Fix handling gif encoding for global palettes.

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -103,7 +103,14 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream, CancellationToken
             {
                 // We avoid dithering by default to preserve the original colors.
                 int transparencyIndex = GetTransparentIndex(quantized, frameMetadata);
-                this.quantizer = new PaletteQuantizer(gifMetadata.GlobalColorTable.Value, new() { Dither = null }, transparencyIndex);
+                if (transparencyIndex >= 0 || gifMetadata.GlobalColorTable.Value.Length < 256)
+                {
+                    this.quantizer = new PaletteQuantizer(gifMetadata.GlobalColorTable.Value, new() { Dither = null }, transparencyIndex);
+                }
+                else
+                {
+                    this.quantizer = KnownQuantizers.Octree;
+                }
             }
             else
             {
@@ -198,19 +205,17 @@ private static GifMetadata GetGifMetadata<TPixel>(Image<TPixel> image)
     private static GifFrameMetadata GetGifFrameMetadata<TPixel>(ImageFrame<TPixel> frame, int transparencyIndex)
         where TPixel : unmanaged, IPixel<TPixel>
     {
+        GifFrameMetadata? metadata = null;
         if (frame.Metadata.TryGetGifMetadata(out GifFrameMetadata? gif))
         {
-            return (GifFrameMetadata)gif.DeepClone();
+            metadata = (GifFrameMetadata)gif.DeepClone();
         }
-
-        GifFrameMetadata? metadata = null;
-        if (frame.Metadata.TryGetPngMetadata(out PngFrameMetadata? png))
+        else if (frame.Metadata.TryGetPngMetadata(out PngFrameMetadata? png))
         {
             AnimatedImageFrameMetadata ani = png.ToAnimatedImageFrameMetadata();
             metadata = GifFrameMetadata.FromAnimatedMetadata(ani);
         }
-
-        if (frame.Metadata.TryGetWebpFrameMetadata(out WebpFrameMetadata? webp))
+        else if (frame.Metadata.TryGetWebpFrameMetadata(out WebpFrameMetadata? webp))
         {
             AnimatedImageFrameMetadata ani = webp.ToAnimatedImageFrameMetadata();
             metadata = GifFrameMetadata.FromAnimatedMetadata(ani);

src/ImageSharp/Formats/Gif/MetadataExtensions.cs

@@ -82,6 +82,9 @@ internal static AnimatedImageFrameMetadata ToAnimatedImageFrameMetadata(this Gif
         // 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);
 
+        // If the color table is global and frame has no transparency. Consider it 'Source' also.
+        blendSource |= source.ColorTableMode == GifColorTableMode.Global && !source.HasTransparency;
+
         return new()
         {
             ColorTable = source.LocalColorTable,

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -499,7 +499,7 @@ public void Encode_AnimatedFormatTransform_FromGif<TPixel>(TestImageProvider<TPi
         // TODO: Find a better way to compare.
         // The image has been visually checked but the quantization pattern used in the png encoder
         // means we cannot use an exact comparison nor replicate using the quantizing processor.
-        ImageComparer.TolerantPercentage(0.46f).VerifySimilarity(output, image);
+        ImageComparer.TolerantPercentage(0.613f).VerifySimilarity(output, image);
 
         GifMetadata gif = image.Metadata.GetGifMetadata();
         PngMetadata png = output.Metadata.GetPngMetadata();

tests/ImageSharp.Tests/TestImages.cs

@@ -488,6 +488,7 @@ public static class 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";
+        public const string Global256NoTrans = "Gif/global-256-no-trans.gif";
 
         // Test images from https://github.com/robert-ancell/pygif/tree/master/test-suite
         public const string ZeroSize = "Gif/image-zero-size.gif";
@@ -535,7 +536,8 @@ public static class Issues
             Issues.Issue2450_B,
             Issues.BadDescriptorWidth,
             Issues.Issue1530,
-            Bit18RGBCube
+            Bit18RGBCube,
+            Global256NoTrans
         };
     }