pnmhisteq

 

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 pnmhisteq(1)                                         pnmhisteq(1)
 
 
 
NAME
        pnmhisteq - histogram equalise a portable anymap
 
SYNOPSIS
        pnmhisteq [-gray] [-rmap pgmfile] [-wmap pgmfile] [-ver­
                bose] [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  com­
        puted,  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 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 val­
                  ues) 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 spec­
                  ified by  the  portable  graymap  mapfile.   The
                  graymap,  usually  created  by an earlier run of
                  pnmhisteq with the -wmap option, contains a sin­
                  gle row with number of columns equal to the max­
                  val (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  pro­
                  cessed with an identical map.
 
        -verbose  Prints the histogram and luminosity map on stan­
                  dard error.
 
        All flags can be abbreviated to their shortest unique pre­
        fix.
 
 BUGS
        Histogram  equalisation  is  effective  for increasing the
        visible detail in scientific imagery and in some  continu­
        ous-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,  sim­
        ply  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 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 soft­
        ware 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)

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