Convert a PPM file to an HP PaintJet file. More information: https://netpbm.sourceforge.net/doc/ppmtopj.html.
ppmtopj path/to/input.ppm > path/to/output.pj
ppmtopj -xpos dx -ypos dy path/to/input.ppm > path/to/output.pj
ppmtopj -gamma gamma path/to/input.ppm > path/to/output.pj