From 58e8efac20aa0c9eee16ad8d1451c81069e89df3 Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Fri, 15 Mar 2024 20:02:00 +1000
Subject: [PATCH 01/15] Ensure VP8X alpha flag is updated correctly.

---
 src/ImageSharp/Common/Helpers/RiffHelper.cs   | 17 +++++++++++-
 .../Formats/Webp/BitWriter/BitWriterBase.cs   | 26 +++++++++++++-----
 .../Formats/Webp/Chunks/WebpVp8X.cs           | 23 +++++++++++++++-
 .../Formats/Webp/Lossless/Vp8LEncoder.cs      | 20 +++++++++-----
 .../Formats/Webp/Lossy/Vp8Encoder.cs          | 24 +++++++++++------
 .../Formats/Webp/WebpChunkParsingUtils.cs     |  1 -
 .../Formats/Webp/WebpDecoderCore.cs           |  2 +-
 .../Formats/Webp/WebpEncoderCore.cs           | 27 ++++++++++++-------
 8 files changed, 105 insertions(+), 35 deletions(-)

diff --git a/src/ImageSharp/Common/Helpers/RiffHelper.cs b/src/ImageSharp/Common/Helpers/RiffHelper.cs
index 8f06e5886..667a47fc9 100644
--- a/src/ImageSharp/Common/Helpers/RiffHelper.cs
+++ b/src/ImageSharp/Common/Helpers/RiffHelper.cs
@@ -3,6 +3,7 @@
 
 using System.Buffers.Binary;
 using System.Text;
+using SixLabors.ImageSharp.Formats.Webp.Chunks;
 
 namespace SixLabors.ImageSharp.Common.Helpers;
 
@@ -107,6 +108,7 @@ internal static class RiffHelper
             position++;
         }
 
+        // Add the size of the encoded file to the Riff header.
         BinaryPrimitives.WriteUInt32LittleEndian(buffer, dataSize);
         stream.Position = sizePosition;
         stream.Write(buffer);
@@ -120,5 +122,18 @@ internal static class RiffHelper
         return sizePosition;
     }
 
-    public static void EndWriteRiffFile(Stream stream, long sizePosition) => EndWriteChunk(stream, sizePosition);
+    public static void EndWriteRiffFile(Stream stream, in WebpVp8X vp8x, bool updateVp8x, long sizePosition)
+    {
+        EndWriteChunk(stream, sizePosition + 4);
+
+        // Write the VP8X chunk if necessary.
+        if (updateVp8x)
+        {
+            long position = stream.Position;
+
+            stream.Position = sizePosition + 12;
+            vp8x.WriteTo(stream);
+            stream.Position = position;
+        }
+    }
 }
diff --git a/src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs b/src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs
index 9ffda0f51..651e68011 100644
--- a/src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs
+++ b/src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs
@@ -88,7 +88,8 @@ internal abstract class BitWriterBase
     /// <param name="iccProfile">The color profile.</param>
     /// <param name="hasAlpha">Flag indicating, if a alpha channel is present.</param>
     /// <param name="hasAnimation">Flag indicating, if an animation parameter is present.</param>
-    public static void WriteTrunksBeforeData(
+    /// <returns>A <see cref="WebpVp8X"/> or a default instance.</returns>
+    public static WebpVp8X WriteTrunksBeforeData(
         Stream stream,
         uint width,
         uint height,
@@ -102,16 +103,19 @@ internal abstract class BitWriterBase
         RiffHelper.BeginWriteRiffFile(stream, WebpConstants.WebpFourCc);
 
         // Write VP8X, header if necessary.
+        WebpVp8X vp8x = default;
         bool isVp8X = exifProfile != null || xmpProfile != null || iccProfile != null || hasAlpha || hasAnimation;
         if (isVp8X)
         {
-            WriteVp8XHeader(stream, exifProfile, xmpProfile, iccProfile, width, height, hasAlpha, hasAnimation);
+            vp8x = WriteVp8XHeader(stream, exifProfile, xmpProfile, iccProfile, width, height, hasAlpha, hasAnimation);
 
             if (iccProfile != null)
             {
                 RiffHelper.WriteChunk(stream, (uint)WebpChunkType.Iccp, iccProfile.ToByteArray());
             }
         }
+
+        return vp8x;
     }
 
     /// <summary>
@@ -124,10 +128,16 @@ internal abstract class BitWriterBase
     /// Write the trunks after data trunk.
     /// </summary>
     /// <param name="stream">The stream to write to.</param>
-    /// <param name="exifProfile">The exif profile.</param>
+    /// <param name="vp8x">The VP8X chunk.</param>
+    /// <param name="updateVp8x">Whether to update the chunk.</param>
+    /// <param name="initialPosition">The initial position of the stream before encoding.</param>
+    /// <param name="exifProfile">The EXIF profile.</param>
     /// <param name="xmpProfile">The XMP profile.</param>
     public static void WriteTrunksAfterData(
         Stream stream,
+        in WebpVp8X vp8x,
+        bool updateVp8x,
+        long initialPosition,
         ExifProfile? exifProfile,
         XmpProfile? xmpProfile)
     {
@@ -141,7 +151,7 @@ internal abstract class BitWriterBase
             RiffHelper.WriteChunk(stream, (uint)WebpChunkType.Xmp, xmpProfile.Data);
         }
 
-        RiffHelper.EndWriteRiffFile(stream, 4);
+        RiffHelper.EndWriteRiffFile(stream, in vp8x, updateVp8x, initialPosition);
     }
 
     /// <summary>
@@ -186,19 +196,21 @@ internal abstract class BitWriterBase
     /// Writes a VP8X header to the stream.
     /// </summary>
     /// <param name="stream">The stream to write to.</param>
-    /// <param name="exifProfile">A exif profile or null, if it does not exist.</param>
-    /// <param name="xmpProfile">A XMP profile or null, if it does not exist.</param>
+    /// <param name="exifProfile">An EXIF profile or null, if it does not exist.</param>
+    /// <param name="xmpProfile">An XMP profile or null, if it does not exist.</param>
     /// <param name="iccProfile">The color profile.</param>
     /// <param name="width">The width of the image.</param>
     /// <param name="height">The height of the image.</param>
     /// <param name="hasAlpha">Flag indicating, if a alpha channel is present.</param>
     /// <param name="hasAnimation">Flag indicating, if an animation parameter is present.</param>
-    protected static void WriteVp8XHeader(Stream stream, ExifProfile? exifProfile, XmpProfile? xmpProfile, IccProfile? iccProfile, uint width, uint height, bool hasAlpha, bool hasAnimation)
+    protected static WebpVp8X WriteVp8XHeader(Stream stream, ExifProfile? exifProfile, XmpProfile? xmpProfile, IccProfile? iccProfile, uint width, uint height, bool hasAlpha, bool hasAnimation)
     {
         WebpVp8X chunk = new(hasAnimation, xmpProfile != null, exifProfile != null, hasAlpha, iccProfile != null, width, height);
 
         chunk.Validate(MaxDimension, MaxCanvasPixels);
 
         chunk.WriteTo(stream);
+
+        return chunk;
     }
 }
diff --git a/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs b/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
index 70d6870ce..41fe4dd07 100644
--- a/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
+++ b/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
@@ -5,7 +5,7 @@ using SixLabors.ImageSharp.Common.Helpers;
 
 namespace SixLabors.ImageSharp.Formats.Webp.Chunks;
 
-internal readonly struct WebpVp8X
+internal readonly struct WebpVp8X : IEquatable<WebpVp8X>
 {
     public WebpVp8X(bool hasAnimation, bool hasXmp, bool hasExif, bool hasAlpha, bool hasIcc, uint width, uint height)
     {
@@ -53,6 +53,24 @@ internal readonly struct WebpVp8X
     /// </summary>
     public uint Height { get; }
 
+    public static bool operator ==(WebpVp8X left, WebpVp8X right) => left.Equals(right);
+
+    public static bool operator !=(WebpVp8X left, WebpVp8X right) => !(left == right);
+
+    public override bool Equals(object? obj) => obj is WebpVp8X x && this.Equals(x);
+
+    public bool Equals(WebpVp8X other)
+        => this.HasAnimation == other.HasAnimation
+        && this.HasXmp == other.HasXmp
+        && this.HasExif == other.HasExif
+        && this.HasAlpha == other.HasAlpha
+        && this.HasIcc == other.HasIcc
+        && this.Width == other.Width
+        && this.Height == other.Height;
+
+    public override int GetHashCode() 
+        => HashCode.Combine(this.HasAnimation, this.HasXmp, this.HasExif, this.HasAlpha, this.HasIcc, this.Width, this.Height);
+
     public void Validate(uint maxDimension, ulong maxCanvasPixels)
     {
         if (this.Width > maxDimension || this.Height > maxDimension)
@@ -67,6 +85,9 @@ internal readonly struct WebpVp8X
         }
     }
 
+    public WebpVp8X WithAlpha(bool hasAlpha)
+        => new(this.HasAnimation, this.HasXmp, this.HasExif, hasAlpha, this.HasIcc, this.Width, this.Height);
+
     public void WriteTo(Stream stream)
     {
         byte flags = 0;
diff --git a/src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs b/src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs
index 518c09ff4..485225ab8 100644
--- a/src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs
+++ b/src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs
@@ -237,7 +237,7 @@ internal class Vp8LEncoder : IDisposable
     /// </summary>
     public Vp8LHashChain HashChain { get; }
 
-    public void EncodeHeader<TPixel>(Image<TPixel> image, Stream stream, bool hasAnimation)
+    public WebpVp8X EncodeHeader<TPixel>(Image<TPixel> image, Stream stream, bool hasAnimation)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         // Write bytes from the bit-writer buffer to the stream.
@@ -247,7 +247,8 @@ internal class Vp8LEncoder : IDisposable
         ExifProfile exifProfile = this.skipMetadata ? null : metadata.ExifProfile;
         XmpProfile xmpProfile = this.skipMetadata ? null : metadata.XmpProfile;
 
-        BitWriterBase.WriteTrunksBeforeData(
+        // The alpha flag is updated following encoding.
+        WebpVp8X vp8x = BitWriterBase.WriteTrunksBeforeData(
             stream,
             (uint)image.Width,
             (uint)image.Height,
@@ -262,9 +263,11 @@ internal class Vp8LEncoder : IDisposable
             WebpMetadata webpMetadata = WebpCommonUtils.GetWebpMetadata(image);
             BitWriterBase.WriteAnimationParameter(stream, webpMetadata.BackgroundColor, webpMetadata.RepeatCount);
         }
+
+        return vp8x;
     }
 
-    public void EncodeFooter<TPixel>(Image<TPixel> image, Stream stream)
+    public void EncodeFooter<TPixel>(Image<TPixel> image, in WebpVp8X vp8x, bool hasAlpha, Stream stream, long initialPosition)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         // Write bytes from the bit-writer buffer to the stream.
@@ -273,7 +276,9 @@ internal class Vp8LEncoder : IDisposable
         ExifProfile exifProfile = this.skipMetadata ? null : metadata.ExifProfile;
         XmpProfile xmpProfile = this.skipMetadata ? null : metadata.XmpProfile;
 
-        BitWriterBase.WriteTrunksAfterData(stream, exifProfile, xmpProfile);
+        bool updateVp8x = hasAlpha && vp8x != default;
+        WebpVp8X updated = updateVp8x ? vp8x.WithAlpha(true) : vp8x;
+        BitWriterBase.WriteTrunksAfterData(stream, in updated, updateVp8x, initialPosition, exifProfile, xmpProfile);
     }
 
     /// <summary>
@@ -285,7 +290,8 @@ internal class Vp8LEncoder : IDisposable
     /// <param name="frameMetadata">The frame metadata.</param>
     /// <param name="stream">The <see cref="Stream"/> to encode the image data to.</param>
     /// <param name="hasAnimation">Flag indicating, if an animation parameter is present.</param>
-    public void Encode<TPixel>(ImageFrame<TPixel> frame, Rectangle bounds, WebpFrameMetadata frameMetadata, Stream stream, bool hasAnimation)
+    /// <returns>A <see cref="bool"/> indicating whether the frame contains an alpha channel.</returns>
+    public bool Encode<TPixel>(ImageFrame<TPixel> frame, Rectangle bounds, WebpFrameMetadata frameMetadata, Stream stream, bool hasAnimation)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         // Convert image pixels to bgra array.
@@ -324,6 +330,8 @@ internal class Vp8LEncoder : IDisposable
         {
             RiffHelper.EndWriteChunk(stream, prevPosition);
         }
+
+        return hasAlpha;
     }
 
     /// <summary>
@@ -502,7 +510,7 @@ internal class Vp8LEncoder : IDisposable
     /// <typeparam name="TPixel">The type of the pixels.</typeparam>
     /// <param name="pixels">The frame pixel buffer to convert.</param>
     /// <returns>true, if the image is non opaque.</returns>
-    private bool ConvertPixelsToBgra<TPixel>(Buffer2DRegion<TPixel> pixels)
+    public bool ConvertPixelsToBgra<TPixel>(Buffer2DRegion<TPixel> pixels)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         bool nonOpaque = false;
diff --git a/src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs b/src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs
index 2b74c300a..7317527f7 100644
--- a/src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs
+++ b/src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs
@@ -311,7 +311,7 @@ internal class Vp8Encoder : IDisposable
     /// </summary>
     private int MbHeaderLimit { get; }
 
-    public void EncodeHeader<TPixel>(Image<TPixel> image, Stream stream, bool hasAlpha, bool hasAnimation)
+    public WebpVp8X EncodeHeader<TPixel>(Image<TPixel> image, Stream stream, bool hasAlpha, bool hasAnimation)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         // Write bytes from the bitwriter buffer to the stream.
@@ -321,7 +321,7 @@ internal class Vp8Encoder : IDisposable
         ExifProfile exifProfile = this.skipMetadata ? null : metadata.ExifProfile;
         XmpProfile xmpProfile = this.skipMetadata ? null : metadata.XmpProfile;
 
-        BitWriterBase.WriteTrunksBeforeData(
+        WebpVp8X vp8x = BitWriterBase.WriteTrunksBeforeData(
             stream,
             (uint)image.Width,
             (uint)image.Height,
@@ -336,9 +336,11 @@ internal class Vp8Encoder : IDisposable
             WebpMetadata webpMetadata = WebpCommonUtils.GetWebpMetadata(image);
             BitWriterBase.WriteAnimationParameter(stream, webpMetadata.BackgroundColor, webpMetadata.RepeatCount);
         }
+
+        return vp8x;
     }
 
-    public void EncodeFooter<TPixel>(Image<TPixel> image, Stream stream)
+    public void EncodeFooter<TPixel>(Image<TPixel> image, in WebpVp8X vp8x, bool hasAlpha, Stream stream, long initialPosition)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         // Write bytes from the bitwriter buffer to the stream.
@@ -347,7 +349,9 @@ internal class Vp8Encoder : IDisposable
         ExifProfile exifProfile = this.skipMetadata ? null : metadata.ExifProfile;
         XmpProfile xmpProfile = this.skipMetadata ? null : metadata.XmpProfile;
 
-        BitWriterBase.WriteTrunksAfterData(stream, exifProfile, xmpProfile);
+        bool updateVp8x = hasAlpha && vp8x != default;
+        WebpVp8X updated = updateVp8x ? vp8x.WithAlpha(true) : vp8x;
+        BitWriterBase.WriteTrunksAfterData(stream, in updated, updateVp8x, initialPosition, exifProfile, xmpProfile);
     }
 
     /// <summary>
@@ -358,9 +362,10 @@ internal class Vp8Encoder : IDisposable
     /// <param name="stream">The stream to encode the image data to.</param>
     /// <param name="bounds">The region of interest within the frame to encode.</param>
     /// <param name="frameMetadata">The frame metadata.</param>
-    public void EncodeAnimation<TPixel>(ImageFrame<TPixel> frame, Stream stream, Rectangle bounds, WebpFrameMetadata frameMetadata)
-        where TPixel : unmanaged, IPixel<TPixel> =>
-        this.Encode(stream, frame, bounds, frameMetadata, true, null);
+    /// <returns>A <see cref="bool"/> indicating whether the frame contains an alpha channel.</returns>
+    public bool EncodeAnimation<TPixel>(ImageFrame<TPixel> frame, Stream stream, Rectangle bounds, WebpFrameMetadata frameMetadata)
+        where TPixel : unmanaged, IPixel<TPixel>
+        => this.Encode(stream, frame, bounds, frameMetadata, true, null);
 
     /// <summary>
     /// Encodes the static image frame to the specified stream.
@@ -385,7 +390,8 @@ internal class Vp8Encoder : IDisposable
     /// <param name="frameMetadata">The frame metadata.</param>
     /// <param name="hasAnimation">Flag indicating, if an animation parameter is present.</param>
     /// <param name="image">The image to encode from.</param>
-    private void Encode<TPixel>(Stream stream, ImageFrame<TPixel> frame, Rectangle bounds, WebpFrameMetadata frameMetadata, bool hasAnimation, Image<TPixel> image)
+    /// <returns>A <see cref="bool"/> indicating whether the frame contains an alpha channel.</returns>
+    private bool Encode<TPixel>(Stream stream, ImageFrame<TPixel> frame, Rectangle bounds, WebpFrameMetadata frameMetadata, bool hasAnimation, Image<TPixel> image)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         int width = bounds.Width;
@@ -515,6 +521,8 @@ internal class Vp8Encoder : IDisposable
         {
             encodedAlphaData?.Dispose();
         }
+
+        return hasAlpha;
     }
 
     /// <inheritdoc/>
diff --git a/src/ImageSharp/Formats/Webp/WebpChunkParsingUtils.cs b/src/ImageSharp/Formats/Webp/WebpChunkParsingUtils.cs
index 80ffe8a99..07f09d45e 100644
--- a/src/ImageSharp/Formats/Webp/WebpChunkParsingUtils.cs
+++ b/src/ImageSharp/Formats/Webp/WebpChunkParsingUtils.cs
@@ -2,7 +2,6 @@
 // Licensed under the Six Labors Split License.
 
 using System.Buffers.Binary;
-using System.Drawing;
 using SixLabors.ImageSharp.Formats.Webp.BitReader;
 using SixLabors.ImageSharp.Formats.Webp.Lossy;
 using SixLabors.ImageSharp.IO;
diff --git a/src/ImageSharp/Formats/Webp/WebpDecoderCore.cs b/src/ImageSharp/Formats/Webp/WebpDecoderCore.cs
index 69a0afcd9..21a25860c 100644
--- a/src/ImageSharp/Formats/Webp/WebpDecoderCore.cs
+++ b/src/ImageSharp/Formats/Webp/WebpDecoderCore.cs
@@ -54,7 +54,7 @@ internal sealed class WebpDecoderCore : IImageDecoderInternals, IDisposable
     /// <summary>
     /// The flag to decide how to handle the background color in the Animation Chunk.
     /// </summary>
-    private BackgroundColorHandling backgroundColorHandling;
+    private readonly BackgroundColorHandling backgroundColorHandling;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="WebpDecoderCore"/> class.
diff --git a/src/ImageSharp/Formats/Webp/WebpEncoderCore.cs b/src/ImageSharp/Formats/Webp/WebpEncoderCore.cs
index e37c1d179..d29759f9a 100644
--- a/src/ImageSharp/Formats/Webp/WebpEncoderCore.cs
+++ b/src/ImageSharp/Formats/Webp/WebpEncoderCore.cs
@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Webp.Chunks;
 using SixLabors.ImageSharp.Formats.Webp.Lossless;
 using SixLabors.ImageSharp.Formats.Webp.Lossy;
 using SixLabors.ImageSharp.Memory;
@@ -143,12 +144,14 @@ internal sealed class WebpEncoderCore : IImageEncoderInternals
                 this.nearLossless,
                 this.nearLosslessQuality);
 
-            encoder.EncodeHeader(image, stream, hasAnimation);
+            long initialPosition = stream.Position;
+            bool hasAlpha = false;
+            WebpVp8X vp8x = encoder.EncodeHeader(image, stream, hasAnimation);
 
             // Encode the first frame.
             ImageFrame<TPixel> previousFrame = image.Frames.RootFrame;
             WebpFrameMetadata frameMetadata = WebpCommonUtils.GetWebpFrameMetadata(previousFrame);
-            encoder.Encode(previousFrame, previousFrame.Bounds(), frameMetadata, stream, hasAnimation);
+            hasAlpha |= encoder.Encode(previousFrame, previousFrame.Bounds(), frameMetadata, stream, hasAnimation);
 
             if (hasAnimation)
             {
@@ -190,14 +193,14 @@ internal sealed class WebpEncoderCore : IImageEncoderInternals
                         this.nearLossless,
                         this.nearLosslessQuality);
 
-                    animatedEncoder.Encode(encodingFrame, bounds, frameMetadata, stream, hasAnimation);
+                    hasAlpha |= animatedEncoder.Encode(encodingFrame, bounds, frameMetadata, stream, hasAnimation);
 
                     previousFrame = currentFrame;
                     previousDisposal = frameMetadata.DisposalMethod;
                 }
             }
 
-            encoder.EncodeFooter(image, stream);
+            encoder.EncodeFooter(image, in vp8x, hasAlpha, stream, initialPosition);
         }
         else
         {
@@ -214,17 +217,20 @@ internal sealed class WebpEncoderCore : IImageEncoderInternals
                 this.spatialNoiseShaping,
                 this.alphaCompression);
 
+            long initialPosition = stream.Position;
+            bool hasAlpha = false;
+            WebpVp8X vp8x = default;
             if (image.Frames.Count > 1)
             {
-                // TODO: What about alpha here?
-                encoder.EncodeHeader(image, stream, false, true);
+                // The alpha flag is updated following encoding.
+                vp8x = encoder.EncodeHeader(image, stream, false, true);
 
                 // Encode the first frame.
                 ImageFrame<TPixel> previousFrame = image.Frames.RootFrame;
                 WebpFrameMetadata frameMetadata = WebpCommonUtils.GetWebpFrameMetadata(previousFrame);
                 WebpDisposalMethod previousDisposal = frameMetadata.DisposalMethod;
 
-                encoder.EncodeAnimation(previousFrame, stream, previousFrame.Bounds(), frameMetadata);
+                hasAlpha |= encoder.EncodeAnimation(previousFrame, stream, previousFrame.Bounds(), frameMetadata);
 
                 // Encode additional frames
                 // This frame is reused to store de-duplicated pixel buffers.
@@ -263,18 +269,19 @@ internal sealed class WebpEncoderCore : IImageEncoderInternals
                         this.spatialNoiseShaping,
                         this.alphaCompression);
 
-                    animatedEncoder.EncodeAnimation(encodingFrame, stream, bounds, frameMetadata);
+                    hasAlpha |= animatedEncoder.EncodeAnimation(encodingFrame, stream, bounds, frameMetadata);
 
                     previousFrame = currentFrame;
                     previousDisposal = frameMetadata.DisposalMethod;
                 }
+
+                encoder.EncodeFooter(image, in vp8x, hasAlpha, stream, initialPosition);
             }
             else
             {
                 encoder.EncodeStatic(stream, image);
+                encoder.EncodeFooter(image, in vp8x, hasAlpha, stream, initialPosition);
             }
-
-            encoder.EncodeFooter(image, stream);
         }
     }
 }

From a197f222a685b3e4b08bf7d83e06cc95b68f09ee Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Sun, 17 Mar 2024 11:05:56 +1000
Subject: [PATCH 02/15] Update WebpVp8X.cs

---
 src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs b/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
index 41fe4dd07..fc88f8faa 100644
--- a/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
+++ b/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
@@ -68,7 +68,7 @@ internal readonly struct WebpVp8X : IEquatable<WebpVp8X>
         && this.Width == other.Width
         && this.Height == other.Height;
 
-    public override int GetHashCode() 
+    public override int GetHashCode()
         => HashCode.Combine(this.HasAnimation, this.HasXmp, this.HasExif, this.HasAlpha, this.HasIcc, this.Width, this.Height);
 
     public void Validate(uint maxDimension, ulong maxCanvasPixels)

From 417667a12c5ba1871b3b33d623e2aaa21020e1d9 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 12:33:23 -1000
Subject: [PATCH 03/15] Add test

---
 tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs          | 3 ++-
 tests/ImageSharp.Tests/TestImages.cs                           | 1 +
 .../Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png      | 3 +++
 .../Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png      | 3 +++
 .../Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png      | 3 +++
 .../Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png      | 3 +++
 .../Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png      | 3 +++
 tests/Images/Input/Png/animated/frame-offset.png               | 3 +++
 8 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png
 create mode 100644 tests/Images/Input/Png/animated/frame-offset.png

diff --git a/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
index 9b165526e..336b89376 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
@@ -87,7 +87,8 @@ public partial class PngDecoderTests
         TestImages.Png.DisposeBackgroundRegion,
         TestImages.Png.DisposePreviousFirst,
         TestImages.Png.DisposeBackgroundBeforeRegion,
-        TestImages.Png.BlendOverMultiple
+        TestImages.Png.BlendOverMultiple,
+        TestImages.Png.FrameOffset
     };
 
     [Theory]
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index fe633fddc..7e6ac9e8d 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -73,6 +73,7 @@ public static class TestImages
         public const string DisposeBackgroundRegion = "Png/animated/15-dispose-background-region.png";
         public const string DisposePreviousFirst = "Png/animated/12-dispose-prev-first.png";
         public const string BlendOverMultiple = "Png/animated/21-blend-over-multiple.png";
+        public const string FrameOffset = "Png/animated/frame-offset.png";
         public const string Issue2666 = "Png/issues/Issue_2666.png";
 
         // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png
new file mode 100644
index 000000000..b9fa24c93
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:84e2353264e3488122f4d488d7c4b198ff5192ad0c662c7fb0a369c957ecc7ea
+size 353
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png
new file mode 100644
index 000000000..6f3a27187
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2916711d3f4d72eb66a5cfc2b40a3318eb4cce5b367658cfc7e3b573fd39cc33
+size 693
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png
new file mode 100644
index 000000000..50911cce5
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:7f9d5503414ccefa6b66661b1e93c2c3f6e4491f14af006a71153cecf43b52f5
+size 806
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png
new file mode 100644
index 000000000..89d2f9570
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:fe42b7dc6524d5589ad680650f4bcd181319b40b258b31e0932d6e936818e980
+size 570
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png
new file mode 100644
index 000000000..c3f2b99b8
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:8002ff5b3451b348f285eec15dd7a093c62d11d8b77c3ead9ac89ca6eb29977d
+size 669
diff --git a/tests/Images/Input/Png/animated/frame-offset.png b/tests/Images/Input/Png/animated/frame-offset.png
new file mode 100644
index 000000000..4eebb44a3
--- /dev/null
+++ b/tests/Images/Input/Png/animated/frame-offset.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:3c019073841b48b02cb07c779fed8654c6052aee700e7620d07f5d775d97088f
+size 2156

From d933db75ed5fff7e3ada9ded4d5562b2982ca260 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 13:50:40 -1000
Subject: [PATCH 04/15] Fix ProcessInterlacedPaletteScanline not obeying
 frameControl.XOffset

---
 src/ImageSharp/Formats/Png/PngScanlineProcessor.cs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs b/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs
index f217515e3..b327ed21b 100644
--- a/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs
+++ b/src/ImageSharp/Formats/Png/PngScanlineProcessor.cs
@@ -198,8 +198,9 @@ internal static class PngScanlineProcessor
         ref byte scanlineSpanRef = ref MemoryMarshal.GetReference(scanlineSpan);
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
         ref Color paletteBase = ref MemoryMarshal.GetReference(palette.Value.Span);
+        uint offset = pixelOffset + frameControl.XOffset;
 
-        for (nuint x = pixelOffset, o = 0; x < frameControl.XMax; x += increment, o++)
+        for (nuint x = offset, o = 0; x < frameControl.XMax; x += increment, o++)
         {
             uint index = Unsafe.Add(ref scanlineSpanRef, o);
             pixel.FromRgba32(Unsafe.Add(ref paletteBase, index).ToRgba32());

From 63b7ecd87e1d6659874123ac7c8e5baefdc3dc53 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 17:49:33 -1000
Subject: [PATCH 05/15] Fix frame dispose operation handling

---
 src/ImageSharp/Formats/Png/PngDecoderCore.cs  | 25 +++++++++++--------
 .../00.png                                    |  4 +--
 .../01.png                                    |  4 +--
 .../02.png                                    |  4 +--
 .../03.png                                    |  4 +--
 .../04.png                                    |  4 +--
 6 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index a96c53c10..caa104c6e 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -240,8 +240,13 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                                 currentFrameControl.Value,
                                 cancellationToken);
 
-                            previousFrame = currentFrame;
-                            previousFrameControl = currentFrameControl;
+                            // if current frame dispose is restore to previous, then from future frame's perspective, it never happened
+                            if (currentFrameControl.Value.DisposeOperation != PngDisposalMethod.RestoreToPrevious)
+                            {
+                                previousFrame = currentFrame;
+                                previousFrameControl = currentFrameControl;
+                            }
+
                             break;
                         case PngChunkType.Data:
 
@@ -639,18 +644,18 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
         out ImageFrame<TPixel> frame)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        // We create a clone of the previous 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
         frame = image.Frames.AddFrame(previousFrame ?? image.Frames.RootFrame);
 
-        // If the first `fcTL` chunk uses a `dispose_op` of APNG_DISPOSE_OP_PREVIOUS it should be treated as APNG_DISPOSE_OP_BACKGROUND.
-        if (previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToBackground
-            || (previousFrame is null && previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToPrevious))
+        // if restoring to before first frame, restore to background
+        if (previousFrame is null && previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToPrevious)
+        {
+            Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion();
+            pixelRegion.Clear();
+        }
+        else if (previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToBackground)
         {
             Rectangle restoreArea = previousFrameControl.Bounds;
-            Rectangle interest = Rectangle.Intersect(frame.Bounds(), restoreArea);
-            Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(interest);
+            Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(restoreArea);
             pixelRegion.Clear();
         }
 
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png
index b9fa24c93..870ed61a4 100644
--- a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:84e2353264e3488122f4d488d7c4b198ff5192ad0c662c7fb0a369c957ecc7ea
-size 353
+oid sha256:b85aaf7153e0ca538856a58d7b069bcc13fadc468ea603c85f8782cc691f86c3
+size 387
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png
index 6f3a27187..cab85d946 100644
--- a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:2916711d3f4d72eb66a5cfc2b40a3318eb4cce5b367658cfc7e3b573fd39cc33
-size 693
+oid sha256:fcb83d6893dcfd869b764ff9846c259eaa0caf26cec3f0fc2cbae2c26f2eeaa5
+size 660
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png
index 50911cce5..1a2c5adcf 100644
--- a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:7f9d5503414ccefa6b66661b1e93c2c3f6e4491f14af006a71153cecf43b52f5
-size 806
+oid sha256:562ec382f6d2af68e66092bf6949f66147d5f608d3c618eea5a7c1ea400737ff
+size 768
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png
index 89d2f9570..d850459ee 100644
--- a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:fe42b7dc6524d5589ad680650f4bcd181319b40b258b31e0932d6e936818e980
-size 570
+oid sha256:d12a7791b960072e32b78bd9aaf456dc99341eea1c66ea05050433d8c082c6ac
+size 579
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png
index c3f2b99b8..000b0567d 100644
--- a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:8002ff5b3451b348f285eec15dd7a093c62d11d8b77c3ead9ac89ca6eb29977d
-size 669
+oid sha256:2db38d7ffcc95c23a5c94a06f10c6cc67406ae581a955c99ede4af97b1a044f8
+size 628

From d15a2e79a4fbfbaa150e436e6afa938da131f05d Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 18:14:23 -1000
Subject: [PATCH 06/15] Add test for default image not animated

---
 tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs          | 3 ++-
 tests/ImageSharp.Tests/TestImages.cs                           | 1 +
 .../00.png                                                     | 3 +++
 .../01.png                                                     | 3 +++
 tests/Images/Input/Png/animated/default-not-animated.png       | 3 +++
 5 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/00.png
 create mode 100644 tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/01.png
 create mode 100644 tests/Images/Input/Png/animated/default-not-animated.png

diff --git a/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
index 336b89376..9ff368d4e 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs
@@ -88,7 +88,8 @@ public partial class PngDecoderTests
         TestImages.Png.DisposePreviousFirst,
         TestImages.Png.DisposeBackgroundBeforeRegion,
         TestImages.Png.BlendOverMultiple,
-        TestImages.Png.FrameOffset
+        TestImages.Png.FrameOffset,
+        TestImages.Png.DefaultNotAnimated
     };
 
     [Theory]
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index 7e6ac9e8d..afb13363d 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -74,6 +74,7 @@ public static class TestImages
         public const string DisposePreviousFirst = "Png/animated/12-dispose-prev-first.png";
         public const string BlendOverMultiple = "Png/animated/21-blend-over-multiple.png";
         public const string FrameOffset = "Png/animated/frame-offset.png";
+        public const string DefaultNotAnimated = "Png/animated/default-not-animated.png";
         public const string Issue2666 = "Png/issues/Issue_2666.png";
 
         // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/00.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/00.png
new file mode 100644
index 000000000..4c5ea8169
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/00.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:8d4716e18655be53630d6d50daebe8c38e0eedb2432c7a73840b55d1473d5944
+size 1050
diff --git a/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/01.png b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/01.png
new file mode 100644
index 000000000..790fe45e4
--- /dev/null
+++ b/tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/01.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:9b5a6d3cf1a777f6b719c2a1cf79bffe2251355d75e6c0f7ce7a973b3d033419
+size 1177
diff --git a/tests/Images/Input/Png/animated/default-not-animated.png b/tests/Images/Input/Png/animated/default-not-animated.png
new file mode 100644
index 000000000..1ed72698d
--- /dev/null
+++ b/tests/Images/Input/Png/animated/default-not-animated.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:647d484c8f320b55824b9219270524df3edc434a4793e1627e0ee14af8d6e4f8
+size 1689

From 9b8ef108d41877464418a16451aac87eec37c8bb Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 18:28:03 -1000
Subject: [PATCH 07/15] Fix handling of case where default image isn't animated

---
 src/ImageSharp/Formats/Png/PngDecoderCore.cs | 22 +++++++++++---------
 src/ImageSharp/Formats/Png/PngMetadata.cs    |  5 +++++
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index caa104c6e..3bc2ccf49 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -228,8 +228,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                                 PngThrowHelper.ThrowMissingFrameControl();
                             }
 
-                            previousFrameControl ??= new((uint)this.header.Width, (uint)this.header.Height);
-                            this.InitializeFrame(previousFrameControl.Value, currentFrameControl.Value, image, previousFrame, out currentFrame);
+                            this.InitializeFrame(previousFrameControl, currentFrameControl.Value, image, previousFrame, out currentFrame);
 
                             this.currentStream.Position += 4;
                             this.ReadScanlines(
@@ -249,7 +248,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
 
                             break;
                         case PngChunkType.Data:
-
+                            pngMetadata.DefaultImageAnimated = currentFrameControl != null;
                             currentFrameControl ??= new((uint)this.header.Width, (uint)this.header.Height);
                             if (image is null)
                             {
@@ -266,9 +265,12 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                                 this.ReadNextDataChunk,
                                 currentFrameControl.Value,
                                 cancellationToken);
+                            if (pngMetadata.DefaultImageAnimated)
+                            {
+                                previousFrame = currentFrame;
+                                previousFrameControl = currentFrameControl;
+                            }
 
-                            previousFrame = currentFrame;
-                            previousFrameControl = currentFrameControl;
                             break;
                         case PngChunkType.Palette:
                             this.palette = chunk.Data.GetSpan().ToArray();
@@ -637,7 +639,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
     /// <param name="previousFrame">The previous frame.</param>
     /// <param name="frame">The created frame</param>
     private void InitializeFrame<TPixel>(
-        FrameControl previousFrameControl,
+        FrameControl? previousFrameControl,
         FrameControl currentFrameControl,
         Image<TPixel> image,
         ImageFrame<TPixel>? previousFrame,
@@ -646,15 +648,15 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
     {
         frame = image.Frames.AddFrame(previousFrame ?? image.Frames.RootFrame);
 
-        // if restoring to before first frame, restore to background
-        if (previousFrame is null && previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToPrevious)
+        // If restoring to before first frame, restore to background. Same if first frame (previousFrameControl null).
+        if (previousFrameControl == null || (previousFrame is null && previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToPrevious))
         {
             Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion();
             pixelRegion.Clear();
         }
-        else if (previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToBackground)
+        else if (previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToBackground)
         {
-            Rectangle restoreArea = previousFrameControl.Bounds;
+            Rectangle restoreArea = previousFrameControl.Value.Bounds;
             Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(restoreArea);
             pixelRegion.Clear();
         }
diff --git a/src/ImageSharp/Formats/Png/PngMetadata.cs b/src/ImageSharp/Formats/Png/PngMetadata.cs
index 93ddcf263..c4ff3bbe2 100644
--- a/src/ImageSharp/Formats/Png/PngMetadata.cs
+++ b/src/ImageSharp/Formats/Png/PngMetadata.cs
@@ -83,6 +83,11 @@ public class PngMetadata : IDeepCloneable
     /// </summary>
     public uint RepeatCount { get; set; } = 1;
 
+    /// <summary>
+    ///  Gets or sets a value indicating whether the default image is shown as part of the animated sequence
+    /// </summary>
+    public bool DefaultImageAnimated { get; set; }
+
     /// <inheritdoc/>
     public IDeepCloneable DeepClone() => new PngMetadata(this);
 

From 85fd4311f1caa30bb37c990842f977ed9a1dec81 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 19:23:35 -1000
Subject: [PATCH 08/15] Fix PngMetadata copy

---
 src/ImageSharp/Formats/Png/PngMetadata.cs              | 3 ++-
 tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngMetadata.cs b/src/ImageSharp/Formats/Png/PngMetadata.cs
index c4ff3bbe2..766377f7c 100644
--- a/src/ImageSharp/Formats/Png/PngMetadata.cs
+++ b/src/ImageSharp/Formats/Png/PngMetadata.cs
@@ -29,6 +29,7 @@ public class PngMetadata : IDeepCloneable
         this.InterlaceMethod = other.InterlaceMethod;
         this.TransparentColor = other.TransparentColor;
         this.RepeatCount = other.RepeatCount;
+        this.DefaultImageAnimated = other.DefaultImageAnimated;
 
         if (other.ColorTable?.Length > 0)
         {
@@ -86,7 +87,7 @@ public class PngMetadata : IDeepCloneable
     /// <summary>
     ///  Gets or sets a value indicating whether the default image is shown as part of the animated sequence
     /// </summary>
-    public bool DefaultImageAnimated { get; set; }
+    public bool DefaultImageAnimated { get; set; } = true;
 
     /// <inheritdoc/>
     public IDeepCloneable DeepClone() => new PngMetadata(this);
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
index b3c122a7a..4f9ba9abe 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
@@ -32,7 +32,8 @@ public class PngMetadataTests
             InterlaceMethod = PngInterlaceMode.Adam7,
             Gamma = 2,
             TextData = new List<PngTextData> { new PngTextData("name", "value", "foo", "bar") },
-            RepeatCount = 123
+            RepeatCount = 123,
+            DefaultImageAnimated = false
         };
 
         PngMetadata clone = (PngMetadata)meta.DeepClone();
@@ -44,6 +45,7 @@ public class PngMetadataTests
         Assert.False(meta.TextData.Equals(clone.TextData));
         Assert.True(meta.TextData.SequenceEqual(clone.TextData));
         Assert.True(meta.RepeatCount == clone.RepeatCount);
+        Assert.True(meta.DefaultImageAnimated == clone.DefaultImageAnimated);
 
         clone.BitDepth = PngBitDepth.Bit2;
         clone.ColorType = PngColorType.Palette;

From addec72467bacd9fc2152549f9ce22e1b6074006 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Thu, 28 Mar 2024 20:11:43 -1000
Subject: [PATCH 09/15] Make PngEncoder respect DefaultImageAnimated

---
 src/ImageSharp/Formats/Png/PngEncoderCore.cs  | 43 +++++++++++++------
 .../Formats/Png/PngEncoderTests.cs            | 23 ++++++++++
 2 files changed, 53 insertions(+), 13 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngEncoderCore.cs b/src/ImageSharp/Formats/Png/PngEncoderCore.cs
index ea94270f2..d1a600737 100644
--- a/src/ImageSharp/Formats/Png/PngEncoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngEncoderCore.cs
@@ -161,6 +161,7 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
 
         ImageFrame<TPixel>? clonedFrame = null;
         ImageFrame<TPixel> currentFrame = image.Frames.RootFrame;
+        int currentFrameIndex = 0;
 
         bool clearTransparency = this.encoder.TransparentColorMode is PngTransparentColorMode.Clear;
         if (clearTransparency)
@@ -190,28 +191,49 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
         if (image.Frames.Count > 1)
         {
             this.WriteAnimationControlChunk(stream, (uint)image.Frames.Count, pngMetadata.RepeatCount);
+        }
+
+        // If the first frame isn't animated, write it as usual and skip it when writing animated frames
+        if (!pngMetadata.DefaultImageAnimated || image.Frames.Count == 1)
+        {
+            FrameControl frameControl = new((uint)this.width, (uint)this.height);
+            this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, false);
+            currentFrameIndex++;
+        }
 
-            // Write the first frame.
+        if (image.Frames.Count > 1)
+        {
+            // Write the first animated frame.
+            currentFrame = image.Frames[currentFrameIndex];
             PngFrameMetadata frameMetadata = GetPngFrameMetadata(currentFrame);
             PngDisposalMethod previousDisposal = frameMetadata.DisposalMethod;
             FrameControl frameControl = this.WriteFrameControlChunk(stream, frameMetadata, currentFrame.Bounds(), 0);
-            this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, false);
+            uint sequenceNumber = 1;
+            if (pngMetadata.DefaultImageAnimated)
+            {
+                this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, false);
+            }
+            else
+            {
+                sequenceNumber += this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, true);
+            }
+
+            currentFrameIndex++;
 
             // Capture the global palette for reuse on subsequent frames.
             ReadOnlyMemory<TPixel>? previousPalette = quantized?.Palette.ToArray();
 
             // Write following frames.
-            uint increment = 0;
             ImageFrame<TPixel> previousFrame = image.Frames.RootFrame;
 
             // This frame is reused to store de-duplicated pixel buffers.
             using ImageFrame<TPixel> encodingFrame = new(image.Configuration, previousFrame.Size());
 
-            for (int i = 1; i < image.Frames.Count; i++)
+            for (; currentFrameIndex < image.Frames.Count; currentFrameIndex++)
             {
                 ImageFrame<TPixel>? prev = previousDisposal == PngDisposalMethod.RestoreToBackground ? null : previousFrame;
-                currentFrame = image.Frames[i];
-                ImageFrame<TPixel>? nextFrame = i < image.Frames.Count - 1 ? image.Frames[i + 1] : null;
+                currentFrame = image.Frames[currentFrameIndex];
+                ImageFrame<TPixel>? nextFrame = currentFrameIndex < image.Frames.Count - 1 ? image.Frames[currentFrameIndex + 1] : null;
 
                 frameMetadata = GetPngFrameMetadata(currentFrame);
                 bool blend = frameMetadata.BlendMethod == PngBlendMethod.Over;
@@ -232,22 +254,17 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
                 }
 
                 // Each frame control sequence number must be incremented by the number of frame data chunks that follow.
-                frameControl = this.WriteFrameControlChunk(stream, frameMetadata, bounds, (uint)i + increment);
+                frameControl = this.WriteFrameControlChunk(stream, frameMetadata, bounds, sequenceNumber);
 
                 // Dispose of previous quantized frame and reassign.
                 quantized?.Dispose();
                 quantized = this.CreateQuantizedImageAndUpdateBitDepth(pngMetadata, encodingFrame, bounds, previousPalette);
-                increment += this.WriteDataChunks(frameControl, encodingFrame.PixelBuffer.GetRegion(bounds), quantized, stream, true);
+                sequenceNumber += this.WriteDataChunks(frameControl, encodingFrame.PixelBuffer.GetRegion(bounds), quantized, stream, true) + 1;
 
                 previousFrame = currentFrame;
                 previousDisposal = frameMetadata.DisposalMethod;
             }
         }
-        else
-        {
-            FrameControl frameControl = new((uint)this.width, (uint)this.height);
-            this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, false);
-        }
 
         this.WriteEndChunk(stream);
 
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
index 950f1d2e3..bbd7ff4e4 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
@@ -586,6 +586,29 @@ public partial class PngEncoderTests
         }
     }
 
+    [Theory]
+    [WithFile(TestImages.Png.DefaultNotAnimated, PixelTypes.Rgba32)]
+    public void Encode_DefaultNotAnimated<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
+        using MemoryStream memStream = new();
+        image.Save(memStream, PngEncoder);
+        memStream.Position = 0;
+
+        image.DebugSave(provider: provider, encoder: PngEncoder, null, false);
+
+        using Image<Rgba32> output = Image.Load<Rgba32>(memStream);
+        ImageComparer.Exact.VerifySimilarity(output, image);
+
+        Assert.Equal(2, image.Frames.Count);
+        Assert.Equal(image.Frames.Count, output.Frames.Count);
+
+        PngMetadata originalMetadata = image.Metadata.GetPngMetadata();
+        PngMetadata outputMetadata = output.Metadata.GetPngMetadata();
+        Assert.Equal(originalMetadata.DefaultImageAnimated, outputMetadata.DefaultImageAnimated);
+    }
+
     [Theory]
     [MemberData(nameof(PngTrnsFiles))]
     public void Encode_PreserveTrns(string imagePath, PngBitDepth pngBitDepth, PngColorType pngColorType)

From 9ae8be4c8afc28f6c648f913b558ba23e6a8e191 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Fri, 29 Mar 2024 21:34:48 -1000
Subject: [PATCH 10/15] Add more tests

---
 .../Formats/Png/PngEncoderTests.Chunks.cs     | 33 +++++++++++++++++++
 .../Formats/Png/PngMetadataTests.cs           | 20 +++++++++++
 2 files changed, 53 insertions(+)

diff --git a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs
index 044da2193..2e3390298 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs
@@ -3,6 +3,7 @@
 
 using System.Buffers.Binary;
 using SixLabors.ImageSharp.Formats.Png;
+using SixLabors.ImageSharp.Formats.Png.Chunks;
 using SixLabors.ImageSharp.PixelFormats;
 
 // ReSharper disable InconsistentNaming
@@ -59,6 +60,38 @@ public partial class PngEncoderTests
         }
     }
 
+    [Theory]
+    [WithFile(TestImages.Png.DefaultNotAnimated, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Png.APng, PixelTypes.Rgba32)]
+    public void AcTL_CorrectlyWritten<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
+        PngMetadata metadata = image.Metadata.GetPngMetadata();
+        int correctFrameCount = image.Frames.Count - (metadata.DefaultImageAnimated ? 0 : 1);
+        using MemoryStream memStream = new();
+        image.Save(memStream, PngEncoder);
+        memStream.Position = 0;
+        Span<byte> bytesSpan = memStream.ToArray().AsSpan(8); // Skip header.
+        bool foundAcTl = false;
+        while (bytesSpan.Length > 0 && !foundAcTl)
+        {
+            int length = BinaryPrimitives.ReadInt32BigEndian(bytesSpan[..4]);
+            PngChunkType type = (PngChunkType)BinaryPrimitives.ReadInt32BigEndian(bytesSpan.Slice(4, 4));
+            if (type == PngChunkType.AnimationControl)
+            {
+                AnimationControl control = AnimationControl.Parse(bytesSpan[8..]);
+                foundAcTl = true;
+                Assert.True(control.NumberFrames == correctFrameCount);
+                Assert.True(control.NumberPlays == metadata.RepeatCount);
+            }
+
+            bytesSpan = bytesSpan[(4 + 4 + length + 4)..];
+        }
+
+        Assert.True(foundAcTl);
+    }
+
     [Theory]
     [InlineData(PngChunkType.Gamma)]
     [InlineData(PngChunkType.Chroma)]
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
index 4f9ba9abe..8308935d0 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
@@ -146,6 +146,26 @@ public class PngMetadataTests
         VerifyExifDataIsPresent(exif);
     }
 
+    [Theory]
+    [WithFile(TestImages.Png.DefaultNotAnimated, PixelTypes.Rgba32)]
+    public void Decode_IdentifiesDefaultFrameNotAnimated<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
+        PngMetadata meta = image.Metadata.GetFormatMetadata(PngFormat.Instance);
+        Assert.False(meta.DefaultImageAnimated);
+    }
+
+    [Theory]
+    [WithFile(TestImages.Png.APng, PixelTypes.Rgba32)]
+    public void Decode_IdentifiesDefaultFrameAnimated<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
+        PngMetadata meta = image.Metadata.GetFormatMetadata(PngFormat.Instance);
+        Assert.True(meta.DefaultImageAnimated);
+    }
+
     [Theory]
     [WithFile(TestImages.Png.PngWithMetadata, PixelTypes.Rgba32)]
     public void Decode_IgnoresExifData_WhenIgnoreMetadataIsTrue<TPixel>(TestImageProvider<TPixel> provider)

From ceb7dc0e7e26ee289cb8e2743ceb5b6793f65c41 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Fri, 29 Mar 2024 21:35:53 -1000
Subject: [PATCH 11/15] Fix incorrect acTL frame count

---
 src/ImageSharp/Formats/Png/PngEncoderCore.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/ImageSharp/Formats/Png/PngEncoderCore.cs b/src/ImageSharp/Formats/Png/PngEncoderCore.cs
index d1a600737..e0f7ed948 100644
--- a/src/ImageSharp/Formats/Png/PngEncoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngEncoderCore.cs
@@ -190,7 +190,7 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
 
         if (image.Frames.Count > 1)
         {
-            this.WriteAnimationControlChunk(stream, (uint)image.Frames.Count, pngMetadata.RepeatCount);
+            this.WriteAnimationControlChunk(stream, (uint)(image.Frames.Count - (pngMetadata.DefaultImageAnimated ? 0 : 1)), pngMetadata.RepeatCount);
         }
 
         // If the first frame isn't animated, write it as usual and skip it when writing animated frames

From 4698e8ca3b4a6a453a45cace6554f40148c53d3e Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Mon, 1 Apr 2024 17:03:25 -0700
Subject: [PATCH 12/15] re-add comment

---
 src/ImageSharp/Formats/Png/PngDecoderCore.cs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index 3bc2ccf49..dd2ce9eb1 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -648,7 +648,8 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
     {
         frame = image.Frames.AddFrame(previousFrame ?? image.Frames.RootFrame);
 
-        // If restoring to before first frame, restore to background. Same if first frame (previousFrameControl null).
+        // If the first `fcTL` chunk uses a `dispose_op` of APNG_DISPOSE_OP_PREVIOUS it should be treated as APNG_DISPOSE_OP_BACKGROUND.
+        // So, if restoring to before first frame, clear entire area. Same if first frame (previousFrameControl null).
         if (previousFrameControl == null || (previousFrame is null && previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToPrevious))
         {
             Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion();

From b1a5593b20e89786343d5d7d71a2c08dd606b1e9 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Mon, 1 Apr 2024 17:28:27 -0700
Subject: [PATCH 13/15] Add test for case with frame offsets

---
 .../Formats/Png/PngEncoderTests.cs            | 31 ++++---------------
 1 file changed, 6 insertions(+), 25 deletions(-)

diff --git a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
index bbd7ff4e4..6c4047be8 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
@@ -447,6 +447,8 @@ public partial class PngEncoderTests
 
     [Theory]
     [WithFile(TestImages.Png.APng, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Png.DefaultNotAnimated, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Png.FrameOffset, PixelTypes.Rgba32)]
     public void Encode_APng<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel>
     {
@@ -458,15 +460,17 @@ public partial class PngEncoderTests
         image.DebugSave(provider: provider, encoder: PngEncoder, null, false);
 
         using Image<Rgba32> output = Image.Load<Rgba32>(memStream);
-        ImageComparer.Exact.VerifySimilarity(output, image);
 
-        Assert.Equal(5, image.Frames.Count);
+        // some loss from original, due to compositing
+        ImageComparer.TolerantPercentage(0.01f).VerifySimilarity(output, image);
+
         Assert.Equal(image.Frames.Count, output.Frames.Count);
 
         PngMetadata originalMetadata = image.Metadata.GetPngMetadata();
         PngMetadata outputMetadata = output.Metadata.GetPngMetadata();
 
         Assert.Equal(originalMetadata.RepeatCount, outputMetadata.RepeatCount);
+        Assert.Equal(originalMetadata.DefaultImageAnimated, outputMetadata.DefaultImageAnimated);
 
         for (int i = 0; i < image.Frames.Count; i++)
         {
@@ -586,29 +590,6 @@ public partial class PngEncoderTests
         }
     }
 
-    [Theory]
-    [WithFile(TestImages.Png.DefaultNotAnimated, PixelTypes.Rgba32)]
-    public void Encode_DefaultNotAnimated<TPixel>(TestImageProvider<TPixel> provider)
-        where TPixel : unmanaged, IPixel<TPixel>
-    {
-        using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
-        using MemoryStream memStream = new();
-        image.Save(memStream, PngEncoder);
-        memStream.Position = 0;
-
-        image.DebugSave(provider: provider, encoder: PngEncoder, null, false);
-
-        using Image<Rgba32> output = Image.Load<Rgba32>(memStream);
-        ImageComparer.Exact.VerifySimilarity(output, image);
-
-        Assert.Equal(2, image.Frames.Count);
-        Assert.Equal(image.Frames.Count, output.Frames.Count);
-
-        PngMetadata originalMetadata = image.Metadata.GetPngMetadata();
-        PngMetadata outputMetadata = output.Metadata.GetPngMetadata();
-        Assert.Equal(originalMetadata.DefaultImageAnimated, outputMetadata.DefaultImageAnimated);
-    }
-
     [Theory]
     [MemberData(nameof(PngTrnsFiles))]
     public void Encode_PreserveTrns(string imagePath, PngBitDepth pngBitDepth, PngColorType pngColorType)

From 848013921281a7381ea4328f4f3d37f3587b01bf Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Mon, 1 Apr 2024 17:56:21 -0700
Subject: [PATCH 14/15] re-add rest of comment

---
 src/ImageSharp/Formats/Png/PngDecoderCore.cs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index dd2ce9eb1..1afb65566 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -646,6 +646,9 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
         out ImageFrame<TPixel> frame)
         where TPixel : unmanaged, IPixel<TPixel>
     {
+        // We create a clone of the previous 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
         frame = image.Frames.AddFrame(previousFrame ?? image.Frames.RootFrame);
 
         // If the first `fcTL` chunk uses a `dispose_op` of APNG_DISPOSE_OP_PREVIOUS it should be treated as APNG_DISPOSE_OP_BACKGROUND.

From c8eab788dd8d496703ba7c82bd595fe2fea81d83 Mon Sep 17 00:00:00 2001
From: SpaceCheetah <space-cheetah@hotmail.com>
Date: Tue, 2 Apr 2024 23:00:15 -0700
Subject: [PATCH 15/15] Rename DefaultImageAnimated to AnimateRootFrame

---
 src/ImageSharp/Formats/Png/PngDecoderCore.cs              | 4 ++--
 src/ImageSharp/Formats/Png/PngEncoderCore.cs              | 6 +++---
 src/ImageSharp/Formats/Png/PngMetadata.cs                 | 6 +++---
 .../Formats/Png/PngEncoderTests.Chunks.cs                 | 2 +-
 tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs     | 2 +-
 tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs    | 8 ++++----
 6 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index 1afb65566..c4b10b4dc 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -248,7 +248,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
 
                             break;
                         case PngChunkType.Data:
-                            pngMetadata.DefaultImageAnimated = currentFrameControl != null;
+                            pngMetadata.AnimateRootFrame = currentFrameControl != null;
                             currentFrameControl ??= new((uint)this.header.Width, (uint)this.header.Height);
                             if (image is null)
                             {
@@ -265,7 +265,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                                 this.ReadNextDataChunk,
                                 currentFrameControl.Value,
                                 cancellationToken);
-                            if (pngMetadata.DefaultImageAnimated)
+                            if (pngMetadata.AnimateRootFrame)
                             {
                                 previousFrame = currentFrame;
                                 previousFrameControl = currentFrameControl;
diff --git a/src/ImageSharp/Formats/Png/PngEncoderCore.cs b/src/ImageSharp/Formats/Png/PngEncoderCore.cs
index e0f7ed948..802e4dd6a 100644
--- a/src/ImageSharp/Formats/Png/PngEncoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngEncoderCore.cs
@@ -190,11 +190,11 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
 
         if (image.Frames.Count > 1)
         {
-            this.WriteAnimationControlChunk(stream, (uint)(image.Frames.Count - (pngMetadata.DefaultImageAnimated ? 0 : 1)), pngMetadata.RepeatCount);
+            this.WriteAnimationControlChunk(stream, (uint)(image.Frames.Count - (pngMetadata.AnimateRootFrame ? 0 : 1)), pngMetadata.RepeatCount);
         }
 
         // If the first frame isn't animated, write it as usual and skip it when writing animated frames
-        if (!pngMetadata.DefaultImageAnimated || image.Frames.Count == 1)
+        if (!pngMetadata.AnimateRootFrame || image.Frames.Count == 1)
         {
             FrameControl frameControl = new((uint)this.width, (uint)this.height);
             this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, false);
@@ -209,7 +209,7 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
             PngDisposalMethod previousDisposal = frameMetadata.DisposalMethod;
             FrameControl frameControl = this.WriteFrameControlChunk(stream, frameMetadata, currentFrame.Bounds(), 0);
             uint sequenceNumber = 1;
-            if (pngMetadata.DefaultImageAnimated)
+            if (pngMetadata.AnimateRootFrame)
             {
                 this.WriteDataChunks(frameControl, currentFrame.PixelBuffer.GetRegion(), quantized, stream, false);
             }
diff --git a/src/ImageSharp/Formats/Png/PngMetadata.cs b/src/ImageSharp/Formats/Png/PngMetadata.cs
index 766377f7c..d9028dd80 100644
--- a/src/ImageSharp/Formats/Png/PngMetadata.cs
+++ b/src/ImageSharp/Formats/Png/PngMetadata.cs
@@ -29,7 +29,7 @@ public class PngMetadata : IDeepCloneable
         this.InterlaceMethod = other.InterlaceMethod;
         this.TransparentColor = other.TransparentColor;
         this.RepeatCount = other.RepeatCount;
-        this.DefaultImageAnimated = other.DefaultImageAnimated;
+        this.AnimateRootFrame = other.AnimateRootFrame;
 
         if (other.ColorTable?.Length > 0)
         {
@@ -85,9 +85,9 @@ public class PngMetadata : IDeepCloneable
     public uint RepeatCount { get; set; } = 1;
 
     /// <summary>
-    ///  Gets or sets a value indicating whether the default image is shown as part of the animated sequence
+    ///  Gets or sets a value indicating whether the root frame is shown as part of the animated sequence
     /// </summary>
-    public bool DefaultImageAnimated { get; set; } = true;
+    public bool AnimateRootFrame { get; set; } = true;
 
     /// <inheritdoc/>
     public IDeepCloneable DeepClone() => new PngMetadata(this);
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs
index 2e3390298..76fd260dd 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.Chunks.cs
@@ -68,7 +68,7 @@ public partial class PngEncoderTests
     {
         using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
         PngMetadata metadata = image.Metadata.GetPngMetadata();
-        int correctFrameCount = image.Frames.Count - (metadata.DefaultImageAnimated ? 0 : 1);
+        int correctFrameCount = image.Frames.Count - (metadata.AnimateRootFrame ? 0 : 1);
         using MemoryStream memStream = new();
         image.Save(memStream, PngEncoder);
         memStream.Position = 0;
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
index 6c4047be8..ca5aae961 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs
@@ -470,7 +470,7 @@ public partial class PngEncoderTests
         PngMetadata outputMetadata = output.Metadata.GetPngMetadata();
 
         Assert.Equal(originalMetadata.RepeatCount, outputMetadata.RepeatCount);
-        Assert.Equal(originalMetadata.DefaultImageAnimated, outputMetadata.DefaultImageAnimated);
+        Assert.Equal(originalMetadata.AnimateRootFrame, outputMetadata.AnimateRootFrame);
 
         for (int i = 0; i < image.Frames.Count; i++)
         {
diff --git a/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs b/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
index 8308935d0..225e4deef 100644
--- a/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs
@@ -33,7 +33,7 @@ public class PngMetadataTests
             Gamma = 2,
             TextData = new List<PngTextData> { new PngTextData("name", "value", "foo", "bar") },
             RepeatCount = 123,
-            DefaultImageAnimated = false
+            AnimateRootFrame = false
         };
 
         PngMetadata clone = (PngMetadata)meta.DeepClone();
@@ -45,7 +45,7 @@ public class PngMetadataTests
         Assert.False(meta.TextData.Equals(clone.TextData));
         Assert.True(meta.TextData.SequenceEqual(clone.TextData));
         Assert.True(meta.RepeatCount == clone.RepeatCount);
-        Assert.True(meta.DefaultImageAnimated == clone.DefaultImageAnimated);
+        Assert.True(meta.AnimateRootFrame == clone.AnimateRootFrame);
 
         clone.BitDepth = PngBitDepth.Bit2;
         clone.ColorType = PngColorType.Palette;
@@ -153,7 +153,7 @@ public class PngMetadataTests
     {
         using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
         PngMetadata meta = image.Metadata.GetFormatMetadata(PngFormat.Instance);
-        Assert.False(meta.DefaultImageAnimated);
+        Assert.False(meta.AnimateRootFrame);
     }
 
     [Theory]
@@ -163,7 +163,7 @@ public class PngMetadataTests
     {
         using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
         PngMetadata meta = image.Metadata.GetFormatMetadata(PngFormat.Instance);
-        Assert.True(meta.DefaultImageAnimated);
+        Assert.True(meta.AnimateRootFrame);
     }
 
     [Theory]