Add option to decode just one frame

src/ImageSharp/Formats/Webp/IWebpDecoderOptions.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
+using SixLabors.ImageSharp.Metadata;
+
 namespace SixLabors.ImageSharp.Formats.Webp
 {
     /// <summary>
@@ -12,5 +14,10 @@ namespace SixLabors.ImageSharp.Formats.Webp
         /// Gets a value indicating whether the metadata should be ignored when the image is being decoded.
         /// </summary>
         bool IgnoreMetadata { get; }
+
+        /// <summary>
+        /// Gets the decoding mode for multi-frame images.
+        /// </summary>
+        FrameDecodingMode DecodingMode { get; }
     }
 }

src/ImageSharp/Formats/Webp/WebpAnimationDecoder.cs

@@ -53,10 +53,12 @@ namespace SixLabors.ImageSharp.Formats.Webp
         /// </summary>
         /// <param name="memoryAllocator">The memory allocator.</param>
         /// <param name="configuration">The global configuration.</param>
-        public WebpAnimationDecoder(MemoryAllocator memoryAllocator, Configuration configuration)
+        /// <param name="decodingMode">The frame decoding mode.</param>
+        public WebpAnimationDecoder(MemoryAllocator memoryAllocator, Configuration configuration, FrameDecodingMode decodingMode)
         {
             this.memoryAllocator = memoryAllocator;
             this.configuration = configuration;
+            this.DecodingMode = decodingMode;
         }
 
         /// <summary>
@@ -64,6 +66,11 @@ namespace SixLabors.ImageSharp.Formats.Webp
         /// </summary>
         public IMemoryOwner<byte> AlphaData { get; set; }
 
+        /// <summary>
+        /// Gets the decoding mode for multi-frame images.
+        /// </summary>
+        public FrameDecodingMode DecodingMode { get; }
+
         /// <summary>
         /// Decodes the animated webp image from the specified stream.
         /// </summary>
@@ -103,7 +110,7 @@ namespace SixLabors.ImageSharp.Formats.Webp
                         break;
                 }
 
-                if (stream.Position == stream.Length)
+                if (stream.Position == stream.Length || this.DecodingMode is FrameDecodingMode.First)
                 {
                     break;
                 }

src/ImageSharp/Formats/Webp/WebpDecoder.cs

@@ -6,6 +6,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Webp
@@ -20,6 +21,12 @@ namespace SixLabors.ImageSharp.Formats.Webp
         /// </summary>
         public bool IgnoreMetadata { get; set; }
 
+        /// <summary>
+        /// Gets or sets the decoding mode for multi-frame images.
+        /// Defaults to All.
+        /// </summary>
+        public FrameDecodingMode DecodingMode { get; set; } = FrameDecodingMode.All;
+
         /// <inheritdoc/>
         public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
             where TPixel : unmanaged, IPixel<TPixel>

src/ImageSharp/Formats/Webp/WebpDecoderCore.cs

@@ -56,10 +56,19 @@ namespace SixLabors.ImageSharp.Formats.Webp
         public WebpDecoderCore(Configuration configuration, IWebpDecoderOptions options)
         {
             this.Configuration = configuration;
+            this.DecodingMode = options.DecodingMode;
             this.memoryAllocator = configuration.MemoryAllocator;
             this.IgnoreMetadata = options.IgnoreMetadata;
         }
 
+        /// <inheritdoc/>
+        public Configuration Configuration { get; }
+
+        /// <summary>
+        /// Gets the decoding mode for multi-frame images.
+        /// </summary>
+        public FrameDecodingMode DecodingMode { get; }
+
         /// <summary>
         /// Gets a value indicating whether the metadata should be ignored when the image is being decoded.
         /// </summary>
@@ -70,9 +79,6 @@ namespace SixLabors.ImageSharp.Formats.Webp
         /// </summary>
         public ImageMetadata Metadata { get; private set; }
 
-        /// <inheritdoc/>
-        public Configuration Configuration { get; }
-
         /// <summary>
         /// Gets the dimensions of the image.
         /// </summary>
@@ -96,7 +102,7 @@ namespace SixLabors.ImageSharp.Formats.Webp
             {
                 if (this.webImageInfo.Features is { Animation: true })
                 {
-                    using var animationDecoder = new WebpAnimationDecoder(this.memoryAllocator, this.Configuration);
+                    using var animationDecoder = new WebpAnimationDecoder(this.memoryAllocator, this.Configuration, this.DecodingMode);
                     return animationDecoder.Decode<TPixel>(stream, this.webImageInfo.Features, this.webImageInfo.Width, this.webImageInfo.Height, fileSize);
                 }
 

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

@@ -3,6 +3,7 @@
 
 using System.IO;
 using SixLabors.ImageSharp.Formats.Webp;
+using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.TestUtilities;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
@@ -234,7 +235,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
 
         // TODO: Reference decoder throws here MagickCorruptImageErrorException, webpinfo also indicates an error here, but decoding the image seems to work.
         // [WithFile(Lossless.GreenTransform5, PixelTypes.Rgba32)]
-        public void WebpDecoder_CanDecode_Lossless_WithSubstractGreenTransform<TPixel>(
+        public void WebpDecoder_CanDecode_Lossless_WithSubtractGreenTransform<TPixel>(
             TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel>
         {
@@ -345,6 +346,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
 
                 Assert.Equal(0, webpMetaData.AnimationLoopCount);
                 Assert.Equal(150U, frameMetaData.FrameDuration);
+                Assert.Equal(12, image.Frames.Count);
             }
         }
 
@@ -363,6 +365,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
 
                 Assert.Equal(0, webpMetaData.AnimationLoopCount);
                 Assert.Equal(150U, frameMetaData.FrameDuration);
+                Assert.Equal(12, image.Frames.Count);
+            }
+        }
+
+        [Theory]
+        [WithFile(Lossless.Animated, PixelTypes.Rgba32)]
+        public void Decode_AnimatedLossless_WithFrameDecodingModeFirst_OnlyDecodesOneFrame<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new WebpDecoder() { DecodingMode = FrameDecodingMode.First }))
+            {
+                Assert.Equal(1, image.Frames.Count);
             }
         }