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  ⟨index.html#commonoptions⟩ ), 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