ppmtopgm
Convert color PPM images to grayscale
TLDR
Convert color to grayscale
$ ppmtopgm [input.ppm] > [output.pgm]
SYNOPSIS
ppmtopgm [file]
DESCRIPTION
ppmtopgm converts PPM color images to PGM grayscale. Uses standard luminance formula to compute gray values. Part of Netpbm toolkit.