This program is part of Netpbm.
pbmtolps reads a PBM image as input and outputs PostScript. The output Postscript uses lines instead of the image operator to generate a (device dependent) picture which will be imaged much faster.
pnmtops is a more general program for converting from the Netpbm formats to Postscript, though it cannot produce the particular arcane subformat that pbmtolps does.
The Postscript path length is constrained to be less that 1000 points so that no limits are overrun on the Apple Laserwriter and (presumably) no other printers.