A whole bunch of utilities for primitive manipulation of
graphic images. Wide array of converters
from one graphics format to another. E.g.
from g3 fax format to jpeg. Many basic graphics
editing tools such as magnifying and cropping.
It lacks a reverse filter tool: $ pnmrefilter origin.pnm filtered.pnm > refiltered.pnm (refiltered)=(origin) + [(origin)-(filtered)]