# pgmmedian

Updated: 28 August 2005
Table Of Contents

## 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.

**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 Two-Dimensional Median Filtering Algorithm" in
"IEEE Transactions on Acoustics, Speech, and Signal
Processing" Vol. ASSP-27, No. 1, February 1979
- "Digital Image Processing Algorithms" by Ioannis
Pitas, Prentice Hall, 1993 ISBN 0-13-145814-0

## SEE ALSO

pgmnoise,
pamaddnoise,
pnmconvol,
pgmmorphconv,
pgm
## 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>

## Table Of Contents