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NAME

       pnmhisteq - histogram equalise a portable anymap

SYNOPSIS

       pnmhisteq [-gray] [-rmap pgmfile] [-wmap pgmfile] [-verbose] [pnmfile]

DESCRIPTION

       pnmhisteq  increases the contrast of a portable graymap or pixmap through the technique of
       histogram equalisation[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 luminosity i in the image and T is  the
       total number of pixels, luminosity j is replaced by:

                j
               ---
               \
                >   N[i] / T

               ---
               i=0

       If  you're  processing  a  related set of images, for example 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 individual 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 colour annotations you don't want modified.  Weather  satellite
                 images  that  show  continent  outlines  in colour 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  portable  graymap
                 mapfile.   The  graymap, 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)  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 portable graymap, 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.

       All flags can be abbreviated to their shortest unique prefix.

BUGS

       Histogram equalisation is effective  for  increasing  the  visible  detail  in  scientific
       imagery  and  in  some  continuous-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 equalisation is an identity operation), the
       only effect of passing the file through pnmhisteq will be the passage of time.

SEE ALSO

       pgmnorm(1), pnm(5), 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 without fee is hereby granted, without  any  conditions  or  restrictions.
       This software is provided ``as is'' without express or implied warranty.

                                          19 March 1995                              pnmhisteq(1)