInfo? webImageInfo;
 
+    /// <summary>
+    /// The flag to decide how to handle the background color in the Animation Chunk.
+    /// </summary>
+    private BackgroundColorHandling backgroundColorHandling;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="WebpDecoderCore"/> class.
     /// </summary>
     /// <param name="options">The decoder options.</param>
-    public WebpDecoderCore(DecoderOptions options)
+    public WebpDecoderCore(WebpDecoderOptions options)
     {
-        this.Options = options;
-        this.configuration = options.Configuration;
-        this.skipMetadata = options.SkipMetadata;
-        this.maxFrames = options.MaxFrames;
+        this.Options = options.GeneralOptions;
+        this.backgroundColorHandling = options.BackgroundColorHandling;
+        this.configuration = options.GeneralOptions.Configuration;
+        this.skipMetadata = options.GeneralOptions.SkipMetadata;
+        this.maxFrames = options.GeneralOptions.MaxFrames;
         this.memoryAllocator = this.configuration.MemoryAllocator;
     }
 
@@ -83,7 +89,7 @@ public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, CancellationToken
             {
                 if (this.webImageInfo.Features is { Animation: true })
                 {
-                    using WebpAnimationDecoder animationDecoder = new(this.memoryAllocator, this.configuration, this.maxFrames);
+                    using WebpAnimationDecoder animationDecoder = new(this.memoryAllocator, this.configuration, this.maxFrames, this.backgroundColorHandling);
                     return animationDecoder.Decode<TPixel>(stream, this.webImageInfo.Features, this.webImageInfo.Width, this.webImageInfo.Height, fileSize);
                 }
 

src/ImageSharp/Formats/Webp/WebpDecoderOptions.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Webp;
+
+/// <summary>
+/// Configuration options for decoding webp images.
+/// </summary>
+public sealed class WebpDecoderOptions : ISpecializedDecoderOptions
+{
+    /// <inheritdoc/>
+    public DecoderOptions GeneralOptions { get; init; } = new();
+
+    /// <summary>
+    /// Gets the flag to decide how to handle the background color Animation Chunk.
+    /// The specification is vague on how to handle the background color of the animation chunk.
+    /// This option let's the user choose how to deal with it.
+    /// </summary>
+    /// <see href="https://developers.google.com/speed/webp/docs/riff_container#animation"/>
+    public BackgroundColorHandling BackgroundColorHandling { get; init; } = BackgroundColorHandling.Standard;
+}