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Decode LZW row by row
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@JimBobSquarePants
JimBobSquarePants committed Jun 27, 2024
1 parent 9dda64a commit e4ba5d3
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Showing 9 changed files with 266 additions and 289 deletions.
196 changes: 95 additions & 101 deletions src/ImageSharp/Formats/Gif/GifDecoderCore.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -427,68 +427,49 @@ private void ReadFrame<TPixel>(BufferedReadStream stream, ref Image<TPixel>? ima
{
this.ReadImageDescriptor(stream);

Buffer2D<byte>? indices = null;
try
{
// Determine the color table for this frame. If there is a local one, use it otherwise use the global color table.
bool hasLocalColorTable = this.imageDescriptor.LocalColorTableFlag;

if (hasLocalColorTable)
{
// Read and store the local color table. We allocate the maximum possible size and slice to match.
int length = this.currentLocalColorTableSize = this.imageDescriptor.LocalColorTableSize * 3;
this.currentLocalColorTable ??= this.configuration.MemoryAllocator.Allocate<byte>(768, AllocationOptions.Clean);
stream.Read(this.currentLocalColorTable.GetSpan()[..length]);
}

indices = this.configuration.MemoryAllocator.Allocate2D<byte>(this.imageDescriptor.Width, this.imageDescriptor.Height, AllocationOptions.Clean);
this.ReadFrameIndices(stream, indices);
// Determine the color table for this frame. If there is a local one, use it otherwise use the global color table.
bool hasLocalColorTable = this.imageDescriptor.LocalColorTableFlag;

Span<byte> rawColorTable = default;
if (hasLocalColorTable)
{
rawColorTable = this.currentLocalColorTable!.GetSpan()[..this.currentLocalColorTableSize];
}
else if (this.globalColorTable != null)
{
rawColorTable = this.globalColorTable.GetSpan();
}

ReadOnlySpan<Rgb24> colorTable = MemoryMarshal.Cast<byte, Rgb24>(rawColorTable);
this.ReadFrameColors(ref image, ref previousFrame, indices, colorTable, this.imageDescriptor);
if (hasLocalColorTable)
{
// Read and store the local color table. We allocate the maximum possible size and slice to match.
int length = this.currentLocalColorTableSize = this.imageDescriptor.LocalColorTableSize * 3;
this.currentLocalColorTable ??= this.configuration.MemoryAllocator.Allocate<byte>(768, AllocationOptions.Clean);
stream.Read(this.currentLocalColorTable.GetSpan()[..length]);
}

// Skip any remaining blocks
SkipBlock(stream);
Span<byte> rawColorTable = default;
if (hasLocalColorTable)
{
rawColorTable = this.currentLocalColorTable!.GetSpan()[..this.currentLocalColorTableSize];
}
finally
else if (this.globalColorTable != null)
{
indices?.Dispose();
rawColorTable = this.globalColorTable.GetSpan();
}
}

/// <summary>
/// Reads the frame indices marking the color to use for each pixel.
/// </summary>
/// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
/// <param name="indices">The 2D pixel buffer to write to.</param>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private void ReadFrameIndices(BufferedReadStream stream, Buffer2D<byte> indices)
{
int minCodeSize = stream.ReadByte();
using LzwDecoder lzwDecoder = new(this.configuration.MemoryAllocator, stream);
lzwDecoder.DecodePixels(minCodeSize, indices);
ReadOnlySpan<Rgb24> colorTable = MemoryMarshal.Cast<byte, Rgb24>(rawColorTable);
this.ReadFrameColors(stream, ref image, ref previousFrame, colorTable, this.imageDescriptor);

// Skip any remaining blocks
SkipBlock(stream);
}

/// <summary>
/// Reads the frames colors, mapping indices to colors.
/// </summary>
/// <typeparam name="TPixel">The pixel format.</typeparam>
/// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
/// <param name="image">The image to decode the information to.</param>
/// <param name="previousFrame">The previous frame.</param>
/// <param name="indices">The indexed pixels.</param>
/// <param name="colorTable">The color table containing the available colors.</param>
/// <param name="descriptor">The <see cref="GifImageDescriptor"/></param>
private void ReadFrameColors<TPixel>(ref Image<TPixel>? image, ref ImageFrame<TPixel>? previousFrame, Buffer2D<byte> indices, ReadOnlySpan<Rgb24> colorTable, in GifImageDescriptor descriptor)
private void ReadFrameColors<TPixel>(
BufferedReadStream stream,
ref Image<TPixel>? image,
ref ImageFrame<TPixel>? previousFrame,
ReadOnlySpan<Rgb24> colorTable,
in GifImageDescriptor descriptor)
where TPixel : unmanaged, IPixel<TPixel>
{
int imageWidth = this.logicalScreenDescriptor.Width;
Expand Down Expand Up @@ -549,73 +530,83 @@ private void ReadFrameColors<TPixel>(ref Image<TPixel>? image, ref ImageFrame<TP
byte transIndex = this.graphicsControlExtension.TransparencyIndex;
int colorTableMaxIdx = colorTable.Length - 1;

for (int y = descriptorTop; y < descriptorBottom && y < imageHeight; y++)
// For a properly encoded gif the descriptor dimensions will never exceed the logical screen dimensions.
// However we have images that exceed this that can be decoded by other libraries. #1530
using IMemoryOwner<byte> indicesRowOwner = this.memoryAllocator.Allocate<byte>(descriptor.Width);
Span<byte> indicesRow = indicesRowOwner.Memory.Span;
ref byte indicesRowRef = ref MemoryMarshal.GetReference(indicesRow);

int minCodeSize = stream.ReadByte();
if (LzwDecoder.IsValidMinCodeSize(minCodeSize))
{
ref byte indicesRowRef = ref MemoryMarshal.GetReference(indices.DangerousGetRowSpan(y - descriptorTop));
using LzwDecoder lzwDecoder = new(this.configuration.MemoryAllocator, stream, minCodeSize);

// Check if this image is interlaced.
int writeY; // the target y offset to write to
if (descriptor.InterlaceFlag)
for (int y = descriptorTop; y < descriptorBottom && y < imageHeight; y++)
{
// If so then we read lines at predetermined offsets.
// When an entire image height worth of offset lines has been read we consider this a pass.
// With each pass the number of offset lines changes and the starting line changes.
if (interlaceY >= descriptor.Height)
// Check if this image is interlaced.
int writeY; // the target y offset to write to
if (descriptor.InterlaceFlag)
{
interlacePass++;
switch (interlacePass)
// If so then we read lines at predetermined offsets.
// When an entire image height worth of offset lines has been read we consider this a pass.
// With each pass the number of offset lines changes and the starting line changes.
if (interlaceY >= descriptor.Height)
{
case 1:
interlaceY = 4;
break;
case 2:
interlaceY = 2;
interlaceIncrement = 4;
break;
case 3:
interlaceY = 1;
interlaceIncrement = 2;
break;
interlacePass++;
switch (interlacePass)
{
case 1:
interlaceY = 4;
break;
case 2:
interlaceY = 2;
interlaceIncrement = 4;
break;
case 3:
interlaceY = 1;
interlaceIncrement = 2;
break;
}
}
}

writeY = interlaceY + descriptor.Top;
interlaceY += interlaceIncrement;
}
else
{
writeY = y;
}
writeY = Math.Min(interlaceY + descriptor.Top, image.Height);
interlaceY += interlaceIncrement;
}
else
{
writeY = y;
}

ref TPixel rowRef = ref MemoryMarshal.GetReference(imageFrame.PixelBuffer.DangerousGetRowSpan(writeY));
lzwDecoder.DecodePixelRow(indicesRow);
ref TPixel rowRef = ref MemoryMarshal.GetReference(imageFrame.PixelBuffer.DangerousGetRowSpan(writeY));

if (!transFlag)
{
// #403 The left + width value can be larger than the image width
for (int x = descriptorLeft; x < descriptorRight && x < imageWidth; x++)
if (!transFlag)
{
int index = Numerics.Clamp(Unsafe.Add(ref indicesRowRef, (uint)(x - descriptorLeft)), 0, colorTableMaxIdx);
ref TPixel pixel = ref Unsafe.Add(ref rowRef, (uint)x);
Rgb24 rgb = colorTable[index];
pixel.FromRgb24(rgb);
// #403 The left + width value can be larger than the image width
for (int x = descriptorLeft; x < descriptorRight && x < imageWidth; x++)
{
int index = Numerics.Clamp(Unsafe.Add(ref indicesRowRef, (uint)(x - descriptorLeft)), 0, colorTableMaxIdx);
ref TPixel pixel = ref Unsafe.Add(ref rowRef, (uint)x);
Rgb24 rgb = colorTable[index];
pixel.FromRgb24(rgb);
}
}
}
else
{
for (int x = descriptorLeft; x < descriptorRight && x < imageWidth; x++)
else
{
int rawIndex = Unsafe.Add(ref indicesRowRef, (uint)(x - descriptorLeft));

// Treat any out of bounds values as transparent.
if (rawIndex > colorTableMaxIdx || rawIndex == transIndex)
for (int x = descriptorLeft; x < descriptorRight && x < imageWidth; x++)
{
continue;
}
int index = Unsafe.Add(ref indicesRowRef, (uint)(x - descriptorLeft));

// Treat any out of bounds values as transparent.
if (index > colorTableMaxIdx || index == transIndex)
{
continue;
}

int index = Numerics.Clamp(rawIndex, 0, colorTableMaxIdx);
ref TPixel pixel = ref Unsafe.Add(ref rowRef, (uint)x);
Rgb24 rgb = colorTable[index];
pixel.FromRgb24(rgb);
ref TPixel pixel = ref Unsafe.Add(ref rowRef, (uint)x);
Rgb24 rgb = colorTable[index];
pixel.FromRgb24(rgb);
}
}
}
}
Expand Down Expand Up @@ -656,8 +647,11 @@ private void ReadFrameMetadata(BufferedReadStream stream, List<ImageFrameMetadat
// Skip the frame indices. Pixels length + mincode size.
// The gif format does not tell us the length of the compressed data beforehand.
int minCodeSize = stream.ReadByte();
using LzwDecoder lzwDecoder = new(this.configuration.MemoryAllocator, stream);
lzwDecoder.SkipIndices(minCodeSize, this.imageDescriptor.Width * this.imageDescriptor.Height);
if (LzwDecoder.IsValidMinCodeSize(minCodeSize))
{
using LzwDecoder lzwDecoder = new(this.configuration.MemoryAllocator, stream, minCodeSize);
lzwDecoder.SkipIndices(this.imageDescriptor.Width * this.imageDescriptor.Height);
}

ImageFrameMetadata currentFrame = new();
frameMetadata.Add(currentFrame);
Expand Down
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