pnmnorm(1)pnmnorm(1)NAMEpnmnorm - normalize the contrast in a Netbpm image
SYNOPSISpnmnorm [-bpercent N | -bvalue N] [-wpercent N | -wvalue
N] [-keephues] [-brightmax]
[ppmfile]
All options can be abbreviated to their shortest unique
prefix. You may use two hyphens instead of one to desig
nate an option. You may use either white space or an
equals sign between an option name and its value.
DESCRIPTION
Reads a PNM image (PBM, PGM, or PPM). Normalizes the con
trast by forcing the lightest pixels to white, the darkest
pixels to black, and linearly rescaling the ones in
between; and produces the same kind of file as output.
This is pretty useless for a PBM image.
The program first determines a mapping of old brightness
to new brightness. For each possible brightness of a
pixel, the program determines a corresponding brightness
for the output image.
Then for each pixel in the image, the program computes a
color which has the desired output brightness and puts
that in the output. With a color image, it is not always
possible to compute such a color and retain any semblance
of the original hue, so the brightest and dimmest pixels
may only approximate the desired brightness.
Note that for a PPM image, this is different from sepa
rately normalizing the individual color components.
OPTIONS
By default, the darkest 2 percent of all pixels are mapped
to black, and the lightest 1 percent are mapped to white.
You can override these percentages by using the -bpercent
and -wpercent flags, or you can specify the exact pixel
values to be mapped by using the -bvalue and -wvalue
flags. Appropriate numbers for the flags can be gotten
from the ppmhist tool. If you just want to enhance the
contrast, then choose values at elbows in the histogram;
e.g. if value 29 represents 3% of the image but value 30
represents 20%, choose 30 for bvalue. If you want to
lighten the image, then set bvalue to 0 and just fiddle
with wvalue; similarly, to darken the image, set wvalue to
maxval and play with bvalue.
The -keephues option says to keep each pixel the same hue
as it is in the input; just adjust its intensity. By
default, pnmnorm normalizes contrast in each component
independently (except that the meaning of the -wpercent
and -bpercent options are based on the overall intensities
of the colors, not each component taken separately). So
if you have a color which is intensely red but dimly
green, pnmnorm would make the red more intense and the
green less intense, so you end up with a different hue
than you started with.
If you specify -keephues, pnmnorm would likely leave this
pixel alone, since its overall intensity is medium.
-keephues can cause clipping, because a certain color may
be below a target intensity while one of its components is
saturated. Where that's the case, pnmnorm uses the maxi
mum representable intensity for the saturated component
and the pixel ends up with less overall intensity, and a
different hue, than it is supposed to have.
This option is meaningless on grayscale images.
Before March 2002, there was no -keephues option.
The -brightmax option says to use the intensity of the
most intense RGB component of a pixel as the pixel's
brightness. By default, pnmnorm uses the luminosity of
the color as its brightness.
This option is meaningless on grayscale images.
Before March 2002, there was no -brightmax option.
SEE ALSOppmhist(1), pgmhist(1), pnmgamma(1), ppmbrighten(1),
ppmdim(1), pnm(5)
7 October 1993 pnmnorm(1)
