Pnmhisteq User Manual(0) Pnmhisteq User Manual(0)NAMEpnmhisteq - histogram equalize a PNM image
SYNOPSISpnmhisteq
[-gray]
[-rmap pgmfile]
[-wmap pgmfile]
[-verbose]
[pnmfile]
DESCRIPTION
This program is part of Netpbm(1)pnmhisteq increases the contrast of a PGM or PPM image through the
technique of 'histogram equalization.'[1]
pnmhisteq computes a histogram of the luminosity of the pixels in the
image. It then calculates a mapping between each luminosity and a new
luminosity such that it spreads out intensity levels around histogram
peaks and compresses them at troughs. I.e. it moves pixels around in
the histogram so as to make it flat. It applies that mapping to the
input image to produce the output image. The effect of this is that
the image has equal numbers of pixels at each possible intensity level,
which means it uses the available levels of intensity more efficiently
and thereby has more visible detail.
Mathematically, the luminosity mapping is this: Assume the pixels are
sorted by luminosity into B buckets numbered from 0 (lowest luminosity)
to B-1. N[i] is the number of pixels in bucket i. T is the total num‐
ber of pixels (sum of N[i] over all i). W is the luminosity of white.
pnmhisteq replaces an input pixel whose luminosity falls into bucket j
with one whose luminosity is:
j
---
\
> (N[i] / T) * W
/
---
i=0
Considering a grayscale image for simplicity, this means that pixels in
the most luminous bucket become white. Pixels in the 10th per centile
of luminosity become 10% of white.
pnmhisteq maps a single luminosity in the input to a single luminosity
in the output. That means if pixels A and B both have luminosity .2 in
the input, and pixel A has luminosity .4 in the output, pixel B also
has luminosity .4 in the output. And since the luminosities in the
input are not continuous, the luminosities in the output aren't either
and pnmhisteq doesn't meet the ideal of having exactly the same number
of pixels of each luminosity in the output.
If you're processing a related set of images, for example frames of an
animation, it's generally best to apply the same luminosity mapping 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 PGM image, the luminosity map it computes from an
image. The -rmap option causes pnmisteq to use such an image as its
luminosity map.
So you can run pnmhisteq with -wmap on a composite you created with
pnmcat of the images you intend to process. Then, you can run pnmisteq
with -rmap on each of the individual images, using the luminosity map
you generated from the composite.
Use pnmhistmap to see the result. Run a color image through ppmtopgm
first so that you see a histogram of the luminosity instead of his‐
tograms of the three color components. It should generally show a flat
histogram. But due to the quantization effects described above, you
might see high bars interleaved with low bars, with the local average
being flat. To see local averages, use the -width option of
pnmhistmap.
OPTIONS
You can abbreviate any option to its shortest unique prefix.
-gray When processing a color image, only gray pixels (those with
identical red, green, and blue values) are included in the his‐
togram and modified in the output image. This is a special pur‐
pose option intended for images where the actual data are gray
scale, with color annotations you don't want modified. Weather
satellite images that show continent outlines in color 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 PGM
file mapfile.
The PGM image, usually created by an earlier run of pnmhisteq
with the -wmap option, contains a single row with number of col‐
umns equal to the maxval (greatest intensity value) of the image
plus one. Each pixel in the image is transformed by looking up
its luminosity in the corresponding column in the map file (col‐
umn number = luminosity) and changing it to the value given by
that column.
-wmap mapfile
Creates a PGM file mapfile, containing the luminosity map com‐
puted 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.
LIMITATIONS
Histogram equalization 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 pnmnorm 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 pamdepth to
agree with the input image.
If the input is a PBM file (on which histogram equalization is an iden‐
tity operation), the only effect of passing the file through pnmhisteq
will be the passage of time.
SEE ALSOpnmnorm(1) , pnmcat(1) , pamdepth(1) , pnmgamma(1) , pnm(1) ,
[1] Russ, John C. The Image Processing Handbook. Boca Raton: CRC
Press, 1992. Pages 105-110.
netpbm documentation 02 February 2010 Pnmhisteq User Manual(0)
