pnmtoddif --version (return code: 0)
pnmtoddif: Using libpbm from Netpbm Version: Netpbm 10.0
pnmtoddif: BSD defined
pnmtoddif: RGB_ENV='RGBDEF'
pnmtoddif: RGBENV= 'RGBDEF' (env vbl is unset)
pnmtoddif -h (return code: 1)
usage: pnmtoddif [-resolution x y] [pnmfile [ddiffile]]