Problem with new GIF/LZW optimizer

[AiZ]

Hello,

There's (again) a new optimizer for GIF files.
https://encode.ru/threads/3008-flexiGIF-lossless-GIF-LZW-optimization
https://create.stephan-brumme.com/flexigif-lossless-gif-lzw-optimization/
GIF files generated with gifski v0.8.5 are problematic with this tool while v0.8.2 and below encodes are OK (took 10 png frames from Big Buck Bunny for the test).
Would you mind participate to the discussion on encode.ru forum?

Thanks in advance,

AiZ

[stbrumme]

As you know, LZW data is split in blocks with at most 255 bytes each.
Unused bits of the last byte of a frame have to be filled with dummy data. However, there are 8 unused bits in the 7th frame, so the entire last byte is redundant. (gifski 0.8.5)

Below is the animation (first 10 frames of Big Bunny are black or almost black, so nothing interesting to see ...)

[kornelski]

gifski encodes GIFs using Piston's encoder, so LZW bugs are best filled there: https://github.com/PistonDevelopers/image

[stbrumme]

I added a workaround in my code for that situation (display an error message, skip the unused/redundant byte and continue).
FYI: I never saw 9 or more unused bits, so it seems to be a typical off-by-one issue.