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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD><TITLE>Pamthreshold User Manual</TITLE></HEAD>
<BODY>
<H1>pamthreshold</H1>
Updated: 06 June 2007
<BR>
<A HREF="#index">Table Of Contents</A>
<H2>NAME</H2>
pamthreshold - threshold grayscale image to black and white
<H2 id="synopsis">SYNOPSIS</H2>
<B>pamthreshold</B>
[<b>-simple</b>]
[<b>-local=</b><i>width</i><b>x</b><i>height</i>]
[<b>-dual=</b><i>width</i><b>x</b><i>height</i>]
[<b>-threshold=</b><i>threshold</i>]
[<b>-contrast=</b><i>threshold</i>]
[<I>inputpamfile</I>]
<P>Minimum unique abbreviations of options are acceptable. You may use
double hyphens instead of a single hyphen to denote options. You may use
white space in place of the equals sign to separate an option name
from its value.
<H2 id="description">DESCRIPTION</H2>
<p>This program is part of <a href="index.html">Netpbm</a>.
<P><b>pamthreshold</b> thresholds a grayscale image. Thresholding means
dividing the image into background and foreground by comparing every pixel
to a thresholding value.
<p>The input should be a PGM image or a PAM image of tuple type
GRAYSCALE. However, pamthreshold doesn't check; it just thresholds the
first channel as if it were grayscale samples. So if you
feed it e.g. a PPM image, it will work but produce probably useless
results.
<p>The output is a PAM with tuple type BLACKANDWHITE. You can turn
this into a PBM (if you need to use it with an older program that
doesn't understand PAM, or you can't afford the 8X amount of space
that PAM uses for the image) with <b>pamtopnm</b>.
<p>The output is to Standard Output.
<p>Another way to convert a grayscale image to black and white is to
dither. Dithering is using clustered black and white pixels such that
if you step back and look at the picture, you see varying levels of
gray. <B>pamditherbw</B> does dithering.
<H2 id="options">OPTIONS</H2>
<p>Without any options, <b>pamthreshold</b> uses an iterative
algorithm found in the <a
href="http://www.wikipedia.org/">wikipedia</a> article
<a href="http://en.wikipedia.org/wiki/Thresholding_%28image_processing%29">
<i>Thresholding (image processing)</i></a> to compute the thresholding
value. (<a
href="http://en.wikipedia.org/w/index.php?title=Thresholding_%28image_processing%29&oldid=132306976">this
version</a> of the Wikipedia article was current at the time of this
writing). It uses this threshold to globally threshold the image.
This should work well for most images. The program issues a message
telling you what threshold it used. (Netpbm messages go to Standard
Error, and you can turn them off with the Netpbm common option
<b>-quiet</b>).
<DL COMPACT>
<dt><b>-simple</b></dt>
<dd>This selects simple or global thresholding,
i.e. <b>pamthreshold</b> compares every pixel to the threshold you
specify with <b>-threshold</b>. Those with a brightness greater than
or equal to the threshold become white; others become black. This
works well for black and white text pages scanned with a flatbed
scanner and is faster than the default method that iteratively
determines the thresholding value first.
<dt><b>-local=</b><i>width</i><b>x</b><i>height</i></dt>
<dd>This selects local adaptive thresholding (also known as dynamic
thresholding) using the neighborhood of <i>width</i> and <i>height</i>
around every pixel. <b>pamthreshold</b> computes the threshold
individually for each pixel of the image. This can accommodate
changing lighting conditions in the image. Depending on the size of
the neighborhood this can be quite slow.
<dt><b>-dual=</b><i>width</i><b>x</b><i>height</i></dt>
<dd>This selects a dual thresholding algorithm using a global threshold
for low contrast neighborhoods and local thresholding otherwise. This
can preserve larger back- respectively foreground areas than local
adaptive thresholding. This algorithm was proposed in the paper
"An Approach To Licence Plate Recognition" by J.R. Parker and Pavol Federl.
<dt><b>-threshold=</b><i>threshold</i></dt>
<dd>This sets the thresholding value for simple or local thresholding.
The value is a floating point number in the range [0, 1] directly
proportional to the Netpbm sample values, , where 0 corresponds to
black and 1 to the maxval of the image.
<p>If you don't specify this option, <b>pamthreshold</b> uses a threshold
of 0.5. Without <b>-simple</b> or <b>-local</b> this option is
meaningless.
<p>The meaning of the threshold depends upon the kind of thresholding
you do (as determined by other options). Roughly, pixels brighter
than the threshold become white in the output while others become black.
<dt><b>-contrast=</b><i>threshold</i></dt>
<dd>This sets the threshold to determine if a neighborhood has low contrast
or not for dual thresholding. The value is a floating point number in
the range [0, 1].
<p>If you don't specify this option, <b>pamthreshold</b> uses a contrast
threshold of 0.05. Without <b>-dual</b> this option is meaningless.
</DL>
<H2 id="seealso">SEE ALSO</H2>
<B><A HREF="pamditherbw.html">pamditherbw</A></B>,
<B><A HREF="ppmtopgm.html">ppmtopgm</A></B>,
<B><A HREF="pamtopnm.html">pamtopnm</A></B>,
<B><A HREF="pam.html">pam</A></B>
<H2 id="history">HISTORY</H2>
<p><b>pamthreshold</b> was new in Netpbm 10.34 (June 2006).
<h2 id="author">AUTHOR</h2>
<p>
<b>pamthreshold</b> is Copyright © 2006 by Erik Auerswald and released
under the <a href="http://www.gnu.org/licenses/gpl.html">GPL</a>.
<HR>
<H2 id="index">Table Of Contents</H2>
<UL>
<LI><A HREF="#name">NAME</A>
<LI><A HREF="#synopsis">SYNOPSIS</A>
<LI><A HREF="#options">OPTIONS</A>
<LI><A HREF="#description">DESCRIPTION</A>
<LI><A HREF="#seealso">SEE ALSO</A>
<LI><A HREF="#history">HISTORY</A>
<LI><A HREF="#author">AUTHOR</A>
</UL>
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