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Pgmmedian User Manual(0) Pgmmedian User Manual(0)
NAME
pgmmedian - apply a median filter to a PGM file
SYNOPSIS
pgmmedian
[-width=n]
[-height=n]
[-type=median_type]
[-cutoff=int]
[pnmfile]
Minimum unique abbreviation of option is acceptable. You may use dou-
ble hyphens instead of 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)
pgmmedian applies a median filter to a PGM image, using either the his-
togram sort or select kth value method to determine the median.
A median filter is a convolution filter in which the value of a pixel
in the output is the median of a certain set of pixels in the neighbor-
hood of the corresponding input pixel. The effect is to eliminate
locally extreme values. Such pixels typically show up as speckles.
See the -type and -cutoff options for information on how pgmmedian
chooses between the two methods.
OPTIONS
-cutoff int
This option provides the cutoff value that pgmmedian uses to
decide between using the histogram sort or select kth value
method to find the median.
If (maxval / ((width * height) - 1)), where maxval is the maxval
of the image and width and height are the dimensions of the
mask, is less than the cutoff value, pgmmedian uses histogram
sort. Otherwise, it uses kth value.
This option has no effect if you specify -type.
The default is 250
-width=n
Width of the median mask to apply.
Default is 3.
-height=n
Height of the median mask to apply.
Default is 3.
-type median_type
This option selects which method to use to find median regard-
less of cutoff value. Choices are histogram_sort and select.
By default, pgmmedian decides which method to use as described
under the -cutoff option.
REFERENCES
o 'Collected Algorithms from ACM' Volume II, Algorithm 489 by
Robert W. Floyd
o 'A Fast Two-Dimensional Median Filtering Algorithm' in 'IEEE
Transactions on Acoustics, Speech, and Signal Processing' Vol.
ASSP-27, No. 1, February 1979
o 'Digital Image Processing Algorithms' by Ioannis Pitas, Prentice
Hall, 1993 ISBN 0-13-145814-0
SEE ALSO
pgmnoise(1) , pamaddnoise(1) , pnmconvol(1) , pgmmorphconv(1) , pgm(5)
HISTORY
pgmmedian was added to Netpbm in Version 10.29 (August 2005). It had
been distributed by Mike Burns via his own web site before that (and
continued to be so).
AUTHOR
Copyright (C) 1996 by Mike Burns <burns@cac.psu.edu>
netpbm documentation 28 August 2005 Pgmmedian User Manual(0)
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