Fix PNG animation encoding and quantizer output

Author: JimBobSquarePants

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -112,6 +112,7 @@ protected override Image<TPixel> Decode<TPixel>(BufferedReadStream stream, Cance
         Image<TPixel>? image = null;
         ImageFrame<TPixel>? previousFrame = null;
         GifDisposalMethod? previousDisposalMethod = null;
+        bool globalColorTableUsed = false;
 
         try
         {
@@ -129,7 +130,7 @@ protected override Image<TPixel> Decode<TPixel>(BufferedReadStream stream, Cance
                         break;
                     }
 
-                    this.ReadFrame(stream, ref image, ref previousFrame, ref previousDisposalMethod, backgroundPixel);
+                    globalColorTableUsed |= this.ReadFrame(stream, ref image, ref previousFrame, ref previousDisposalMethod, backgroundPixel);
 
                     // Reset per-frame state.
                     this.imageDescriptor = default;
@@ -164,6 +165,13 @@ protected override Image<TPixel> Decode<TPixel>(BufferedReadStream stream, Cance
                     break;
                 }
             }
+
+            // We cannot always trust the global GIF palette has actually been used.
+            // https://github.com/SixLabors/ImageSharp/issues/2866
+            if (!globalColorTableUsed)
+            {
+                this.gifMetadata.ColorTableMode = GifColorTableMode.Local;
+            }
         }
         finally
         {
@@ -425,7 +433,7 @@ private void ReadComments(BufferedReadStream stream)
     /// <param name="previousFrame">The previous frame.</param>
     /// <param name="previousDisposalMethod">The previous disposal method.</param>
     /// <param name="backgroundPixel">The background color pixel.</param>
-    private void ReadFrame<TPixel>(
+    private bool ReadFrame<TPixel>(
         BufferedReadStream stream,
         ref Image<TPixel>? image,
         ref ImageFrame<TPixel>? previousFrame,
@@ -461,6 +469,8 @@ private void ReadFrame<TPixel>(
 
         // Skip any remaining blocks
         SkipBlock(stream);
+
+        return !hasLocalColorTable;
     }
 
     /// <summary>
@@ -809,6 +819,7 @@ private void ReadLogicalScreenDescriptorAndGlobalColorTable(BufferedReadStream s
         if (table is not null && index < table.Value.Length)
         {
             this.backgroundColor = table.Value.Span[index];
+            this.gifMetadata.BackgroundColorIndex = index;
         }
         else
         {

src/ImageSharp/Formats/Gif/MetadataExtensions.cs

@@ -60,15 +60,20 @@ public static bool TryGetGifMetadata(this ImageFrameMetadata source, [NotNullWhe
         => source.TryGetFormatMetadata(GifFormat.Instance, out metadata);
 
     internal static AnimatedImageMetadata ToAnimatedImageMetadata(this GifMetadata source)
+    {
+        bool global = source.ColorTableMode == GifColorTableMode.Global;
+        Color color = global && source.GlobalColorTable.HasValue && source.GlobalColorTable.Value.Span.Length > source.BackgroundColorIndex
+            ? source.GlobalColorTable.Value.Span[source.BackgroundColorIndex]
+            : Color.Transparent;
 
-        // We cannot trust the global GIF palette so don't use it.
-        // https://github.com/SixLabors/ImageSharp/issues/2866
-        => new()
+        return new()
         {
             ColorTableMode = source.ColorTableMode == GifColorTableMode.Global ? FrameColorTableMode.Global : FrameColorTableMode.Local,
+            ColorTable = global ? source.GlobalColorTable : null,
             RepeatCount = source.RepeatCount,
-            BackgroundColor = Color.Transparent,
+            BackgroundColor = color,
         };
+    }
 
     internal static AnimatedImageFrameMetadata ToAnimatedImageFrameMetadata(this GifFrameMetadata source)
     {

src/ImageSharp/Formats/Png/PngEncoder.cs

@@ -2,25 +2,13 @@
 // Licensed under the Six Labors Split License.
 #nullable disable
 
-using SixLabors.ImageSharp.Processing.Processors.Quantization;
-
 namespace SixLabors.ImageSharp.Formats.Png;
 
 /// <summary>
 /// Image encoder for writing image data to a stream in png format.
 /// </summary>
 public class PngEncoder : QuantizingImageEncoder
 {
-    /// <summary>
-    /// Initializes a new instance of the <see cref="PngEncoder"/> class.
-    /// </summary>
-    public PngEncoder()
-
-        // Hack. TODO: Investigate means to fix/optimize the Wu quantizer.
-        // The Wu quantizer does not handle the default sampling strategy well for some larger images.
-        // It's expensive and the results are not better than the extensive strategy.
-        => this.PixelSamplingStrategy = new ExtensivePixelSamplingStrategy();
-
     /// <summary>
     /// Gets the number of bits per sample or per palette index (not per pixel).
     /// Not all values are allowed for all <see cref="ColorType" /> values.