pnmhisteq (1)
histogram equalise a portable anymap
SYNOPSIS
pnmhisteq
'ti 15
[ -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:
9p
j
---
\\
gt; 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
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
AUTHOR
2
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.
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