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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 cal-
       culates	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 lev-
       els 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 number of pixels (sum of N[i] over all i).	W is the luminos-
       ity 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.

       pnmhisteq  maps	a  single  luminosity  in the input to a single luminosity in the output.
       That means if pixels A and B both have luminosity .2 in the input, and pixel A  has  lumi-
       nosity  .4  in  the  output,  pixel B also has luminosity .4 in the output.  And since the
       luminosities in the input are not continuous, the luminosities in the output aren't either
       and  pnmhisteq  doesn't meet the ideal of having exactly the same number of pixels of each
       luminosity in the output.

       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.

       Use pnmhistmap to see the result.  Run a color image through ppmtopgm first  so	that  you
       see a histogram of the luminosity instead of histograms of the three color components.  It
       should generally show a flat histogram.	But due to  the  quantization  effects	described
       above,  you  might  see	high bars interleaved with low bars, with the local average being
       flat.  To see local averages, use the -width option of pnmhistmap.

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 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
	      looking up its luminosity in the corresponding column in the map file (column  num-
	      ber = luminosity) and changing it to the value given by that column.

       -wmap mapfile
	      Creates  a  PGM  file mapfile, containing the luminosity map computed from the his-
	      togram of the input image.  This map file can be read on subsequent runs of pnmhis-
	      teq with the -rmap option, allowing a group of images to be processed with an iden-
	      tical 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 identity 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.

netpbm documentation			 02 February 2010		 Pnmhisteq User Manual(0)
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