I recently came across some assets of old games, back from when PNG images don't get an alpha channel, and assets must be provided with another greyscale image, white as opaque, black as full transparent, and grey as semi-transparent.
While editing them with C++ file write could get the job done, that's a last resort considering the efforts involved.
I'd like to load and process them to be a render-ready texture, or alternatively somehow process them to be before loading. Pure SDL would be great, as I'm not quite yet ready to pull my own rendering implement. But if that's not possible/very inelegant to, any neat workaround like an image processing library/software is good too.