Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen to denote options. You may use white space in place of the equals sign to separate an option name from its value.
This program is part of Netpbm.
pngtopnm was obsoleted by pngtopam, introduced with Netpbm 10.44 (September 2008). pngtopam is backward compatible with pngtopnm, plus adds many additional functions, including the ability to produce a PAM image that includes a transparency (alpha) channel. Starting in Release 10.48, pngtopnm is just an alias for pngtopam.
pngtopnm remained in the Netpbm package through Release 10.47 because it may have fewer bugs than pngtopam in those releases, and may be faster in some environments. But pngtopnm's incompatibility with the most current PNG libraries makes it impractical to maintain along with pngtopam now.
In releases before 10.48, you can use the pngtopam documentation for pngtopnm, considering the following differences: