VP8X reserved bytes use Position += 3 instead of Write, causing 3-byte truncation in alpha WebP images #3035
Description
Prerequisites
- I have written a descriptive issue title
- I have verified that I am running the latest version of ImageSharp
- I have verified if the problem exist in both
DEBUGandRELEASEmode - I have searched open and closed issues to ensure it has not already been reported
ImageSharp version
3.1.12
Other ImageSharp packages and versions
Seems to have been introduced around v3.1.0
Environment (Operating system, version and so on)
Windows and Linux
.NET Framework version
net8
Description
Some webp images with an alpha channel result in a broken image. I've included an example of an image as well as the resulting encoded result.
This was the result of using the following encoder:
new WebpEncoder {
Quality = 70,
TransparentColorMode = TransparentColorMode.Preserve
}
The issue appears to be here: https://github.com/SixLabors/ImageSharp/blob/main/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs#L126
specifically
stream.Position += 3; // Reserved bytes
The stream position is being advanced 3 bytes but the stream is not actually being written to. This appears to result in the RIFF size and the corresponding VP8 chunk to be 3 bytes too large.
Changing this stream.Position += 3; to instead write zeros to advance the stream
Span<byte> reserved = stackalloc byte[3];
stream.Write(reserved);
seems to resolve the issue.
I am currently running a private fork for this to get us moving forward but would rather someone with more experience with webp review this finding.
Steps to Reproduce
Try to do a simple encoding of the original.webp provided that preserves the alpha channel.
Using something like
new WebpEncoder {
Quality = 70,
TransparentColorMode = TransparentColorMode.Preserve
}
Compare the results
original.webp (valid)
| Property | Value |
|---|---|
| File size | 9484 bytes |
| Declared size | 9476 + 8 = 9484 bytes |
| VP8X chunk | 10 bytes (metadata) |
| ALPH chunk | 3483 bytes (alpha/transparency) |
| VP8 chunk | 5954 bytes (image data) |
after-encoding.webp (corrupted)
| Property | Value |
|---|---|
| File size | 9371 bytes |
| Declared size | 9366 + 8 = 9374 bytes |
| VP8X chunk | 10 bytes |
| ALPH chunk | 3483 bytes |
| VP8 chunk | Declared 5844 bytes, but only 5841 bytes present |
Images
