Merge branch 'release/3.1.x' into af/memlimit-01

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;
+        }
+    }
 }

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(
@@ -240,11 +239,16 @@ 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:
-
+                            pngMetadata.AnimateRootFrame = currentFrameControl != null;
                             currentFrameControl ??= new((uint)this.header.Width, (uint)this.header.Height);
                             if (image is null)
                             {
@@ -261,9 +265,12 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                                 this.ReadNextDataChunk,
                                 currentFrameControl.Value,
                                 cancellationToken);
+                            if (pngMetadata.AnimateRootFrame)
+                            {
+                                previousFrame = currentFrame;
+                                previousFrameControl = currentFrameControl;
+                            }
 
-                            previousFrame = currentFrame;
-                            previousFrameControl = currentFrameControl;
                             break;
                         case PngChunkType.Palette:
                             this.palette = chunk.Data.GetSpan().ToArray();
@@ -632,7 +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,
@@ -645,12 +652,16 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
         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))
+        // 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();
+            pixelRegion.Clear();
+        }
+        else if (previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToBackground)
         {
-            Rectangle restoreArea = previousFrameControl.Bounds;
-            Rectangle interest = Rectangle.Intersect(frame.Bounds(), restoreArea);
-            Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(interest);
+            Rectangle restoreArea = previousFrameControl.Value.Bounds;
+            Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(restoreArea);
             pixelRegion.Clear();
         }
 

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)
@@ -189,29 +190,50 @@ 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.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.AnimateRootFrame || 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.AnimateRootFrame)
+            {
+                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);
 

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.AnimateRootFrame = other.AnimateRootFrame;
 
         if (other.ColorTable?.Length > 0)
         {
@@ -83,6 +84,11 @@ public class PngMetadata : IDeepCloneable
     /// </summary>
     public uint RepeatCount { get; set; } = 1;
 
+    /// <summary>
+    ///  Gets or sets a value indicating whether the root frame is shown as part of the animated sequence
+    /// </summary>
+    public bool AnimateRootFrame { get; set; } = true;
+
     /// <inheritdoc/>
     public IDeepCloneable DeepClone() => new PngMetadata(this);
 

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());

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;
     }
 }

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;

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;

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/>

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;

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.

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);
         }
     }
 }

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

@@ -87,7 +87,9 @@ public partial class PngDecoderTests
         TestImages.Png.DisposeBackgroundRegion,
         TestImages.Png.DisposePreviousFirst,
         TestImages.Png.DisposeBackgroundBeforeRegion,
-        TestImages.Png.BlendOverMultiple
+        TestImages.Png.BlendOverMultiple,
+        TestImages.Png.FrameOffset,
+        TestImages.Png.DefaultNotAnimated
     };
 
     [Theory]

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.AnimateRootFrame ? 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)]

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.AnimateRootFrame, outputMetadata.AnimateRootFrame);
 
         for (int i = 0; i < image.Frames.Count; i++)
         {

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,
+            AnimateRootFrame = 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.AnimateRootFrame == clone.AnimateRootFrame);
 
         clone.BitDepth = PngBitDepth.Bit2;
         clone.ColorType = PngColorType.Palette;
@@ -144,6 +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.AnimateRootFrame);
+    }
+
+    [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.AnimateRootFrame);
+    }
+
     [Theory]
     [WithFile(TestImages.Png.PngWithMetadata, PixelTypes.Rgba32)]
     public void Decode_IgnoresExifData_WhenIgnoreMetadataIsTrue<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -73,6 +73,8 @@ 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 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

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

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

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:b85aaf7153e0ca538856a58d7b069bcc13fadc468ea603c85f8782cc691f86c3
+size 387

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:fcb83d6893dcfd869b764ff9846c259eaa0caf26cec3f0fc2cbae2c26f2eeaa5
+size 660

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 sha256:562ec382f6d2af68e66092bf6949f66147d5f608d3c618eea5a7c1ea400737ff
+size 768

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:d12a7791b960072e32b78bd9aaf456dc99341eea1c66ea05050433d8c082c6ac
+size 579

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 sha256:2db38d7ffcc95c23a5c94a06f10c6cc67406ae581a955c99ede4af97b1a044f8
+size 628

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

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