pgmtoppm colorspec [pgmfile] pgmtoppm colorspec1-colorspec2 [pgmfile]
pgmtoppm -map=mapfile [pgmfile]

Minimum unique abbreviation of option is acceptable. You may use dou-
ble hyphens instead of single hyphen to denote options. You may use
white space in place of the equals sign to separate an option name
from its value.

This program is part of Netpbm.

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 inten-
sities of the specified color.

If you specify two color arguments (separated by a hyphen), 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_parsecolor() 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 in between. The maxval of the output image is the maxval of
the map image.

A more direct way to specify a particular color to replace each par-
ticular gray level is to use pamlookup. You make an index file that
explicitly associates a color with each possible gray level.

When you don't use -map, 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 pamdepth 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, pamdepth 16
would do it. Or spare yourself the unnecessary thinking and just say
pamdepth 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 nec-
essary to represent a PBM image, pgmtoppm, like all Netpbm programs,
in fact treats it as a maxval of 255.

pamdepth, rgb3toppm, ppmtopgm, ppmtorgb3, ppm, pgm

Copyright (C) 1991 by Jef Poskanzer.

netpbm documentation 10 December 2006 Pgmtoppm User Manual(0)