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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//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>] <I>N</I>
[<b>-randomseed=</b><i>integer</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 <tt>pamscale |
pamditherbw</tt>, 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>
<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>-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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