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pnmhisteq(1)									     pnmhisteq(1)

       pnmhisteq - histogram equalise a portable anymap

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

       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  avail-
       able  levels of intensity more efficiently and thereby increases the detail visible in the

       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:

		>   N[i] / T


       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 (usu-
       ally  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.

       -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 chang-
		 ing 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.

       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 sub-
       tle 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.

       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

			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)
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