Provided by: netpbm_11.08.02-1_amd64 
NAME
pamthreshold - threshold grayscale image to black and white
SYNOPSIS
pamthreshold [-simple] [-local=widthxheight] [-dual=widthxheight] [-threshold=threshold]
[-contrast=threshold] [inputpamfile]
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.
DESCRIPTION
This program is part of Netpbm(1).
pamthreshold thresholds a grayscale image. Thresholding means dividing the image into
background and foreground by comparing every pixel to a thresholding value.
The input should be a PGM image or a PAM image of tuple type GRAYSCALE or GRAYSCALE_ALPHA.
However, pamthreshold doesn't check; it just thresholds the first channel as if it were
grayscale samples and if there is a second channel, processes it as if it is a
transparency (alpha) channel. So if you feed it e.g. a PPM image, it will work but
produce probably useless results.
The output is a PAM with tuple type BLACKANDWHITE or BLACKANDWHITE_ALPHA, depending on
whether the input has a transparency channel. 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 pamtopnm.
The output is to Standard Output.
When the input has a transparency channel, pamthreshold includes a transparency channel in
the output. Since the output has maxval 1, the transparency channel can indicate only
fully transparent or fully opaque. pamthreshold make it fully transparent where the input
is more than half transparent and fully opaque where it isn't.
The transparency function was new in Netpbm 10.43 (June 2008). Before that, pamthreshold
ignores any transparency channel in the input.
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. pamditherbw does dithering.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet,
see
Common Options ⟨index.html#commonoptions⟩ ), pamthreshold recognizes the following
command line options:
Without any options, pamthreshold uses a method based on the iterative algorithm found in
the wikipedia ⟨http://www.wikipedia.org/⟩ article
Thresholding (image processing)
⟨http://en.wikipedia.org/wiki/Thresholding_%28image_processing%29⟩ to compute the
thresholding value. ( this version
⟨http://en.wikipedia.org/w/index.php?title=Thresholding_%28image_processing%29&oldid=132306976⟩
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 -quiet).
Options -simple, -local, and -dual select other methods.
-simple
This selects simple or global thresholding, i.e. pamthreshold compares every pixel
to the threshold you specify with -threshold. 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.
-local=widthxheight
This selects local adaptive thresholding (also known as dynamic thresholding) using
the neighborhood which is the square width pixels wide and height pixels high
centered on the pixel in question. pamthreshold 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.
-dual=widthxheight
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.
-threshold=threshold
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.
If you don't specify this option, pamthreshold uses a threshold of 0.5.
Without -simple, -local, or -dual, this option is meaningless.
The meaning of the threshold depends upon the kind of thresholding you do (as
determined by other options). Roughly, pixels at least as bright as the threshold
become white in the output while others become black.
-contrast=threshold
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].
If you don't specify this option, pamthreshold uses a contrast threshold of 0.05.
Without -dual this option is meaningless.
SEE ALSO
pamditherbw(1), ppmtopgm(1), pamtopnm(1), pam(1)
HISTORY
pamthreshold was new in Netpbm 10.34 (June 2006).
AUTHOR
pamthreshold is Copyright © 2006 by Erik Auerswald and released under the GPL(1).
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master
documentation is at
http://netpbm.sourceforge.net/doc/pamthreshold.html