Acro

May be a bit late, but a useful circumstance would be when you have a light background. At least being able to change the color to black would be nice.

I can't say whether or not your first idea is possible, but I think it would be better and maybe easier to either: 1. Add an option to simply toggle...

That would be my preferred solution, but if you wanted to go with 2, you could make the level of opacity configurable(not just completely transparent) by adding a slider or...

image-rs/image#1765 should no longer be an issue. The image crate should now support jxl via the jxl-oxide crate. Please see https://github.com/image-rs/image/pull/2372 and https://github.com/tirr-c/jxl-oxide/pull/480.