This program is part of Netpbm.
ppmtorgb3 reads a PPM image as input and writes three PGM images as output, one each for red, green, and blue.
ppmtorgb3 constructs the output filenames by taking the input filename, stripping off any extension, and appending .red, .grn, .blu. For example, separating lenna.ppm would result in lenna.red, lenna.grn, and lenna.blu. If the input comes from stdin, the names are noname.red, noname.grn, and noname.blu.
There are no command line options defined specifically for ppmtorgb3, but it recognizes the options common to all programs based on libnetpbm (See Common Options.)