NAME
pnmhisteq - histogram equalise a portable anymap
SYNOPSIS
pnmhisteq [-gray] [-rmap pgmfile] [-wmap pgmfile] [-verbose] [pnmfile]
DESCRIPTION
pnmhisteq increases the contrast of a portable graymap or pixmap
through the technique of histogram equalisation[1]. A histogram of the
luminance of pixels in the map is computed, from which a transfer
function 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:
j
---
\
> N[i] / T
---
i=0
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
-rmap option.
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 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.
-rmap mapfile
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
(greatest 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 that column.
-wmap mapfile
Creates a portable graymap, mapfile, containing the
luminosity 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.
BUGS
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
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 equalisation is an
identity operation), the only effect of passing the file through
pnmhisteq will be the passage of time.
SEE ALSO
pgmnorm(1), pnm(5), pnmcat(1), pnmdepth(1), pnmgamma(1), pnmnorm(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,
without any conditions or restrictions. This software is provided ‘‘as
is’’ without express or implied warranty.
19 March 1995 pnmhisteq(1)