pgmtoppm – colorize a portable graymap into a portable pixmap
pgmtoppm colorspec [pgmfile] pgmtoppm colorspec1-colorspec2 [pgmfile] pgmtoppm -map mapfile [pgmfile]
Reads a PGM as input. 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.
You can specify the color in one of five ways:
o A name, from an X11-style color names file.
o An X11-style hexadecimal specifier: rgb:r/g/b, where r g and b
are each 1- to 4-digit hexadecimal numbers.
o An X11-style decimal specifier: rgbi:r/g/b, where r g and b are
floating point numbers between 0 and 1.
o For backwards compatibility, an old-X11-style hexadecimal num‐
ber: #rgb, #rrggbb, #rrrgggbbb, or #rrrrggggbbbb.
o For backwards compatibility, a triplet of numbers separated by
commas: r,g,b, where r g and b are floating point numbers
between 0 and 1. (This style was added before MIT came up with
the similar rgbi style.)
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
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 (in fact, let’s say it’s a PBM
file, since pgmtoppm, like all Netpbm programs, can accept a PBM file
as if it were PGM. The maxval of this image is 1, because only two
gray values are needed: black and white. Run this image through pgm‐
toppm 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 .
Copyright (C) 1991 by Jef Poskanzer.
24 January 2001 pgmtoppm