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



Table Of Contents


NAME
       pnmhisteq - histogram equalise 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 portable graymap
       or pixmap through the technique of  histogram  equalisa-
       tion[1].

       A  histogram  of  the  luminance of pixels in the map is
       computed, from which a transfer function  is  calculated
       which  spreads  out  intensity  levels  around histogram
       peaks and compresses them  at  troughs.   This  has  the
       effect  of  using the available levels of intensity more
       efficiently and thereby increases the detail visible  in
       the image.

       Mathematically, if N[i] is the number of pixels of lumi-
       nosity i in the image and T is the total number of  pix-
       els, luminosity j is replaced by:


             j
            ---
                  > N[i] / T
            /
            ---
            i=0

       If  you're processing a related set of images, for exam-
       ple frames of an animation, it's generally best to apply
       the  same  intensity map 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 portable graymap, the  luminosity  map
       computed  from  an  image  (usually  a  composite of the
       images you  intend  to  process  created  with  pnmcat).
       Then,  you can subsequently process each of the individ-
       ual images using the luminosity map saved in  the  file,
       supplied with the -rmap option.


OPTIONS
       -gray  When processing a pixmap, 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 speci-
              fied 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.
              Each pixel in the image is transformed by looking
              up its luminosity in the corresponding column  in
              the  map  file and changing it to the value given
              by that column.


       -wmap mapfile
              Creates a PGM file mapfile, containing the  lumi-
              nosity  map  computed  from  the histogram of the
              input image.  This map file can be read on subse-
              quent  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 stan-
              dard error.



       You can abbreviate any option  to  its  shortest  unique
       prefix.


LIMITATIONS
       Histogram  equalisation  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 pgnnorm, ppmnorm,  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
       pnmdepth to agree with the input image.

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


SEE ALSO
       pgmnorm(1), pnm(1), pnmcat(1), pnmdepth(1), pnmgamma(1),
       pnmnorm(1)



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




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

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



netpbm documentation     19 March 1995 Pnmhisteq User Manual(0)
