Handle missing global color table. Fix #1963

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -376,7 +376,17 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 indices = this.Configuration.MemoryAllocator.Allocate2D<byte>(this.imageDescriptor.Width, this.imageDescriptor.Height, AllocationOptions.Clean);
 
                 this.ReadFrameIndices(indices);
-                ReadOnlySpan<Rgb24> colorTable = MemoryMarshal.Cast<byte, Rgb24>((localColorTable ?? this.globalColorTable).GetSpan());
+                Span<byte> rawColorTable = default;
+                if (localColorTable != null)
+                {
+                    rawColorTable = localColorTable.GetSpan();
+                }
+                else if (this.globalColorTable != null)
+                {
+                    rawColorTable = this.globalColorTable.GetSpan();
+                }
+
+                ReadOnlySpan<Rgb24> colorTable = MemoryMarshal.Cast<byte, Rgb24>(rawColorTable);
                 this.ReadFrameColors(ref image, ref previousFrame, indices, colorTable, this.imageDescriptor);
 
                 // Skip any remaining blocks
@@ -415,6 +425,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         {
             int imageWidth = this.logicalScreenDescriptor.Width;
             int imageHeight = this.logicalScreenDescriptor.Height;
+            bool transFlag = this.graphicsControlExtension.TransparencyFlag;
 
             ImageFrame<TPixel> prevFrame = null;
             ImageFrame<TPixel> currentFrame = null;
@@ -422,8 +433,15 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             if (previousFrame is null)
             {
-                // This initializes the image to become fully transparent because the alpha channel is zero.
-                image = new Image<TPixel>(this.Configuration, imageWidth, imageHeight, this.metadata);
+                if (!transFlag)
+                {
+                    image = new Image<TPixel>(this.Configuration, imageWidth, imageHeight, Color.Black.ToPixel<TPixel>(), this.metadata);
+                }
+                else
+                {
+                    // This initializes the image to become fully transparent because the alpha channel is zero.
+                    image = new Image<TPixel>(this.Configuration, imageWidth, imageHeight, this.metadata);
+                }
 
                 this.SetFrameMetadata(image.Frames.RootFrame.Metadata);
 
@@ -445,6 +463,11 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 this.RestoreToBackground(imageFrame);
             }
 
+            if (colorTable.Length == 0)
+            {
+                return;
+            }
+
             int interlacePass = 0; // The interlace pass
             int interlaceIncrement = 8; // The interlacing line increment
             int interlaceY = 0; // The current interlaced line
@@ -452,7 +475,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
             int descriptorBottom = descriptorTop + descriptor.Height;
             int descriptorLeft = descriptor.Left;
             int descriptorRight = descriptorLeft + descriptor.Width;
-            bool transFlag = this.graphicsControlExtension.TransparencyFlag;
             byte transIndex = this.graphicsControlExtension.TransparencyIndex;
             int colorTableMaxIdx = colorTable.Length - 1;
 
@@ -635,10 +657,13 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 int globalColorTableLength = this.logicalScreenDescriptor.GlobalColorTableSize * 3;
                 this.gifMetadata.GlobalColorTableLength = globalColorTableLength;
 
-                this.globalColorTable = this.MemoryAllocator.Allocate<byte>(globalColorTableLength, AllocationOptions.Clean);
+                if (globalColorTableLength > 0)
+                {
+                    this.globalColorTable = this.MemoryAllocator.Allocate<byte>(globalColorTableLength, AllocationOptions.Clean);
 
-                // Read the global color table data from the stream
-                stream.Read(this.globalColorTable.GetSpan());
+                    // Read the global color table data from the stream
+                    stream.Read(this.globalColorTable.GetSpan());
+                }
             }
         }
     }

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

@@ -247,5 +247,17 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
                     "Disco")
                 .Dispose();
         }
+
+        // https://github.com/SixLabors/ImageSharp/issues/1962
+        [Theory]
+        [WithFile(TestImages.Gif.Issues.Issue1962NoColorTable, PixelTypes.Rgba32)]
+        public void Issue1962<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage();
+            image.DebugSave(provider);
+
+            image.CompareFirstFrameToReferenceOutput(ImageComparer.Exact, provider);
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -451,6 +451,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string Issue1505 = "Gif/issues/issue1505_argumentoutofrange.png";
                 public const string Issue1530 = "Gif/issues/issue1530.gif";
                 public const string InvalidColorIndex = "Gif/issues/issue1668_invalidcolorindex.gif";
+                public const string Issue1962NoColorTable = "Gif/issues/issue1962_tiniest_gif_1st.gif";
             }
 
             public static readonly string[] All = { Rings, Giphy, Cheers, Trans, Kumin, Leo, Ratio4x1, Ratio1x4 };

tests/Images/External/ReferenceOutput/GifDecoderTests/Issue1962_Rgba32_issue1962_tiniest_gif_1st.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:4f8c6d416f09671777934e57bc67fb52ccc97145dc6f1869e628d9ffd7d8f6e7
+size 119

tests/Images/Input/Gif/issues/issue1962_tiniest_gif_1st.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:3b7b8a4b411ddf8db9bacc2f3aabf406f8e4c0c087829b336ca331c40adfdff1
+size 26