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.

       Pixels at the edges of the image, pixels where the convolution kernel would go off the edge of the image,
       are  just  copied.  For example, if -height is 9, the first 4 and last 4 rows of the input image are just
       copied to the output.

       See the -type and -cutoff options for information on how pgmmedian chooses between the two methods.

OPTIONS

       In addition to the options common to all programs based on libnetpbm (most notably  -quiet,  see   Common
       Options ), pgmmedian recognizes the following command line 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.

              Maximum allowed is the width of the input image.

              Default is 3.

       -height=n
              Height of the median mask to apply.

              Maximum allowed is the height of the input image.

              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(1), pamaddnoise(1), pnmconvol(1), pgmmorphconv(1), pgm(1)

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>

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/pgmmedian.html

netpbm documentation                             20 January 2022                        Pgmmedian User Manual(1)