Upstream fixes.

src/ImageSharp/Formats/Cur/CurFrameMetadata.cs

@@ -104,7 +104,6 @@ public class CurFrameMetadata : IFormatFrameMetadata<CurFrameMetadata>
             Compression = compression,
             EncodingWidth = ClampEncodingDimension(metadata.EncodingWidth),
             EncodingHeight = ClampEncodingDimension(metadata.EncodingHeight),
-            ColorTable = compression == IconFrameCompression.Bmp ? metadata.ColorTable : null
         };
     }
 
@@ -113,7 +112,6 @@ public class CurFrameMetadata : IFormatFrameMetadata<CurFrameMetadata>
         => new()
         {
             PixelTypeInfo = this.GetPixelTypeInfo(),
-            ColorTable = this.ColorTable,
             EncodingWidth = this.EncodingWidth,
             EncodingHeight = this.EncodingHeight
         };

src/ImageSharp/Formats/Cur/CurMetadata.cs

@@ -71,8 +71,7 @@ public class CurMetadata : IFormatMetadata<CurMetadata>
         return new CurMetadata
         {
             BmpBitsPerPixel = bbpp,
-            Compression = compression,
-            ColorTable = compression == IconFrameCompression.Bmp ? metadata.ColorTable : null
+            Compression = compression
         };
     }
 
@@ -145,8 +144,7 @@ public class CurMetadata : IFormatMetadata<CurMetadata>
             EncodingType = this.Compression == IconFrameCompression.Bmp && this.BmpBitsPerPixel <= BmpBitsPerPixel.Bit8
                 ? EncodingType.Lossy
                 : EncodingType.Lossless,
-            PixelTypeInfo = this.GetPixelTypeInfo(),
-            ColorTable = this.ColorTable
+            PixelTypeInfo = this.GetPixelTypeInfo()
         };
 
     /// <inheritdoc/>

src/ImageSharp/Formats/FormatConnectingFrameMetadata.cs

@@ -15,11 +15,6 @@ public class FormatConnectingFrameMetadata
     /// </summary>
     public PixelTypeInfo? PixelTypeInfo { get; init; }
 
-    /// <summary>
-    /// Gets the frame color table if any.
-    /// </summary>
-    public ReadOnlyMemory<Color>? ColorTable { get; init; }
-
     /// <summary>
     /// Gets the frame color table mode.
     /// </summary>

src/ImageSharp/Formats/FormatConnectingMetadata.cs

@@ -28,11 +28,6 @@ public class FormatConnectingMetadata
     /// </summary>
     public PixelTypeInfo PixelTypeInfo { get; init; }
 
-    /// <summary>
-    /// Gets the shared color table if any.
-    /// </summary>
-    public ReadOnlyMemory<Color>? ColorTable { get; init; }
-
     /// <summary>
     /// Gets the shared color table mode.
     /// </summary>

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -90,9 +90,9 @@ internal sealed class GifDecoderCore : ImageDecoderCore
     private GifMetadata? gifMetadata;
 
     /// <summary>
-    /// The background color used to fill the frame.
+    /// The background color index.
     /// </summary>
-    private Color backgroundColor;
+    private byte backgroundColorIndex;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="GifDecoderCore"/> class.
@@ -115,11 +115,11 @@ internal sealed class GifDecoderCore : ImageDecoderCore
         ImageFrame<TPixel>? previousFrame = null;
         FrameDisposalMode? previousDisposalMode = null;
         bool globalColorTableUsed = false;
+        Color backgroundColor = Color.Transparent;
 
         try
         {
             this.ReadLogicalScreenDescriptorAndGlobalColorTable(stream);
-            TPixel backgroundPixel = this.backgroundColor.ToPixel<TPixel>();
 
             // Loop though the respective gif parts and read the data.
             int nextFlag = stream.ReadByte();
@@ -132,7 +132,7 @@ internal sealed class GifDecoderCore : ImageDecoderCore
                         break;
                     }
 
-                    globalColorTableUsed |= this.ReadFrame(stream, ref image, ref previousFrame, ref previousDisposalMode, backgroundPixel);
+                    globalColorTableUsed |= this.ReadFrame(stream, ref image, ref previousFrame, ref previousDisposalMode, ref backgroundColor);
 
                     // Reset per-frame state.
                     this.imageDescriptor = default;
@@ -442,14 +442,14 @@ internal sealed class GifDecoderCore : ImageDecoderCore
     /// <param name="image">The image to decode the information to.</param>
     /// <param name="previousFrame">The previous frame.</param>
     /// <param name="previousDisposalMode">The previous frame disposal mode.</param>
-    /// <param name="backgroundPixel">The background color pixel.</param>
+    /// <param name="backgroundColor">The background color.</param>
     /// <returns>Whether the frame has a global color table.</returns>
     private bool ReadFrame<TPixel>(
         BufferedReadStream stream,
         ref Image<TPixel>? image,
         ref ImageFrame<TPixel>? previousFrame,
         ref FrameDisposalMode? previousDisposalMode,
-        TPixel backgroundPixel)
+        ref Color backgroundColor)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         this.ReadImageDescriptor(stream);
@@ -471,7 +471,47 @@ internal sealed class GifDecoderCore : ImageDecoderCore
         }
 
         ReadOnlySpan<Rgb24> colorTable = MemoryMarshal.Cast<byte, Rgb24>(rawColorTable);
-        this.ReadFrameColors(stream, ref image, ref previousFrame, ref previousDisposalMode, colorTable, backgroundPixel);
+
+        // First frame
+        if (image is null)
+        {
+            if (this.backgroundColorIndex < colorTable.Length)
+            {
+                backgroundColor = Color.FromPixel(colorTable[this.backgroundColorIndex]);
+            }
+            else
+            {
+                backgroundColor = Color.Transparent;
+            }
+
+            if (this.graphicsControlExtension.TransparencyFlag)
+            {
+                backgroundColor = backgroundColor.WithAlpha(0);
+            }
+        }
+
+        this.ReadFrameColors(stream, ref image, ref previousFrame, ref previousDisposalMode, colorTable, backgroundColor.ToPixel<TPixel>());
+
+        // Update from newly decoded frame.
+        if (this.graphicsControlExtension.DisposalMethod != FrameDisposalMode.RestoreToPrevious)
+        {
+            if (this.backgroundColorIndex < colorTable.Length)
+            {
+                backgroundColor = Color.FromPixel(colorTable[this.backgroundColorIndex]);
+            }
+            else
+            {
+                backgroundColor = Color.Transparent;
+            }
+
+            // TODO: I don't understand why this is always set to alpha of zero.
+            // This should be dependent on the transparency flag of the graphics
+            // control extension. ImageMagick does the same.
+            // if (this.graphicsControlExtension.TransparencyFlag)
+            {
+                backgroundColor = backgroundColor.WithAlpha(0);
+            }
+        }
 
         // Skip any remaining blocks
         SkipBlock(stream);
@@ -504,8 +544,9 @@ internal sealed class GifDecoderCore : ImageDecoderCore
         bool transFlag = this.graphicsControlExtension.TransparencyFlag;
         FrameDisposalMode disposalMethod = this.graphicsControlExtension.DisposalMethod;
         ImageFrame<TPixel> currentFrame;
+        ImageFrame<TPixel>? restoreFrame = null;
 
-        if (previousFrame is null)
+        if (previousFrame is null && previousDisposalMode is null)
         {
             image = transFlag
                 ? new Image<TPixel>(this.configuration, imageWidth, imageHeight, this.metadata)
@@ -516,26 +557,42 @@ internal sealed class GifDecoderCore : ImageDecoderCore
         }
         else
         {
-            // We create a clone of the frame and add it.
-            // We will overpaint the difference of pixels on the current frame to create a complete image.
-            // This ensures that we have enough pixel data to process without distortion. #2450
-            currentFrame = image!.Frames.AddFrame(previousFrame);
+            if (previousFrame != null)
+            {
+                currentFrame = image!.Frames.AddFrame(previousFrame);
+            }
+            else
+            {
+                currentFrame = image!.Frames.CreateFrame(backgroundPixel);
+            }
 
             this.SetFrameMetadata(currentFrame.Metadata);
 
+            if (this.graphicsControlExtension.DisposalMethod == FrameDisposalMode.RestoreToPrevious)
+            {
+                restoreFrame = previousFrame;
+            }
+
             if (previousDisposalMode == FrameDisposalMode.RestoreToBackground)
             {
                 this.RestoreToBackground(currentFrame, backgroundPixel, transFlag);
             }
         }
 
-        Rectangle interest = Rectangle.Intersect(image.Bounds, new(descriptor.Left, descriptor.Top, descriptor.Width, descriptor.Height));
-        previousFrame = currentFrame;
+        if (this.graphicsControlExtension.DisposalMethod == FrameDisposalMode.RestoreToPrevious)
+        {
+            previousFrame = restoreFrame;
+        }
+        else
+        {
+            previousFrame = currentFrame;
+        }
+
         previousDisposalMode = disposalMethod;
 
         if (disposalMethod == FrameDisposalMode.RestoreToBackground)
         {
-            this.restoreArea = interest;
+            this.restoreArea = Rectangle.Intersect(image.Bounds, new(descriptor.Left, descriptor.Top, descriptor.Width, descriptor.Height));
         }
 
         if (colorTable.Length == 0)
@@ -814,19 +871,9 @@ internal sealed class GifDecoderCore : ImageDecoderCore
             }
         }
 
-        // If the global color table is present, we can set the background color
-        // otherwise we default to transparent to match browser behavior.
-        ReadOnlyMemory<Color>? table = this.gifMetadata.GlobalColorTable;
         byte index = this.logicalScreenDescriptor.BackgroundColorIndex;
-        if (table is not null && index < table.Value.Length)
-        {
-            this.backgroundColor = table.Value.Span[index];
-            this.gifMetadata.BackgroundColorIndex = index;
-        }
-        else
-        {
-            this.backgroundColor = Color.Transparent;
-        }
+        this.backgroundColorIndex = index;
+        this.gifMetadata.BackgroundColorIndex = index;
     }
 
     private unsafe struct ScratchBuffer

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;
@@ -77,34 +76,12 @@ public class GifFrameMetadata : IFormatFrameMetadata<GifFrameMetadata>
 
     /// <inheritdoc />
     public static GifFrameMetadata FromFormatConnectingFrameMetadata(FormatConnectingFrameMetadata metadata)
-    {
-        int index = -1;
-        const float background = 1f;
-        if (metadata.ColorTable.HasValue)
+        => new()
         {
-            ReadOnlySpan<Color> colorTable = metadata.ColorTable.Value.Span;
-            for (int i = 0; i < colorTable.Length; i++)
-            {
-                Vector4 vector = colorTable[i].ToScaledVector4();
-                if (vector.W < background)
-                {
-                    index = i;
-                }
-            }
-        }
-
-        bool hasTransparency = index >= 0;
-
-        return new()
-        {
-            LocalColorTable = metadata.ColorTable,
             ColorTableMode = metadata.ColorTableMode,
             FrameDelay = (int)Math.Round(metadata.Duration.TotalMilliseconds / 10),
             DisposalMode = metadata.DisposalMode,
-            HasTransparency = hasTransparency,
-            TransparencyIndex = hasTransparency ? unchecked((byte)index) : byte.MinValue,
         };
-    }
 
     /// <inheritdoc />
     public FormatConnectingFrameMetadata ToFormatConnectingFrameMetadata()
@@ -118,7 +95,6 @@ public class GifFrameMetadata : IFormatFrameMetadata<GifFrameMetadata>
 
         return new()
         {
-            ColorTable = this.LocalColorTable,
             ColorTableMode = this.ColorTableMode,
             Duration = TimeSpan.FromMilliseconds(this.FrameDelay * 10),
             DisposalMode = this.DisposalMode,

src/ImageSharp/Formats/Gif/GifMetadata.cs

@@ -71,32 +71,14 @@ public class GifMetadata : IFormatMetadata<GifMetadata>
 
     /// <inheritdoc/>
     public static GifMetadata FromFormatConnectingMetadata(FormatConnectingMetadata metadata)
-    {
-        int index = 0;
-        Color background = metadata.BackgroundColor;
-        if (metadata.ColorTable.HasValue)
-        {
-            ReadOnlySpan<Color> colorTable = metadata.ColorTable.Value.Span;
-            for (int i = 0; i < colorTable.Length; i++)
-            {
-                if (background != colorTable[i])
-                {
-                    continue;
-                }
-
-                index = i;
-                break;
-            }
-        }
-
-        return new()
+        => 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,
             RepeatCount = metadata.RepeatCount,
-            BackgroundColorIndex = (byte)Numerics.Clamp(index, 0, 255),
         };
-    }
 
     /// <inheritdoc/>
     public PixelTypeInfo GetPixelTypeInfo()
@@ -114,22 +96,13 @@ public class GifMetadata : IFormatMetadata<GifMetadata>
 
     /// <inheritdoc/>
     public FormatConnectingMetadata ToFormatConnectingMetadata()
-    {
-        bool global = this.ColorTableMode == FrameColorTableMode.Global;
-        Color color = global && this.GlobalColorTable.HasValue && this.GlobalColorTable.Value.Span.Length > this.BackgroundColorIndex
-            ? this.GlobalColorTable.Value.Span[this.BackgroundColorIndex]
-            : Color.Transparent;
-
-        return new FormatConnectingMetadata()
+        => new()
         {
             AnimateRootFrame = true,
-            ColorTable = global ? this.GlobalColorTable : null,
-            BackgroundColor = color,
             ColorTableMode = this.ColorTableMode,
             PixelTypeInfo = this.GetPixelTypeInfo(),
             RepeatCount = this.RepeatCount,
         };
-    }
 
     /// <inheritdoc/>
     public void AfterImageApply<TPixel>(Image<TPixel> destination)

src/ImageSharp/Formats/Ico/IcoFrameMetadata.cs

@@ -96,8 +96,7 @@ public class IcoFrameMetadata : IFormatFrameMetadata<IcoFrameMetadata>
             BmpBitsPerPixel = bbpp,
             Compression = compression,
             EncodingWidth = ClampEncodingDimension(metadata.EncodingWidth),
-            EncodingHeight = ClampEncodingDimension(metadata.EncodingHeight),
-            ColorTable = compression == IconFrameCompression.Bmp ? metadata.ColorTable : null
+            EncodingHeight = ClampEncodingDimension(metadata.EncodingHeight)
         };
     }
 
@@ -106,7 +105,6 @@ public class IcoFrameMetadata : IFormatFrameMetadata<IcoFrameMetadata>
         => new()
         {
             PixelTypeInfo = this.GetPixelTypeInfo(),
-            ColorTable = this.ColorTable,
             EncodingWidth = this.EncodingWidth,
             EncodingHeight = this.EncodingHeight
         };

src/ImageSharp/Formats/Ico/IcoMetadata.cs

@@ -71,8 +71,7 @@ public class IcoMetadata : IFormatMetadata<IcoMetadata>
         return new IcoMetadata
         {
             BmpBitsPerPixel = bbpp,
-            Compression = compression,
-            ColorTable = compression == IconFrameCompression.Bmp ? metadata.ColorTable : null
+            Compression = compression
         };
     }
 
@@ -145,8 +144,7 @@ public class IcoMetadata : IFormatMetadata<IcoMetadata>
             EncodingType = this.Compression == IconFrameCompression.Bmp && this.BmpBitsPerPixel <= BmpBitsPerPixel.Bit8
                 ? EncodingType.Lossy
                 : EncodingType.Lossless,
-            PixelTypeInfo = this.GetPixelTypeInfo(),
-            ColorTable = this.ColorTable
+            PixelTypeInfo = this.GetPixelTypeInfo()
         };
 
     /// <inheritdoc/>

src/ImageSharp/Formats/Icon/IconEncoderCore.cs

@@ -120,17 +120,17 @@ internal abstract class IconEncoderCore
         this.entries = this.iconFileType switch
         {
             IconFileType.ICO =>
-            image.Frames.Select(i =>
+            [.. image.Frames.Select(i =>
             {
                 IcoFrameMetadata metadata = i.Metadata.GetIcoMetadata();
                 return new EncodingFrameMetadata(metadata.Compression, metadata.BmpBitsPerPixel, metadata.ColorTable, metadata.ToIconDirEntry(i.Size));
-            }).ToArray(),
+            })],
             IconFileType.CUR =>
-            image.Frames.Select(i =>
+            [.. image.Frames.Select(i =>
             {
                 CurFrameMetadata metadata = i.Metadata.GetCurMetadata();
                 return new EncodingFrameMetadata(metadata.Compression, metadata.BmpBitsPerPixel, metadata.ColorTable, metadata.ToIconDirEntry(i.Size));
-            }).ToArray(),
+            })],
             _ => throw new NotSupportedException(),
         };
     }
@@ -149,9 +149,15 @@ internal abstract class IconEncoderCore
 
         if (metadata.ColorTable is null)
         {
+            int count = metadata.Entry.ColorCount;
+            if (count == 0)
+            {
+                count = 256;
+            }
+
             return new WuQuantizer(new()
             {
-                MaxColors = metadata.Entry.ColorCount
+                MaxColors = count
             });
         }
 

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -1570,7 +1570,8 @@ internal sealed class PngEncoderCore : IDisposable
             }
             else
             {
-                // Don't use transparency threshold for quantization PNG can handle multiple transparent colors.
+                // Don't use the default transparency threshold for quantization as PNG can handle multiple transparent colors.
+                // We choose a value that is close to zero so that edge cases causes by lower bit depths for the alpha channel are handled correctly.
                 this.quantizer = new WuQuantizer(new QuantizerOptions { TransparencyThreshold = 0, MaxColors = ColorNumerics.GetColorCountForBitDepth(bitDepth) });
             }
         }

src/ImageSharp/Formats/Png/PngMetadata.cs

@@ -93,25 +93,6 @@ public class PngMetadata : IFormatMetadata<PngMetadata>
     /// <inheritdoc/>
     public static PngMetadata FromFormatConnectingMetadata(FormatConnectingMetadata 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)
-        {
-            for (int i = 0; i < colorTable.Length; i++)
-            {
-                ref Color c = ref colorTable[i];
-                if (c != metadata.BackgroundColor)
-                {
-                    continue;
-                }
-
-                // Png treats background as fully empty
-                c = Color.Transparent;
-                break;
-            }
-        }
-
         PngColorType color;
         PixelColorType colorType = metadata.PixelTypeInfo.ColorType;
 
@@ -152,7 +133,6 @@ public class PngMetadata : IFormatMetadata<PngMetadata>
         {
             ColorType = color,
             BitDepth = bitDepth,
-            ColorTable = colorTable,
             RepeatCount = metadata.RepeatCount,
         };
     }
@@ -241,7 +221,6 @@ public class PngMetadata : IFormatMetadata<PngMetadata>
     public FormatConnectingMetadata ToFormatConnectingMetadata()
         => new()
         {
-            ColorTable = this.ColorTable,
             ColorTableMode = FrameColorTableMode.Global,
             PixelTypeInfo = this.GetPixelTypeInfo(),
             RepeatCount = (ushort)Numerics.Clamp(this.RepeatCount, 0, ushort.MaxValue),

src/ImageSharp/Formats/Webp/Lossy/Vp8Decoder.cs

@@ -69,11 +69,11 @@ internal class Vp8Decoder : IDisposable
         this.YuvBuffer = memoryAllocator.Allocate<byte>((WebpConstants.Bps * 17) + (WebpConstants.Bps * 9) + extraY);
         this.CacheY = memoryAllocator.Allocate<byte>((16 * this.CacheYStride) + extraY, AllocationOptions.Clean);
         int cacheUvSize = (16 * this.CacheUvStride) + extraUv;
-        this.CacheU = memoryAllocator.Allocate<byte>(cacheUvSize, AllocationOptions.Clean);
-        this.CacheV = memoryAllocator.Allocate<byte>(cacheUvSize, AllocationOptions.Clean);
-        this.TmpYBuffer = memoryAllocator.Allocate<byte>((int)width, AllocationOptions.Clean);
-        this.TmpUBuffer = memoryAllocator.Allocate<byte>((int)width, AllocationOptions.Clean);
-        this.TmpVBuffer = memoryAllocator.Allocate<byte>((int)width, AllocationOptions.Clean);
+        this.CacheU = memoryAllocator.Allocate<byte>(cacheUvSize);
+        this.CacheV = memoryAllocator.Allocate<byte>(cacheUvSize);
+        this.TmpYBuffer = memoryAllocator.Allocate<byte>((int)width);
+        this.TmpUBuffer = memoryAllocator.Allocate<byte>((int)width);
+        this.TmpVBuffer = memoryAllocator.Allocate<byte>((int)width);
         this.Pixels = memoryAllocator.Allocate<byte>((int)(width * height * 4), AllocationOptions.Clean);
 
 #if DEBUG

src/ImageSharp/Processing/Processors/Quantization/EuclideanPixelMap{TPixel}.cs

@@ -116,7 +116,7 @@ internal sealed class EuclideanPixelMap<TPixel> : IDisposable
         }
 
         // Now I have the index, pop it into the cache for next time
-        this.cache.Add(rgba, (byte)index);
+        this.cache.Add(rgba, (short)index);
         match = Unsafe.Add(ref paletteRef, (uint)index);
 
         return index;
@@ -131,9 +131,11 @@ internal sealed class EuclideanPixelMap<TPixel> : IDisposable
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static float DistanceSquared(Rgba32 a, Rgba32 b)
     {
-        Vector4 va = new(a.R, a.G, a.B, a.A);
-        Vector4 vb = new(b.R, b.G, b.B, b.A);
-        return Vector4.DistanceSquared(va, vb);
+        float deltaR = a.R - b.R;
+        float deltaG = a.G - b.G;
+        float deltaB = a.B - b.B;
+        float deltaA = a.A - b.A;
+        return (deltaR * deltaR) + (deltaG * deltaG) + (deltaB * deltaB) + (deltaA * deltaA);
     }
 
     public void Dispose() => this.cache.Dispose();

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

@@ -34,6 +34,41 @@ public class GifDecoderTests
         image.CompareToReferenceOutputMultiFrame(provider, ImageComparer.Exact);
     }
 
+    [Theory]
+    [WithFile(TestImages.Gif.AnimatedLoop, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.AnimatedLoopInterlaced, PixelTypes.Rgba32)]
+    public void Decode_Animated<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage();
+        image.DebugSaveMultiFrame(provider);
+        image.CompareToReferenceOutputMultiFrame(provider, ImageComparer.Exact);
+    }
+
+    [Theory]
+    [WithFile(TestImages.Gif.AnimatedTransparentNoRestore, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.AnimatedTransparentRestorePrevious, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.AnimatedTransparentLoop, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.AnimatedTransparentFirstFrameRestorePrev, PixelTypes.Rgba32)]
+    public void Decode_Animated_WithTransparency<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage();
+        image.DebugSaveMultiFrame(provider);
+        image.CompareToReferenceOutputMultiFrame(provider, ImageComparer.Exact);
+    }
+
+    [Theory]
+    [WithFile(TestImages.Gif.StaticNontransparent, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.StaticTransparent, PixelTypes.Rgba32)]
+    public void Decode_Static_No_Animation<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage();
+        image.DebugSave(provider);
+        image.CompareFirstFrameToReferenceOutput(ImageComparer.Exact, provider);
+    }
+
     [Theory]
     [WithFile(TestImages.Gif.Issues.Issue2450_A, PixelTypes.Rgba32)]
     [WithFile(TestImages.Gif.Issues.Issue2450_B, PixelTypes.Rgba32)]

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

@@ -422,12 +422,18 @@ public class GifEncoderTests
 
     [Theory]
     [WithFile(TestImages.Gif.Issues.Issue2866, PixelTypes.Rgba32)]
-    public void GifEncoder_CanDecode_Issue2866<TPixel>(TestImageProvider<TPixel> provider)
-    where TPixel : unmanaged, IPixel<TPixel>
+    public void GifEncoder_CanDecode_AndEncode_Issue2866<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
     {
         using Image<TPixel> image = provider.GetImage();
 
-        // image.DebugSaveMultiFrame(provider);
+        // Save the image for visual inspection.
         provider.Utility.SaveTestOutputFile(image, "gif", new GifEncoder(), "animated");
+
+        // Now compare the debug output with the reference output.
+        // We do this because the gif encoding is lossy and encoding will lead to differences in the 10s of percent.
+        // From the unencoded image, we can see that the image is visually the same.
+        static bool Predicate(int i, int _) => i % 8 == 0; // Image has many frames, only compare a selection of them.
+        image.CompareDebugOutputToReferenceOutputMultiFrame(provider, ImageComparer.Exact, extension: "gif", predicate: Predicate);
     }
 }

tests/ImageSharp.Tests/Formats/Icon/Cur/CurEncoderTests.cs

@@ -4,6 +4,7 @@
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Cur;
 using SixLabors.ImageSharp.Formats.Ico;
+using SixLabors.ImageSharp.Formats.Png;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using static SixLabors.ImageSharp.Tests.TestImages.Cur;
@@ -49,8 +50,8 @@ public class CurEncoderTests
         using Image<TPixel> encoded = Image.Load<TPixel>(memStream);
         encoded.DebugSaveMultiFrame(provider);
 
-        // Despite preservation of the palette. The process can still be lossy
-        encoded.CompareToOriginalMultiFrame(provider, ImageComparer.TolerantPercentage(.23f), IcoDecoder.Instance);
+        // Color palettes are not preserved when transcoding.
+        encoded.CompareToOriginalMultiFrame(provider, ImageComparer.TolerantPercentage(.05F), IcoDecoder.Instance);
 
         for (int i = 0; i < image.Frames.Count; i++)
         {

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

@@ -442,40 +442,42 @@ public partial class PngEncoderTests
     }
 
     [Theory]
-    [WithFile(TestImages.Gif.Leo, PixelTypes.Rgba32, 0.613F)]
-    [WithFile(TestImages.Gif.Issues.Issue2866, PixelTypes.Rgba32, 1.06F)]
-    public void Encode_AnimatedFormatTransform_FromGif<TPixel>(TestImageProvider<TPixel> provider, float percentage)
+    [WithFile(TestImages.Gif.Leo, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.Issues.Issue2866, PixelTypes.Rgba32)]
+    public void Encode_AnimatedFormatTransform_FromGif<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        if (TestEnvironment.RunsOnCI)
+        if (TestEnvironment.RunsOnCI && !TestEnvironment.IsWindows)
         {
             return;
         }
 
         using Image<TPixel> image = provider.GetImage(GifDecoder.Instance);
 
+        // Save the image for visual inspection.
+        provider.Utility.SaveTestOutputFile(image, "png", PngEncoder, "animated");
+
+        // Now compare the debug output with the reference output.
+        // We do this because the transcoding encoding is lossy and encoding will lead to differences.
+        // From the unencoded image, we can see that the image is visually the same.
+        static bool Predicate(int i, int _) => i % 8 == 0; // Image has many frames, only compare a selection of them.
+        image.CompareDebugOutputToReferenceOutputMultiFrame(provider, ImageComparer.Exact, extension: "png", encoder: PngEncoder, predicate: Predicate);
+
+        // Now save the image and load it again to compare the metadata.
         using MemoryStream memStream = new();
         image.Save(memStream, PngEncoder);
         memStream.Position = 0;
 
-        image.DebugSave(provider: provider, extension: "png", encoder: PngEncoder);
-
-        using Image<TPixel> output = Image.Load<TPixel>(memStream);
-
-        // TODO: Find a better way to compare.
-        // The image has been visually checked but the coarse cache used by the palette quantizer
-        // can lead to minor differences between frames.
-        ImageComparer.TolerantPercentage(percentage).VerifySimilarity(output, image);
-
+        using Image<TPixel> encoded = Image.Load<TPixel>(memStream);
         GifMetadata gif = image.Metadata.GetGifMetadata();
-        PngMetadata png = output.Metadata.GetPngMetadata();
+        PngMetadata png = encoded.Metadata.GetPngMetadata();
 
         Assert.Equal(gif.RepeatCount, png.RepeatCount);
 
         for (int i = 0; i < image.Frames.Count; i++)
         {
             GifFrameMetadata gifF = image.Frames[i].Metadata.GetGifMetadata();
-            PngFrameMetadata pngF = output.Frames[i].Metadata.GetPngMetadata();
+            PngFrameMetadata pngF = encoded.Frames[i].Metadata.GetPngMetadata();
 
             Assert.Equal(gifF.FrameDelay, (int)(pngF.FrameDelay.ToDouble() * 100));
 

tests/ImageSharp.Tests/Formats/WebP/WebpEncoderTests.cs

@@ -8,6 +8,8 @@ using SixLabors.ImageSharp.Formats.Png;
 using SixLabors.ImageSharp.Formats.Webp;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
 using SixLabors.ImageSharp.Tests.TestUtilities;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using SixLabors.ImageSharp.Tests.TestUtilities.ReferenceCodecs;
@@ -110,6 +112,63 @@ public class WebpEncoderTests
         }
     }
 
+    [Theory]
+    // [WithFile(AlphaBlend, PixelTypes.Rgba32)]
+    // [WithFile(AlphaBlend2, PixelTypes.Rgba32)]
+    [WithFile(AlphaBlend3, PixelTypes.Rgba32)]
+    // [WithFile(AlphaBlend4, PixelTypes.Rgba32)]
+    public void Encode_AlphaBlended<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage();
+        WebpEncoder encoder = new()
+        {
+            FileFormat = WebpFileFormatType.Lossless
+        };
+
+        QuantizerOptions options = new()
+        {
+            TransparencyThreshold = 128 / 255F
+        };
+
+        // First save as gif to gif using different quantizers with default options.
+        // Alpha thresholding is 64/255F.
+        GifEncoder gifEncoder = new()
+        {
+            Quantizer = new OctreeQuantizer(options)
+        };
+        provider.Utility.SaveTestOutputFile(image, "gif", gifEncoder, "octree");
+
+        gifEncoder = new GifEncoder()
+        {
+            Quantizer = new WuQuantizer(options)
+        };
+        provider.Utility.SaveTestOutputFile(image, "gif", gifEncoder, "wu");
+
+        // Now clone and quantize the image using the same quantizers  without alpha thresholding and save as webp.
+        options = new()
+        {
+            TransparencyThreshold = 0
+        };
+
+        using Image<TPixel> cloned1 = image.Clone();
+        cloned1.Mutate(c => c.Quantize(new OctreeQuantizer(options)));
+        provider.Utility.SaveTestOutputFile(cloned1, "webp", encoder, "octree");
+
+        using Image<TPixel> cloned2 = image.Clone();
+        cloned2.Mutate(c => c.Quantize(new WuQuantizer(options)));
+        provider.Utility.SaveTestOutputFile(cloned2, "webp", encoder, "wu");
+
+        // Now blend the images with a blue background and save as webp.
+        using Image<Rgba32> background1 = new(image.Width, image.Height, Color.White.ToPixel<Rgba32>());
+        background1.Mutate(c => c.DrawImage(cloned1, 1));
+        provider.Utility.SaveTestOutputFile(background1, "webp", encoder, "octree-blended");
+
+        using Image<Rgba32> background2 = new(image.Width, image.Height, Color.White.ToPixel<Rgba32>());
+        background2.Mutate(c => c.DrawImage(cloned2, 1));
+        provider.Utility.SaveTestOutputFile(background2, "webp", encoder, "wu-blended");
+    }
+
     [Theory]
     [WithFile(TestImages.Png.APng, PixelTypes.Rgba32)]
     public void Encode_AnimatedFormatTransform_FromPng<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -508,6 +508,31 @@ public static class TestImages
         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/peterdn/gif-test-suite.git
+        // Animated gif with 4 frames, looping forever, no transparency.
+        public const string AnimatedLoop = "Gif/animated_loop.gif";
+
+        // Animated gif with 4 frames, interlaced, looping forever, no transparency.
+        public const string AnimatedLoopInterlaced = "Gif/animated_loop_interlaced.gif";
+
+        // Transparent gif with 4 frames, loops forever.
+        public const string AnimatedTransparentLoop = "Gif/animated_transparent_loop.gif";
+
+        // Transparent gif with 4 frames, loops forever, first frame restore previous.
+        public const string AnimatedTransparentFirstFrameRestorePrev = "Gif/animated_transparent_firstframerestoreprev_loop.gif";
+
+        // Transparent gif with 4 transparent frames, loops forever, no dispose
+        public const string AnimatedTransparentNoRestore = "Gif/animated_transparent_frame_norestore_loop.gif";
+
+        // Transparent gif with 4 transparent frames, loops forever, restore previous.
+        public const string AnimatedTransparentRestorePrevious = "Gif/animated_transparent_frame_restoreprev_loop.gif";
+
+        // Static gif with no animation, no transparency.
+        public const string StaticNontransparent = "Gif/static_nontransparent.gif";
+
+        // Static transparent gif with no animation.
+        public const string StaticTransparent = "Gif/static_transparent.gif";
+
         // Test images from https://github.com/robert-ancell/pygif/tree/master/test-suite
         public const string ZeroSize = "Gif/image-zero-size.gif";
         public const string ZeroHeight = "Gif/image-zero-height.gif";
@@ -833,6 +858,11 @@ public static class TestImages
             public const string Issue2801 = "Webp/issues/Issue2801.webp";
             public const string Issue2866 = "Webp/issues/Issue2866.webp";
         }
+
+        public const string AlphaBlend = "Webp/alpha-blend.webp";
+        public const string AlphaBlend2 = "Webp/alpha-blend-2.webp";
+        public const string AlphaBlend3 = "Webp/alpha-blend-3.webp";
+        public const string AlphaBlend4 = "Webp/alpha-blend-4.webp";
     }
 
     public static class Tiff

tests/ImageSharp.Tests/TestUtilities/TestImageExtensions.cs

@@ -107,6 +107,7 @@ public static class TestImageExtensions
         ITestImageProvider provider,
         object testOutputDetails = null,
         string extension = "png",
+        IImageEncoder encoder = null,
         bool appendPixelTypeToFileName = true,
         Func<int, int, bool> predicate = null)
         where TPixel : unmanaged, IPixel<TPixel>
@@ -119,6 +120,7 @@ public static class TestImageExtensions
         provider.Utility.SaveTestOutputFileMultiFrame(
             image,
             extension,
+            encoder: encoder,
             testOutputDetails: testOutputDetails,
             appendPixelTypeToFileName: appendPixelTypeToFileName,
             predicate: predicate);
@@ -277,6 +279,47 @@ public static class TestImageExtensions
         return image;
     }
 
+    public static Image<TPixel> CompareDebugOutputToReferenceOutputMultiFrame<TPixel>(
+        this Image<TPixel> image,
+        ITestImageProvider provider,
+        ImageComparer comparer,
+        object testOutputDetails = null,
+        string extension = "png",
+        IImageEncoder encoder = null,
+        bool appendPixelTypeToFileName = true,
+        Func<int, int, bool> predicate = null)
+    where TPixel : unmanaged, IPixel<TPixel>
+    {
+        image.DebugSaveMultiFrame(
+            provider,
+            testOutputDetails,
+            extension,
+            encoder,
+            appendPixelTypeToFileName,
+            predicate: predicate);
+
+        using (Image<TPixel> debugImage = GetDebugOutputImageMultiFrame<TPixel>(
+            provider,
+            image.Frames.Count,
+            testOutputDetails,
+            extension,
+            appendPixelTypeToFileName,
+            predicate: predicate))
+
+        using (Image<TPixel> referenceImage = GetReferenceOutputImageMultiFrame<TPixel>(
+            provider,
+            image.Frames.Count,
+            testOutputDetails,
+            extension,
+            appendPixelTypeToFileName,
+            predicate: predicate))
+        {
+            comparer.VerifySimilarity(referenceImage, debugImage);
+        }
+
+        return image;
+    }
+
     public static Image<TPixel> CompareToReferenceOutputMultiFrame<TPixel>(
         this Image<TPixel> image,
         ITestImageProvider provider,
@@ -375,6 +418,54 @@ public static class TestImageExtensions
         return result;
     }
 
+    public static Image<TPixel> GetDebugOutputImageMultiFrame<TPixel>(
+        this ITestImageProvider provider,
+        int frameCount,
+        object testOutputDetails = null,
+        string extension = "png",
+        bool appendPixelTypeToFileName = true,
+        Func<int, int, bool> predicate = null)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        (int Index, string FileName)[] frameFiles = [.. provider.Utility.GetTestOutputFileNamesMultiFrame(
+            frameCount,
+            extension,
+            testOutputDetails,
+            appendPixelTypeToFileName,
+            predicate: predicate)];
+
+        List<Image<TPixel>> temporaryFrameImages = [];
+
+        IImageDecoder decoder = TestEnvironment.GetReferenceDecoder(frameFiles[0].FileName);
+
+        for (int i = 0; i < frameFiles.Length; i++)
+        {
+            string path = frameFiles[i].FileName;
+            if (!File.Exists(path))
+            {
+                throw new FileNotFoundException("Reference output file missing: " + path);
+            }
+
+            using FileStream stream = File.OpenRead(path);
+            Image<TPixel> tempImage = decoder.Decode<TPixel>(DecoderOptions.Default, stream);
+            temporaryFrameImages.Add(tempImage);
+        }
+
+        Image<TPixel> firstTemp = temporaryFrameImages[0];
+
+        Image<TPixel> result = new(firstTemp.Width, firstTemp.Height);
+
+        foreach (Image<TPixel> fi in temporaryFrameImages)
+        {
+            result.Frames.AddFrame(fi.Frames.RootFrame);
+            fi.Dispose();
+        }
+
+        // Remove the initial empty frame:
+        result.Frames.RemoveFrame(0);
+        return result;
+    }
+
     public static IEnumerable<ImageSimilarityReport> GetReferenceOutputSimilarityReports<TPixel>(
         this Image<TPixel> image,
         ITestImageProvider provider,

tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_Rgba32_animated_loop.gif/00.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_Rgba32_animated_loop.gif/01.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_Rgba32_animated_loop_interlaced.gif/00.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_Rgba32_animated_loop_interlaced.gif/01.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_Rgba32_animated_loop_interlaced.gif/02.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_Rgba32_animated_loop_interlaced.gif/03.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_firstframerestoreprev_loop.gif/00.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_firstframerestoreprev_loop.gif/01.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_firstframerestoreprev_loop.gif/02.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_norestore_loop.gif/00.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_norestore_loop.gif/01.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_norestore_loop.gif/02.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_norestore_loop.gif/03.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_restoreprev_loop.gif/00.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_restoreprev_loop.gif/01.png

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tests/Images/External/ReferenceOutput/GifDecoderTests/Decode_Animated_WithTransparency_Rgba32_animated_transparent_frame_restoreprev_loop.gif/02.png

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