pnmconvol --version (return code: 0)
pnmconvol: Using libpbm from Netpbm Version: Netpbm 10.0
pnmconvol: BSD defined
pnmconvol: RGB_ENV='RGBDEF'
pnmconvol: RGBENV= 'RGBDEF' (env vbl is unset)
pnmconvol (return code: 1)
usage: pnmconvol <convolutionfile> [pnmfile]