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Fix PNG decoding of empty data chunks and short palettes, and use big-endian 16-bit tRNS #3228

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Fix PNG decoding of empty data chunks and short palettes, and use big-endian 16-bit tRNS #3228

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78 changes: 47 additions & 31 deletions src/ImageSharp/Formats/Png/PngDecoderCore.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1338,7 +1338,9 @@ private void ProcessInterlacedDefilteredScanline<TPixel>(
/// <param name="pngMetadata">The png metadata.</param>
private static void AssignColorPalette(ReadOnlySpan<byte> palette, ReadOnlySpan<byte> alpha, PngMetadata pngMetadata)
{
if (palette.Length == 0)
// A palette chunk too short to hold a single entry would produce an empty color table, which the
// scanline processors would index outside of. Treat it as missing so indexed decoding reports it.
if (palette.Length < Unsafe.SizeOf<Rgb24>())
{
return;
}
Expand Down Expand Up @@ -1380,9 +1382,11 @@ private void AssignTransparentMarkers(ReadOnlySpan<byte> alpha, PngMetadata pngM
{
if (this.header.BitDepth == 16)
{
ushort rc = BinaryPrimitives.ReadUInt16LittleEndian(alpha[..2]);
ushort gc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(2, 2));
ushort bc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(4, 2));
// tRNS samples are stored big-endian like all other PNG multi-byte values,
// matching the byte order used for the 16-bit samples in the image data.
ushort rc = BinaryPrimitives.ReadUInt16BigEndian(alpha[..2]);
ushort gc = BinaryPrimitives.ReadUInt16BigEndian(alpha.Slice(2, 2));
ushort bc = BinaryPrimitives.ReadUInt16BigEndian(alpha.Slice(4, 2));

pngMetadata.TransparentColor = Color.FromPixel(new Rgb48(rc, gc, bc));
return;
Expand All @@ -1400,7 +1404,7 @@ private void AssignTransparentMarkers(ReadOnlySpan<byte> alpha, PngMetadata pngM
{
if (this.header.BitDepth == 16)
{
pngMetadata.TransparentColor = Color.FromPixel(new L16(BinaryPrimitives.ReadUInt16LittleEndian(alpha[..2])));
pngMetadata.TransparentColor = Color.FromPixel(new L16(BinaryPrimitives.ReadUInt16BigEndian(alpha[..2])));
}
else
{
Expand Down Expand Up @@ -2183,24 +2187,30 @@ private int ReadNextDataChunk()

Span<byte> buffer = stackalloc byte[20];

int length = this.currentStream.Read(buffer, 0, 4);
if (length == 0)
// The compressed stream is the concatenation of the payloads of all consecutive data chunks, and the
// specification permits zero-length data chunks. A return value of 0 tells the inflater that the
// compressed stream has ended, so empty chunks are skipped rather than reported.
while (true)
{
return 0;
}
// Skip the CRC of the previous chunk.
int length = this.currentStream.Read(buffer, 0, 4);
if (length == 0 || !this.TryReadChunk(buffer, out PngChunk chunk))
{
return 0;
}

if (this.TryReadChunk(buffer, out PngChunk chunk))
{
if (chunk.Type is PngChunkType.Data or PngChunkType.FrameData)
if (chunk.Type is not (PngChunkType.Data or PngChunkType.FrameData))
{
chunk.Data?.Dispose();
return chunk.Length;
this.nextChunk = chunk;
return 0;
}

this.nextChunk = chunk;
chunk.Data?.Dispose();
if (chunk.Length > 0)
{
return chunk.Length;
}
}

return 0;
}

/// <summary>
Expand All @@ -2216,26 +2226,32 @@ private int ReadNextFrameDataChunk()

Span<byte> buffer = stackalloc byte[20];

int length = this.currentStream.Read(buffer, 0, 4);
if (length == 0)
{
return 0;
}

if (this.TryReadChunk(buffer, out PngChunk chunk))
// As with data chunks, frame data chunks with no payload after the sequence number are valid and are
// skipped, so that a return value of 0 only ever means that the frame's compressed stream has ended.
while (true)
{
if (chunk.Type is PngChunkType.FrameData)
// Skip the CRC of the previous chunk.
int length = this.currentStream.Read(buffer, 0, 4);
if (length == 0 || !this.TryReadChunk(buffer, out PngChunk chunk))
{
chunk.Data?.Dispose();
return 0;
}

this.currentStream.Position += 4; // Skip sequence number
return chunk.Length - 4;
if (chunk.Type is not PngChunkType.FrameData)
{
this.nextChunk = chunk;
return 0;
}

this.nextChunk = chunk;
}
chunk.Data?.Dispose();

return 0;
this.currentStream.Position += 4; // Skip sequence number
int dataLength = chunk.Length - 4;
if (dataLength > 0)
{
return dataLength;
}
}
}

/// <summary>
Expand Down
9 changes: 5 additions & 4 deletions src/ImageSharp/Formats/Png/PngEncoderCore.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1247,10 +1247,11 @@ private void WriteTransparencyChunk(Stream stream, PngMetadata pngMetadata)
{
if (this.use16Bit)
{
// tRNS samples are stored big-endian like all other PNG multi-byte values.
Rgb48 rgb = pngMetadata.TransparentColor.Value.ToPixel<Rgb48>();
BinaryPrimitives.WriteUInt16LittleEndian(alpha, rgb.R);
BinaryPrimitives.WriteUInt16LittleEndian(alpha.Slice(2, 2), rgb.G);
BinaryPrimitives.WriteUInt16LittleEndian(alpha.Slice(4, 2), rgb.B);
BinaryPrimitives.WriteUInt16BigEndian(alpha, rgb.R);
BinaryPrimitives.WriteUInt16BigEndian(alpha.Slice(2, 2), rgb.G);
BinaryPrimitives.WriteUInt16BigEndian(alpha.Slice(4, 2), rgb.B);

this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer, 0, 6);
}
Expand All @@ -1269,7 +1270,7 @@ private void WriteTransparencyChunk(Stream stream, PngMetadata pngMetadata)
if (this.use16Bit)
{
L16 l16 = pngMetadata.TransparentColor.Value.ToPixel<L16>();
BinaryPrimitives.WriteUInt16LittleEndian(alpha, l16.PackedValue);
BinaryPrimitives.WriteUInt16BigEndian(alpha, l16.PackedValue);
this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer, 0, 2);
}
else
Expand Down
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