This program is part of Netpbm(1).
pnmtopnm simply copies a PNM image to Standard Output. The
output has the same major PNM format (PBM, PGM, or PPM) and maxval as
the input. This may seem an unnecessary duplication of cat,
but it lets you convert between the plain (ASCII) and raw (binary)
subformats of PNM. Use the -plain Netpbm common option to
ensure the output is plain PNM, and don't use -plain to ensure
the output is raw PNM. See
Common Options .
You don't normally need to convert between the PNM subformats, because any program that uses the Netpbm library to read a PNM image will read all of them directly. But there are a lot of programs that don't use the Netpbm library and understand only the raw format. Plain format is nice because it is human readable; people often use it to debug programs that process PNM images.
pnmtopnm is really just another name for the program pamtopnm. The latter does the job because like any Netpbm program that takes PAM input via the Netpbm programming library facilities, it also takes PNM input.
pnmtopnm was new in Netpbm 10.23 (July 2004). It obsoleted pnmtoplainpnm, which specifically did the conversion to plain PNM. There was no program to explicitly convert to raw PNM, but many Netpbm programs can be made, with the right options, to be idempotent (i.e. to do the same thing as pnmtopnm).
Then David Jones realized that the existing pamtopnm already did everything that pnmtopnm did and more, so in Netpbm 10.27 (March 2005), pnmtopnm became simply an alternate name for pamtopnm.