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Pgmtoppm User Manual
.SH 1
pgmtoppm
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Updated: 24 January 2001
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Table Of Contents
.SH 2
NAME
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pgmtoppm - colorize a PGM (grayscale) image into a PPM (color) image
.SH 2
SYNOPSIS
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\fBpgmtoppm\fR
\fIcolorspec\fR
[\fIpgmfile\fR]
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\fBpgmtoppm\fR
\fIcolorspec1\fR\fB-\fR\fIcolorspec2\fR
[\fIpgmfile\fR]
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\fBpgmtoppm -map\fR
\fImapfile\fR
[\fIpgmfile\fR]
.SH 2
DESCRIPTION
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This program is part of Netpbm.
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\fBpgmtoppm\fR reads a PGM as input and produces a PPM file as
output with a specific color assigned to each gray value in the input.
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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
intensities of the specified color.
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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.
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Specify the color (\fIcolor\fR) as described for the argument of the \fBppm_parsecolor()\fR
library routine.
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Also, you can specify an entire colormap with the \fB-map\fR
option.  The mapfile is just a \fBppm\fR 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.
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A more direct way to specify a particular color to replace each
particular gray level is to use \fBpamlookup\fR.  You make an index
file that explicitly associates a color with each possible gray level.
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NOTE - MAXVAL
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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 \fBpgmtoppm 0f/00/00\fR 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.  \fBpgmtoppm\fR rounds the color
0f/00/00 down to black, and you get an output image that is nothing
but black.
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The fix is easy: Pass the input through \fBpamdepth\fR on the way
into \fBpgmtoppm\fR to increase its depth to something that would
give you the resolution you need to get your desired color.  In this
case, \fBpamdepth 16\fR would do it.  Or spare yourself the
unnecessary thinking and just say \fBpamdepth 255\fR.
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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, \fBpgmtoppm\fR, like all Netpbm
programs, in fact treats it as a maxval of 255.
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SEE ALSO
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\fBpamdepth\fR,
\fBrgb3toppm\fR,
\fBppmtopgm\fR,
\fBppmtorgb3\fR,
\fBppm\fR,
\fBpgm\fR
.SH 2
AUTHOR
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Copyright (C) 1991 by Jef Poskanzer.
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Table Of Contents
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NAME
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SYNOPSIS
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DESCRIPTION
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NOTE - MAXVAL
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SEE ALSO
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AUTHOR
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