Pgmtoppm User Manual(0) Pgmtoppm User Manual(0)
pgmtoppm - colorize a PGM (grayscale) image into a PPM (color) image
[pgmfile] pgmtoppm -map
This program is part of Netpbm(1).
pgmtoppm reads a PGM as input and produces a PPM file as output with a
specific color assigned to each gray value in the input.
If you specify one color argument, black in the pgm file stays black
and white in the pgm file turns into the specified color in the ppm
file. Gray values in between are linearly mapped to differing intensi-
ties of the specified color.
If you specify two color arguments (separated by a dash), then black
gets mapped to the first color and white gets mapped to the second and
gray values in between get mapped linearly (across a three dimensional
space) to colors in between.
Specify the color (color) as described for the argument of the ppm_par-
secolor() library routine .
Also, you can specify an entire colormap with the -map option. The
mapfile is just a ppm file; it can be any shape, all that matters is
the colors in it and their order. In this case, black gets mapped into
the first color in the map file, and white gets mapped to the last and
gray values in between are mapped linearly onto the sequence of colors
A more direct way to specify a particular color to replace each partic-
ular gray level is to use pamlookup. You make an index file that
explicitly associates a color with each possible gray level.
NOTE - MAXVAL
The ’maxval,’ or depth, of the output image is the same as that of the
input image. The maxval affects the color resolution, which may cause
quantization errors you don’t anticipate in your output. For example,
you have a simple black and white image as a PGM with maxval 1. Run
this image through pgmtoppm 0f/00/00 to try to make the image black and
faint red. Because the output image will also have maxval 1, there is
no such thing as faint red. It has to be either full-on red or black.
pgmtoppm rounds the color 0f/00/00 down to black, and you get an output
image that is nothing but black.
The fix is easy: Pass the input through pnmdepth on the way into pgm-
toppm to increase its depth to something that would give you the reso-
lution you need to get your desired color. In this case, pnmdepth 16
would do it. Or spare yourself the unnecessary thinking and just say
pnmdepth 255 .
PBM input is a special case. While you might think this would be
equivalent to a PGM with maxval 1 since only two gray levels are neces-
sary to represent a PBM image, pgmtoppm, like all Netpbm programs, in
fact treats it as a maxval of 255.
pnmdepth(1), rgb3toppm(1), ppmtopgm(1), ppmtorgb3(1), ppm(1), pgm(1)
Copyright (C) 1991 by Jef Poskanzer.
netpbm documentation 24 January 2001 Pgmtoppm User Manual(0)
Man(1) output converted with