This program is part of Netpbm.

ppmtoppm simply copies a PPM image from Standard Input to Standard
Output. This may seem an unnecessary duplication of cat, but remember
that a PPM program can read a PBM or PGM image, and the right kind of
PAM, as if it were PPM. So ppmtoppm can read either a PBM, PGM, or
PPM image and produce a PPM image as output.

Even that is of limited usefulness because of the fact that almost any
program to which you would feed the resulting PPM image could also
just take the original image directly. However, sometimes you really
need a true PPM image.

When you know you have a PGM image and want a PPM image, pgmtoppm is a
more general way to do that conversion. When you know you have a PBM
image, use that and pbmtopgm.

pgmtopgm, pamtopam, pamtopnm, pgmtoppm, pbmtopgm,
ppm, ppm, ppm,

This program was added to Netpbm in Release 10.9 (September 2002).

netpbm documentation Feburary 2007 Ppmtoppm User Manual(0)