This program is part of
Netpbm(1).
ppmtopgm reads a PPM as input and produces a PGM as output.
The output is a 'black and white' rendering of the original
image, as in a black and white photograph. The quantization formula
ppmtopgm uses is g = .299 r + .587 g + .114 b.
Note that although there is a pgmtoppm program, it is not
necessary for simple conversions from pgm to ppm , because any ppm
program can read pgm (and pbm ) files automatically. pgmtoppm
is for colorizing a pgm file. Also, see ppmtorgb3 for a
different way of converting color to gray. And ppmdist
generates a grayscale image from a color image, but in a way that
makes it easy to differentiate the original colors, not necessarily a
way that looks like a black and white photograph.