Do not copy color tables when transcoding.

Author: JimBobSquarePants

src/ImageSharp/Formats/Gif/GifFrameMetadata.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-using System.Numerics;
 using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Gif;
@@ -79,35 +78,15 @@ private GifFrameMetadata(GifFrameMetadata other)
     public IDeepCloneable DeepClone() => new GifFrameMetadata(this);
 
     internal static GifFrameMetadata FromAnimatedMetadata(AnimatedImageFrameMetadata metadata)
-    {
-        // TODO: v4 How do I link the parent metadata to the frame metadata to get the global color table?
-        int index = -1;
-        float background = 1f;
-        if (metadata.ColorTable.HasValue)
-        {
-            ReadOnlySpan<Color> colorTable = metadata.ColorTable.Value.Span;
-            for (int i = 0; i < colorTable.Length; i++)
-            {
-                Vector4 vector = (Vector4)colorTable[i];
-                if (vector.W < background)
-                {
-                    index = i;
-                }
-            }
-        }
-
-        bool hasTransparency = index >= 0;
-
-        return new()
+        => new()
         {
-            LocalColorTable = metadata.ColorTable,
+            // Do not copy the color table or transparency data.
+            // This will lead to a mismatch when the image is comprised of frames
+            // extracted individually from a multi-frame image.
             ColorTableMode = metadata.ColorTableMode == FrameColorTableMode.Global ? GifColorTableMode.Global : GifColorTableMode.Local,
             FrameDelay = (int)Math.Round(metadata.Duration.TotalMilliseconds / 10),
-            DisposalMethod = GetMode(metadata.DisposalMode),
-            HasTransparency = hasTransparency,
-            TransparencyIndex = hasTransparency ? unchecked((byte)index) : byte.MinValue,
+            DisposalMethod = GetMode(metadata.DisposalMode)
         };
-    }
 
     private static GifDisposalMethod GetMode(FrameDisposalMode mode) => mode switch
     {

src/ImageSharp/Formats/Gif/GifMetadata.cs

@@ -73,28 +73,12 @@ private GifMetadata(GifMetadata other)
     public IDeepCloneable DeepClone() => new GifMetadata(this);
 
     internal static GifMetadata FromAnimatedMetadata(AnimatedImageMetadata metadata)
-    {
-        int index = 0;
-        Color background = metadata.BackgroundColor;
-        if (metadata.ColorTable.HasValue)
+        => new()
         {
-            ReadOnlySpan<Color> colorTable = metadata.ColorTable.Value.Span;
-            for (int i = 0; i < colorTable.Length; i++)
-            {
-                if (background == colorTable[i])
-                {
-                    index = i;
-                    break;
-                }
-            }
-        }
-
-        return new()
-        {
-            GlobalColorTable = metadata.ColorTable,
+            // Do not copy the color table or bit depth.
+            // This will lead to a mismatch when the image is comprised of frames
+            // extracted individually from a multi-frame image.
             ColorTableMode = metadata.ColorTableMode == FrameColorTableMode.Global ? GifColorTableMode.Global : GifColorTableMode.Local,
             RepeatCount = metadata.RepeatCount,
-            BackgroundColorIndex = (byte)Numerics.Clamp(index, 0, 255),
         };
-    }
 }

src/ImageSharp/Formats/Png/PngMetadata.cs

@@ -93,32 +93,12 @@ private PngMetadata(PngMetadata other)
     public IDeepCloneable DeepClone() => new PngMetadata(this);
 
     internal static PngMetadata FromAnimatedMetadata(AnimatedImageMetadata metadata)
-    {
-        // Should the conversion be from a format that uses a 24bit palette entries (gif)
-        // we need to clone and adjust the color table to allow for transparency.
-        Color[]? colorTable = metadata.ColorTable?.ToArray();
-        if (colorTable != null)
+        => new()
         {
-            for (int i = 0; i < colorTable.Length; i++)
-            {
-                ref Color c = ref colorTable[i];
-                if (c == metadata.BackgroundColor)
-                {
-                    // Png treats background as fully empty
-                    c = Color.Transparent;
-                    break;
-                }
-            }
-        }
-
-        return new()
-        {
-            ColorType = colorTable != null ? PngColorType.Palette : null,
-            BitDepth = colorTable != null
-                        ? (PngBitDepth)Numerics.Clamp(ColorNumerics.GetBitsNeededForColorDepth(colorTable.Length), 1, 8)
-                        : null,
-            ColorTable = colorTable,
+            // Do not copy the color table or bit depth.
+            // This will lead to a mismatch when the image is comprised of frames
+            // extracted individually from a multi-frame image.
+            ColorType = metadata.ColorTable != null ? PngColorType.Palette : null,
             RepeatCount = metadata.RepeatCount,
         };
-    }
 }

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

@@ -484,7 +484,7 @@ public void Encode_APng<TPixel>(TestImageProvider<TPixel> provider)
     }
 
     [Theory]
-    [WithFile(TestImages.Gif.Leo, PixelTypes.Rgba32, 0.613F)]
+    [WithFile(TestImages.Gif.Leo, PixelTypes.Rgba32, 0.7629F)]
     [WithFile(TestImages.Gif.Issues.Issue2866, PixelTypes.Rgba32, 1.06F)]
     public void Encode_AnimatedFormatTransform_FromGif<TPixel>(TestImageProvider<TPixel> provider, float percentage)
         where TPixel : unmanaged, IPixel<TPixel>