> Is there an image format that's simpler than PPM and that supports > alpha transparency?
If you want a format that works with existing tools, TGA can be simple. Just don't use run-length encoding or indexed colour or extension information (colour correction table, gamma field, premultiplied alpha flag, pixel aspect ratio). # write a sample TGA image: printf '\x00' # no image ID field printf '\x00' # no color map printf '\x02' # unmapped, uncompressed RGB printf '\x00\x00\x00\x00\x00' # color map info (ignored) printf '\x00\x00' # x origin printf '\x00\x00' # y origin printf '\x10\x00' # width printf '\x10\x00' # height printf '\x20' # bits per pixel printf '\x28' # 8 alpha bits per pixel, origin top left (0x20), no interleaving # origin bottom left (0x00) is common. for i in 0 1 2 3 4 5 6 7 8 9 a b c d e f; do for j in 0 1 2 3 4 5 6 7 8 9 a b c d e f; do printf "\xff\x00\x00\x$i$j" # BGRA - bright blue, varying alpha done done # optional footer: # printf '\x00\x00\x00\x00' # no extension area # printf '\x00\x00\x00\x00' # no developer directory # printf 'TRUEVISION-XFILE.\x00'