ppmdither --version (return code: 0)
ppmdither: Using libpbm from Netpbm Version: Netpbm 10.0
ppmdither: BSD defined
ppmdither: RGB_ENV='RGBDEF'
ppmdither: RGBENV= 'RGBDEF' (env vbl is unset)
ppmdither -h (return code: 1)
usage: ppmdither [-dim <num>] [-red <num>] [-green <num>] [-blue <num>] [ppmfile]