This program is part of Netpbm.
This program is obsolete since Netpbm 10.23 (July 2004). Use pamditherbw to do what this program used to do.
pgmtopbm never was the simple converter it appeared to be. It was a dithering program. Unfortunately, it didn't do the dithering properly because it treated the PGM input samples as if they were directly proportional to light intensity, but they are actually gamma-adjusted.
pamditherbw is backward compatible with pgmtopbm except that it does the correct gamma adjustments.