ppmtopgm (1) Linux Manual Page
ppmtopgm – convert a portable pixmap into a portable graymap
Synopsis
ppmtopgm [ppmfile]Description
Reads a portable pixmap as input. Produces a portable graymap as output. The output is a "black and white" rendering of the original image, as in a black and white photograph. The quantization formula used is .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.
Quote
Cold-hearted orb that rules the night
Removes the colors from our sight
Red is gray, and yellow white
But we decide which is right
And which is a quantization error.