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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.3//EN">
<html><head><title>Pbmreduce User Manual</title></head>
<body>
<h1>pbmreduce</h1>
Updated: 13 April 2016
<br>
<a href="#index">Table Of Contents</a>
<h2>NAME</h2>
pbmreduce - read a PBM image and reduce it N times
<h2 id="synopsis">SYNOPSIS</h2>
<b>pbmreduce</b>
[<b>-floyd</b>|<b>-fs</b>|<b>-threshold</b>]
[<b>-value</b> <i>val</i>]
[<b>-randomseed=</b><i>integer</i>]
<i>N</i>
[<i>pbmfile</i>]
<p>You can abbreviate any option to its shortest unique prefix.
<h2 id="description">DESCRIPTION</h2>
<p>This program is part of <a href="index.html">Netpbm</a>.
<p><b>pbmreduce</b> reads a PBM image as input and reduces it by a
factor of <i>N</i>, producing a PBM image as output.
<p><b>pbmreduce</b> duplicates a lot of the functionality of
<b>pamditherbw</b>; you could do something like <kbd>pamscale |
pamditherbw</kbd>, but <b>pbmreduce</b> is a lot faster.
<p>You can use <b>pbmreduce</b> to "re-halftone" an image.
Let's say you have a scanner that only produces black&white, not
grayscale, and it does a terrible job of halftoning (most b&w
scanners fit this description). One way to fix the halftoning is to
scan at the highest possible resolution, say 300 dpi, and then reduce
by a factor of three or so using <b>pbmreduce</b>. You can even
correct the brightness of an image, by using the <b>-value</b> option.
<h2 id="options">OPTIONS</h2>
<p>In addition to the options common to all programs based on libnetpbm
(most notably <b>-quiet</b>, see <a href="index.html#commonoptions">
Common Options</a>), <b>pbmreduce</b> recognizes the following
command line options:
<dl>
<dt><b>-threshold</b>
<dd>
By default, <b>pbmreduce</b> does the halftoning after the reduction via
boustrophedonic Floyd-Steinberg error diffusion; however, you can use this
option to specify simple thresholding. This gives better results when
reducing line drawings.
<dt><b>-floyd</b>, <b>-fs</b>
<dd>Specify the Floyd-Steinberg error diffusion method. This is the
default.
<dt><b>-value</b>
<dd>
<p>This option alters the thresholding value for all quantizations. It should
be a real number between 0 and 1. Above 0.5 means darker images; below 0.5
means lighter.
<dt><b>-randomseed=</b><i>integer</i>
<dd>This is the seed for the random number generator that controls the
halftoning.
<p>Use this to ensure you get the same image on separate invocations.
<p>This option was new in Netpbm 10.75 (June 2016).
</dl>
<h2 id="seealso">SEE ALSO</h2>
<a href="pamenlarge.html">pamenlarge</a>,
<a href="pamscale.html">pamscale</a>,
<a href="pamditherbw.html">pamditherbw</a>,
<a href="pbm.html">pbm</a>
<h2 id="author">AUTHOR</h2>
Copyright (C) 1988 by Jef Poskanzer.
<hr>
<h2 id="index">Table Of Contents</h2>
<ul>
<li><a href="#synopsis">SYNOPSIS</a>
<li><a href="#description">DESCRIPTION</a>
<li><a href="#options">OPTIONS</a>
<li><a href="#seealso">SEE ALSO</a>
<li><a href="#author">AUTHOR</a>
</ul>
</body>
</html>
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