Pgmtoppm User Manual

# pgmtoppm

Updated: 02 October 2021
Table Of Contents

## NAME

pgmtoppm - colorize a PGM (grayscale) image into a PPM (color) image

## SYNOPSIS

pgmtoppm [-black=colorspec1] [-white=colorspec2] [pgmfile] pgmtoppm -map=filename [pgmfile] pgmtoppm colorspec [pgmfile]
pgmtoppm colorspec1-colorspec2 [pgmfile]

Minimum unique abbreviation of option is acceptable. You may use double 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.

## DESCRIPTION

This program is part of Netpbm.

If all you want to do is convert PGM to PPM, keeping the same gray pixels, you may not need to. All Netpbm programs that expect PPM input also recognize PGM. And if you must have a PPM file, use ppmtoppm instead. It is more efficient and easier to use.

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.

You can specify the color in the output to which black in the input maps, and the color to which white maps. All the gray values in between map linearly (across a three dimensional space) to colors between the black and white colors you specify.

Use the -black and -white options for this. For historical reasons, you can alternatively use a non-option argument to specify the colors. If you do that, pgmtoppm interprets the color argument like this: if the argument takes the form black-white, it has the effect of -black=black -white=white If instead there is no hyphen in the color argument, it has the effect of -white=color_argument.

Because of the historical syntax, it is not possible to let both -black and -white default (but you shouldn't want to -- see below for advice on making such a null conversion).

You can alternatively specify an entire colormap with the -map option.

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

## OPTIONS

In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options), pgmtoppm recognizes the following command line options:

-black=colorspec
  The program maps black pixels in the input to this color in the output. The default is black.

  Specify the color (color) as described for the argument of the pnm_parsecolor() library routine.

  You cannot specify this together with -map.

  This option was new in Netpbm 10.97 (December 2021). Before that, use the color argument.

-white=colorspec
  The program maps white pixels in the input to this color in the output. The default is white.

  Specify the color (color) as described for the argument of the pnm_parsecolor() library routine.

  You cannot specify this together with -map.

  This option was new in Netpbm 10.97 (December 2021). Before that, use the color argument.

-map=filename
  This option specifies a complete mapping of gray values in the input to color values in the output. The map file (named filename) 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.

## NOTE - MAXVAL

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 pgmtoppm to increase its depth to something that would give you the resolution 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 necessary to represent a PBM image, pgmtoppm, like all Netpbm programs, in fact treats it as a maxval of 255.

## SEE ALSO

ppmtoppm, pamdepth, rgb3toppm, ppmtopgm, ppmtorgb3, ppm, pgm

## AUTHOR

Copyright (C) 1991 by Jef Poskanzer.

## Table Of Contents

- SYNOPSIS
- DESCRIPTION
- OPTIONS
- NOTE - MAXVAL
- SEE ALSO
- AUTHOR