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 numbered from 0 (lowest luminosity)
       to B-1.	N[i] is the number of pixels in bucket i.  T is the total num-
       ber 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  his-
	      togram and modified in the output image.	This is a special pur-
	      pose 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 col-
	      umns 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 (col-
	      umn number = luminosity) and changing it to the value  given  by
	      that column.


       -wmap mapfile
	      Creates  a  PGM file mapfile, containing the luminosity map com-
	      puted 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  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  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 iden-
       tity  operation), the only effect of passing 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 (kelvin@fourmilab.ch).  WWW home
       page: http://www.fourmilab.ch/ <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, with-
       out any conditions or restrictions.  This software is provided 'as  is'
       without express or implied warranty.



netpbm documentation	       10 September 2005      Pnmhisteq User Manual(0)