pnmhisteq



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




NAME

       pnmhisteq - histogram equalize a PNM image



SYNOPSIS

       pnmhisteq

       [-gray]

       [-rmap pgmfile]

       [-wmap pgmfile]

       [-verbose]

       [pnmfile]

       You can abbreviate any option to its shortest unique prefix.




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.  It applies that mapping to the
       input image to produce the output image.  The effect of  this  is  that
       the  image  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

       -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 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
              columns equal to the maxval (greatest intensity  value)  of  the
              image plus one.  Each pixel in the image is transformed by look-
              ing up its luminosity in the corresponding  column  in  the  map
              file  (column  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 pnmdepth  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), pnmdepth(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/

       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)

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