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Pnmhisteq User Manual(0)                              Pnmhisteq User Manual(0)



NAME
       pnmhisteq - histogram equalize a PNM image


SYNOPSIS
       pnmhisteq

       [-gray]

       [-rmap pgmfile]

       [-wmap pgmfile]

       [-verbose]

       [pnmfile]



DESCRIPTION
       This program is part of Netpbm(1).

       pnmhisteq increases the contrast of a PGM or PPM image through the technique of 'histogram equalization.'[1]

       pnmhisteq  computes  a  histogram  of  the luminosity of the pixels in the image.  It then calculates a mapping
       between each luminosity and a new luminosity such that it spreads out intensity levels around  histogram  peaks
       and  compresses  them  at  troughs.   I.e.   it moves pixels around in the histogram so as to make it flat.  It
       applies that mapping to the input image to produce the output image.  The effect of this is that the image  has
       equal numbers of pixels at each possible intensity level, which means it uses the available levels of intensity
       more efficiently and thereby has more visible detail.

       Mathematically, the luminosity mapping is this: Assume the pixels are sorted by luminosity into B buckets  num-
       bered  from  0 (lowest luminosity) to B-1.  N[i] is the number of pixels in bucket i.  T is the total number of
       pixels (sum of N[i] over all i).  W is the luminosity of white.

       pnmhisteq replaces an input pixel whose luminosity falls into bucket j with one whose luminosity is:


             j
            ---
            \
             > (N[i] / T) * W
            /
            ---
            i=0

       Considering a grayscale image for simplicity, this means that pixels in the most luminous bucket become  white.
       Pixels in the 10th per centile of luminosity become 10% of white.

       If  you're processing a related set of images, for example frames of an animation, it's generally best to apply
       the same luminosity mapping to every frame, since otherwise you'll get distracting  frame-to-frame  changes  in
       the  brightness of objects.  pnmhisteq's -wmap option allows you to save, as a PGM image, the luminosity map it
       computes from an image.  The -rmap option causes pnmisteq to use such an image as its luminosity map.

       So you can run pnmhisteq with -wmap on a composite you created with pnmcat of the images you intend to process.
       Then,  you can run pnmisteq with -rmap on each of the individual images, using the luminosity map you generated
       from the composite.



OPTIONS
       You can abbreviate any option to its shortest unique prefix.



       -gray  When processing a color image, only gray pixels (those with identical red, green, and blue  values)  are
              included  in  the histogram and modified in the output image.  This is a special purpose option intended
              for images where the actual data are gray  scale,  with  color  annotations  you  don't  want  modified.
              Weather  satellite  images  that  show continent outlines in color are best processed using this option.
              The option has no effect when the input is a graymap.


       -rmap mapfile
              Process the image using the luminosity map specified by the PGM file mapfile.

              The PGM image, usually created by an earlier run of pnmhisteq with the -wmap option, contains  a  single
              row  with  number of columns equal to the maxval (greatest intensity value) of the image plus one.  Each
              pixel in the image is transformed by looking up its luminosity in the corresponding column  in  the  map
              file (column number = luminosity) and changing it to the value given by that column.


       -wmap mapfile
              Creates  a  PGM  file  mapfile,  containing  the luminosity map computed from the histogram of the input
              image.  This map file can be read on subsequent runs of pnmhisteq with  the  -rmap  option,  allowing  a
              group of images to be processed with an identical map.


       -verbose
              Prints the histogram and luminosity map on Standard Error.




LIMITATIONS
       Histogram equalization is effective for increasing the visible detail in scientific imagery and in some contin-
       uous-tone pictures.  It is often too drastic, however, for scanned halftone images, where it does an  excellent
       job  of  making  halftone  artifacts apparent.  You might want to experiment with pnmnorm and pnmgamma for more
       subtle contrast enhancement.

       The luminosity map file supplied by the -rmap option must have the same maxval as the  input  image.   This  is
       always  the  case when the map file was created by the -wmap option of pnmhisteq.  If this restriction causes a
       problem, simply adjust the maxval of the map with pamdepth to agree with the input image.

       If the input is a PBM file (on which histogram equalization is an identity operation), the only effect of pass-
       ing the file through pnmhisteq will be the passage of time.


SEE ALSO
       pnmnorm(1), pnmcat(1), pamdepth(1), pnmgamma(1), pnm(1),



       [1]    Russ, John C.  The Image Processing Handbook.  Boca Raton: CRC Press, 1992.  Pages 105-110.




AUTHOR
       Copyright (C) 1995 by John Walker (kelvinATfourmilab.ch).  WWW home page: http://www.fourmilab.ch/

       Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without
       fee is hereby granted, without any conditions or restrictions.  This  software  is  provided  'as  is'  without
       express or implied warranty.



netpbm documentation           10 September 2005      Pnmhisteq User Manual(0)