Netpbm (the spec) supports alpha. It's in the PAM format: https://netpbm.sourceforge.net/doc/pam.html https://netpbm.sourceforge.net/doc/pam.html I've never tr

Netpbm (the spec) supports alpha. It's in the PAM format: https://netpbm.sourceforge.net/doc/pam.html https://netpbm.sourceforge.net/doc/pam.html I've never tried that variant, nor do I think I've seen files in the wild, so IDK how widely stuff supports it. If I have need for an alpha channel, I usually reach for a PNG encoder at that point…