This program is part of Netpbm.
This program is almost entirely 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
Although pgmtopbm is mostly obsolete, the simple threshold method (specified by -threshold) is an exception. Unlike the other conversion methods, gamma-adjustment is not an issue with simple thresholding and pgmtopbm can produce PBM output directly. Note that even in this basic mode of operation pgmtopbm may produce output which is slightly different from what pamditherbw does because of differences in internal calculation.
If you just want to convert a PGM image with maxval 1 to PBM, use pamtopnm.