pgmmedian (1)  Linux Manuals
pgmmedian: apply a median filter to a PGM file
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 double 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 histogram 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 neighborhood 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 regardless
of cutoff value. Choices are histogram_sort and select.
By default, pgmmedian decides which method to use as described under the cutoff option.
REFERENCES
 •

'Collected Algorithms from ACM' Volume II, Algorithm 489
by Robert W. Floyd
 •

'A Fast TwoDimensional Median Filtering Algorithm' in
'IEEE Transactions on Acoustics, Speech, and Signal
Processing' Vol. ASSP27, No. 1, February 1979
 •

'Digital Image Processing Algorithms' by Ioannis
Pitas, Prentice Hall, 1993 ISBN 0131458140
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 [at] cac.psu.edu>