Add FrameDecodingMode. Fix #255

src/ImageSharp/Formats/Gif/FrameDecodingMode.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Formats.Gif
+{
+    /// <summary>
+    /// Enumerated frame process modes to apply to multi-frame images.
+    /// </summary>
+    public enum FrameDecodingMode
+    {
+        /// <summary>
+        /// Decodes all the frames of a multi-frame image.
+        /// </summary>
+        All,
+
+        /// <summary>
+        /// Decodes only the first frame of a multi-frame image.
+        /// </summary>
+        First
+    }
+}

src/ImageSharp/Formats/Gif/GifDecoder.cs

@@ -24,6 +24,11 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         public Encoding TextEncoding { get; set; } = GifConstants.DefaultEncoding;
 
+        /// <summary>
+        /// Gets or sets the decoding mode for multi-frame images
+        /// </summary>
+        public FrameDecodingMode DecodingMode { get; set; } = FrameDecodingMode.All;
+
         /// <inheritdoc/>
         public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
             where TPixel : struct, IPixel<TPixel>

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -84,6 +84,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         {
             this.TextEncoding = options.TextEncoding ?? GifConstants.DefaultEncoding;
             this.IgnoreMetadata = options.IgnoreMetadata;
+            this.DecodingMode = options.DecodingMode;
             this.configuration = configuration ?? Configuration.Default;
         }
 
@@ -97,6 +98,11 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         public Encoding TextEncoding { get; }
 
+        /// <summary>
+        /// Gets the decoding mode for multi-frame images
+        /// </summary>
+        public FrameDecodingMode DecodingMode { get; }
+
         /// <summary>
         /// Decodes the stream to the image.
         /// </summary>
@@ -129,6 +135,11 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 {
                     if (nextFlag == GifConstants.ImageLabel)
                     {
+                        if (this.previousFrame != null && this.DecodingMode == FrameDecodingMode.First)
+                        {
+                            break;
+                        }
+
                         this.ReadFrame();
                     }
                     else if (nextFlag == GifConstants.ExtensionIntroducer)
@@ -238,14 +249,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
             {
                 throw new ImageFormatException($"Invalid gif colormap size '{this.logicalScreenDescriptor.GlobalColorTableSize}'");
             }
-
-            /* // No point doing this as the max width/height is always int.Max and that always bigger than the max size of a gif which is stored in a short.
-            if (this.logicalScreenDescriptor.Width > Image<TPixel>.MaxWidth || this.logicalScreenDescriptor.Height > Image<TPixel>.MaxHeight)
-            {
-                throw new ArgumentOutOfRangeException(
-                    $"The input gif '{this.logicalScreenDescriptor.Width}x{this.logicalScreenDescriptor.Height}' is bigger then the max allowed size '{Image<TPixel>.MaxWidth}x{Image<TPixel>.MaxHeight}'");
-            }
-            */
         }
 
         /// <summary>

src/ImageSharp/Formats/Gif/IGifDecoderOptions.cs

@@ -19,5 +19,10 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// Gets the encoding that should be used when reading comments.
         /// </summary>
         Encoding TextEncoding { get; }
+
+        /// <summary>
+        /// Gets the decoding mode for multi-frame images
+        /// </summary>
+        FrameDecodingMode DecodingMode { get; }
     }
 }

tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.Text;
-using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Gif;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Primitives;
@@ -45,12 +44,12 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void Decode_IgnoreMetadataIsFalse_CommentsAreRead()
         {
-            GifDecoder options = new GifDecoder()
+            var options = new GifDecoder
             {
                 IgnoreMetadata = false
             };
 
-            TestFile testFile = TestFile.Create(TestImages.Gif.Rings);
+            var testFile = TestFile.Create(TestImages.Gif.Rings);
 
             using (Image<Rgba32> image = testFile.CreateImage(options))
             {
@@ -63,12 +62,12 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void Decode_IgnoreMetadataIsTrue_CommentsAreIgnored()
         {
-            GifDecoder options = new GifDecoder()
+            var options = new GifDecoder
             {
                 IgnoreMetadata = true
             };
 
-            TestFile testFile = TestFile.Create(TestImages.Gif.Rings);
+            var testFile = TestFile.Create(TestImages.Gif.Rings);
 
             using (Image<Rgba32> image = testFile.CreateImage(options))
             {
@@ -79,12 +78,12 @@ namespace SixLabors.ImageSharp.Tests
         [Fact]
         public void Decode_TextEncodingSetToUnicode_TextIsReadWithCorrectEncoding()
         {
-            GifDecoder options = new GifDecoder()
+            var options = new GifDecoder
             {
                 TextEncoding = Encoding.Unicode
             };
 
-            TestFile testFile = TestFile.Create(TestImages.Gif.Rings);
+            var testFile = TestFile.Create(TestImages.Gif.Rings);
 
             using (Image<Rgba32> image = testFile.CreateImage(options))
             {
@@ -92,5 +91,27 @@ namespace SixLabors.ImageSharp.Tests
                 Assert.Equal("浉条卥慨灲", image.MetaData.Properties[0].Value);
             }
         }
+
+        [Theory]
+        [WithFile(TestImages.Gif.Giphy, PixelTypes.Rgba32)]
+        public void CanDecodeJustOneFrame<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new GifDecoder { DecodingMode = FrameDecodingMode.First }))
+            {
+                Assert.Equal(1, image.Frames.Count);
+            }
+        }
+
+        [Theory]
+        [WithFile(TestImages.Gif.Giphy, PixelTypes.Rgba32)]
+        public void CanDecodeAllFrames<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new GifDecoder { DecodingMode = FrameDecodingMode.All }))
+            {
+                Assert.True(image.Frames.Count > 1);
+            }
+        }
     }
-}
+}