convert a portable pixmap into a portable graymap
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.
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.
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.
1989 by Jef Poskanzer.