pnmhisteq
histogram equalise a portable anymap
see also :
pgmnorm - pnmcat - pnmdepth - pnmgamma - pnmnorm
Synopsis
pnmhisteq
[-gray] [-rmap pgmfile]
[-wmap pgmfile] [-verbose]
[pnmfile]
add an example, a script, a trick and tips
examples
no example yet ...
... Feel free to add your own example above to help other Linux-lovers !
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 ,
pnm, pnmcat , pnmdepth ,
pnmgamma , pnmnorm
[1]
Russ, John C. The Image
Processing Handbook. Boca Raton: CRC Press, 1992. Pages
105-110.
author
Copyright (C)
1995 by John Walker (kelvin[:at:]fourmilab[:dot:]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.