Handle dedup of local palette of 256 length

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -380,18 +380,42 @@ private static GifFrameMetadata GetGifFrameMetadata<TPixel>(ImageFrame<TPixel> f
                 // We can use the color data from the decoded metadata here.
                 // We avoid dithering by default to preserve the original colors.
                 ReadOnlyMemory<Color> palette = metadata.LocalColorTable.Value;
-
                 if (hasDuplicates && !metadata.HasTransparency)
                 {
-                    // A difference was captured but the metadata does not have transparency.
+                    // Duplicates were captured but the metadata does not have transparency.
                     metadata.HasTransparency = true;
-                    transparencyIndex = palette.Length;
-                    metadata.TransparencyIndex = ClampIndex(transparencyIndex);
-                }
 
-                PaletteQuantizer quantizer = new(palette, new() { Dither = null }, transparencyIndex);
-                using IQuantizer<TPixel> frameQuantizer = quantizer.CreatePixelSpecificQuantizer<TPixel>(this.configuration, quantizer.Options);
-                quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(encodingFrame, bounds);
+                    if (palette.Length < 256)
+                    {
+                        // We can use the existing palette and set the transparent index as the length.
+                        // decoders will ignore this value.
+                        transparencyIndex = palette.Length;
+                        metadata.TransparencyIndex = ClampIndex(transparencyIndex);
+
+                        PaletteQuantizer quantizer = new(palette, new() { Dither = null }, transparencyIndex);
+                        using IQuantizer<TPixel> frameQuantizer = quantizer.CreatePixelSpecificQuantizer<TPixel>(this.configuration, quantizer.Options);
+                        quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(encodingFrame, bounds);
+                    }
+                    else
+                    {
+                        // We must quantize the frame to generate a local color table.
+                        IQuantizer quantizer = this.hasQuantizer ? this.quantizer! : KnownQuantizers.Octree;
+                        using IQuantizer<TPixel> frameQuantizer = quantizer.CreatePixelSpecificQuantizer<TPixel>(this.configuration, quantizer.Options);
+                        quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(encodingFrame, bounds);
+
+                        // The transparency index derived by the quantizer will differ from the index
+                        // within the metadata. We need to update the metadata to reflect this.
+                        int derivedTransparencyIndex = GetTransparentIndex(quantized, null);
+                        metadata.TransparencyIndex = ClampIndex(derivedTransparencyIndex);
+                    }
+                }
+                else
+                {
+                    // Just use the local palette.
+                    PaletteQuantizer quantizer = new(palette, new() { Dither = null }, transparencyIndex);
+                    using IQuantizer<TPixel> frameQuantizer = quantizer.CreatePixelSpecificQuantizer<TPixel>(this.configuration, quantizer.Options);
+                    quantized = frameQuantizer.BuildPaletteAndQuantizeFrame(encodingFrame, bounds);
+                }
             }
             else
             {

src/ImageSharp/Formats/Gif/MetadataExtensions.cs

@@ -77,14 +77,20 @@ internal static AnimatedImageMetadata ToAnimatedImageMetadata(this GifMetadata s
     }
 
     internal static AnimatedImageFrameMetadata ToAnimatedImageFrameMetadata(this GifFrameMetadata source)
-        => new()
+    {
+        // For most scenarios we would consider the blend method to be 'Over' however if a frame has a disposal method of 'RestoreToBackground' or
+        // has a local palette with 256 colors and is not transparent we should use 'Source'.
+        bool blendSource = source.DisposalMethod == GifDisposalMethod.RestoreToBackground || (source.LocalColorTable?.Length == 256 && !source.HasTransparency);
+
+        return new()
         {
             ColorTable = source.LocalColorTable,
             ColorTableMode = source.ColorTableMode == GifColorTableMode.Global ? FrameColorTableMode.Global : FrameColorTableMode.Local,
             Duration = TimeSpan.FromMilliseconds(source.FrameDelay * 10),
             DisposalMode = GetMode(source.DisposalMethod),
-            BlendMode = source.DisposalMethod == GifDisposalMethod.RestoreToBackground ? FrameBlendMode.Source : FrameBlendMode.Over,
+            BlendMode = blendSource ? FrameBlendMode.Source : FrameBlendMode.Over,
         };
+    }
 
     private static FrameDisposalMode GetMode(GifDisposalMethod method) => method switch
     {

src/ImageSharp/Formats/Webp/BitReader/Vp8LBitReader.cs

@@ -71,7 +71,7 @@ public Vp8LBitReader(IMemoryOwner<byte> data)
         this.Eos = false;
 
         ulong currentValue = 0;
-        System.Span<byte> dataSpan = this.Data.Memory.Span;
+        Span<byte> dataSpan = this.Data.Memory.Span;
         for (int i = 0; i < 8; i++)
         {
             currentValue |= (ulong)dataSpan[i] << (8 * i);
@@ -103,7 +103,7 @@ public Vp8LBitReader(Stream inputStream, uint imageDataSize, MemoryAllocator mem
         }
 
         ulong currentValue = 0;
-        System.Span<byte> dataSpan = this.Data.Memory.Span;
+        Span<byte> dataSpan = this.Data.Memory.Span;
         for (int i = 0; i < length; i++)
         {
             currentValue |= (ulong)dataSpan[i] << (8 * i);

tests/ImageSharp.Tests/TestImages.cs

@@ -487,6 +487,7 @@ public static class Gif
         public const string GlobalQuantizationTest = "Gif/GlobalQuantizationTest.gif";
         public const string MixedDisposal = "Gif/mixed-disposal.gif";
         public const string M4nb = "Gif/m4nb.gif";
+        public const string Bit18RGBCube = "Gif/18-bit_RGB_Cube.gif";
 
         // Test images from https://github.com/robert-ancell/pygif/tree/master/test-suite
         public const string ZeroSize = "Gif/image-zero-size.gif";
@@ -533,7 +534,8 @@ public static class Issues
             Issues.Issue2450_A,
             Issues.Issue2450_B,
             Issues.BadDescriptorWidth,
-            Issues.Issue1530
+            Issues.Issue1530,
+            Bit18RGBCube
         };
     }