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. A histogram of the
luminance of pixels in the map is computed, from which a transfer func‐
tion is calculated which spreads out intensity levels around histogram
peaks and compresses them at troughs. This has the effect of using the
available levels of intensity more efficiently and thereby increases
the detail visible in the image.
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
(usually 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
-gray When processing a pixmap, only gray pixels (those with iden‐
tical red, green, and blue values) are included in the his‐
togram 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.
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 (great‐
est 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 changing it to the value given by
Creates a portable graymap, mapfile, containing the luminos‐
ity 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 subtle con‐
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 iden‐
tity operation), the only effect of passing the file through pnmhisteq
will be the passage of time.
 Russ, John C. The Image Processing Handbook. Boca Raton: CRC
Press, 1992. Pages 105-110.
Copyright (C) 1995 by John Walker (email@example.com).
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.
19 March 1995 pnmhisteq