This program is part of Netpbm.
pnmhisteq increases the contrast of a PGM or PPM image through the
technique of 'histogram equalization.'
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 luminos-
ity) 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 luminosity
pnmhisteq replaces an input pixel whose luminosity falls into bucket j
with one whose luminosity is:
Considering a grayscale image for simplicity, this means that pixels
in the most luminous bucket become white. Pixels in the 10th per cen-
tile 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
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 pnmis-
teq with -rmap on each of the individual images, using the luminosity
map you generated from the composite.
You can abbreviate any option to its shortest unique prefix.
-gray When processing a color image, only gray pixels (those with
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
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 restric-
tion 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.
pnmnorm, pnmcat, pamdepth, pnmgamma, pnm,
 Russ, John C. The Image Processing Handbook. Boca Raton: CRC
Copyright (C) 1995 by John Walker (email@example.com). WWW home
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)