Pamthreshold User Manual(0) Pamthreshold User Manual(0)
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 an alpha
(transparency) 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 an alpha
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 an alpha channel, pamthreshold includes an alpha
channel in the output. Since the output has maxval 1, the alpha
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 alpha function was new in Netpbm 10.43 (June 2008). Before that,
pamthreshold ignores any alpha 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
Without any options, pamthreshold uses a method based on the iterative
algorithm found in the wikipedia article
Thresholding (image processing)
to
compute the thresholding value. ( this version
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(5)
HISTORY
pamthreshold was new in Netpbm 10.34 (June 2006).
AUTHOR
pamthreshold is Copyright (C) 2006 by Erik Auerswald and released under
the GPL(1)
netpbm documentation 06 June 2007 Pamthreshold User Manual(0)