img2pdf?

[mara004]

Just wondering whether you are aware of img2pdf?
It's always lossless, supports alpha channels through SMasks, and can often avoid CPU-hungry re-encoding.
I think it's probably the best OSS tool in existence for this task (although I might have a few code style / design decision disagreements).

[stefan6419846]

Thanks for opening this issue.

I am aware of img2pdf and it is being used in the same project, but for some reasons (probably historical) this specific task had been implemented using Pillow back then. If I remember correctly, img2pdf would have its own limitations/issues, although I am not completely sure out of my head without digging further. (I just stumbled upon the required dependency on pikepdf, although it being optional internally.)

In this specific case, I needed a quick solution and focused on the Pillow issue linked in the README. In the future, this might change and I re-evaluate img2pdf for the corresponding use-cases, but for now I will probably stay with the current approach and this package.

[mara004]

Yeah, historically, img2pdf did not support alpha channels. It was contributed by Tamas Zahola some time ago.

Current limitations I'm aware of include lack of support for >8bpp TIFFs, and >8bpp PNGs with alpha channel. Due to issues in Pillow, these images would be implicitly truncated to 8 bpp, and the author strictly refuses the idea of adding an option to let them pass through lossily.

So if one needs to convert such an image and the quality loss is acceptable, falling back to Pillow (rsp. this derivative) is probably a good choice.