pnmhisteq(1)
NAME
pnmhisteq - histogram equalise a portable anymap
SYNOPSIS
pnmhisteq [-gray] [-rmap pgmfile] [-wmap pgmfile] [-ver-
bose] [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 com-
puted, 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 lumi-
nosity 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 val-
ues) 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 sin-
gle row with number of columns equal to the max-
val (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 pro-
cessed with an identical map.
-verbose Prints the histogram and luminosity map on stan-
dard error.
All flags can be abbreviated to their shortest unique pre-
fix.
BUGS
Histogram equalisation is effective for increasing the
visible detail in scientific imagery and in some continu-
ous-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, sim-
ply adjust the maxval of the map with pnmdepth to agree
with the input image.
If the input is a PBM file (on which histogram equalisa-
tion 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 soft-
ware 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.