Pnmnorm User Manual(0) Pnmnorm User Manual(0)
NAME
pnmnorm - normalize the contrast in a Netbpm image
SYNOPSIS
pnmnorm
[-bpercent=N | -bvalue=N]
[-wpercent=N | -wvalue=N]
[-maxexpand
[-keephues]
[-luminosity | -colorvalue | -saturation]
[ppmfile]
All options can be abbreviated to their shortest unique prefix. You
may use two hyphens instead of one to designate an option. You may use
either white space or an equals sign between an option name and its
value.
DESCRIPTION
This program is part of Netpbm(1).
pnmnorm reads a PNM image (PBM, PGM, or PPM). It normalizes the con-
trast by forcing the brightest 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 bright-
ness. 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.
For a PPM image, you have a choice of three different ways to define
brightness:
· luminosity
· color value
· saturation
In the case of saturation, ’brightness’ is pretty much a mis-
nomer, but you can use the brightness analogy to see what it
does. In the analogy, bright means saturated and dark means
unsaturated.
Note that all of these are different from separately normalizing the
individual color components.
An alternative way to spread out the brightnesses in an image is
pnmhisteq. pnmhisteq stretches the brightest pixels to white and the
darkest pixels to black, but rather than linearly adjusting the ones in
between, it adjusts them so that there are an equal number of pixels of
each brightness throughout the range. This gives you more contrast
than pnmnorm does, but can considerably change the picture in exchange.
OPTIONS
By default, the darkest 2 percent of all pixels are mapped to black,
and the brightest 1 percent are mapped to white. You can override
these percentages by using the -bpercent and -wpercent options, or you
can specify the exact pixel values to be mapped by using the -bvalue
and -wvalue options. You can get appropriate numbers for the options
from ppmhist. 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 brighten 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.
If you specify both -bvalue and -bpercent, pnmnorm uses the one that
produces the least change. The same goes for -wvalue and -wpercent.
If you want to maximize the change instead of minimizing it, just cas-
cade two runs of pnmnorm, specifying values for the first and percent-
ages for the second.
This is further constrained by the -maxexpand option. Sometimes, too
much contrast is a bad thing. If your intensities are all concentrated
in the middle, -bpercent=2 and -wpercent=1 might mean that an intensity
of 60 gets stretched up to 100 and and intensity of 40 gets stretched
down to zero, for a range expansion of 150% (from a range of 40 to a
range of 100). That much stretching means two adjacent pixels that
used to differ in intensity by 4 units now differ by 10, and that might
be unsightly.
So that you can put a limit on the amount of expansion without having
to examine the image first, there is the -maxexpand option. It speci-
fies the maximum expansion you will tolerate, as an additional per cen-
tage. In the example above, you could say -maxexpand=50 to say you
want the range to expand by at most 50%, regardless of your other
options. pnmnorm figures out what intensity to stretch to full inten-
sity and what intensity to stretch to zero intensity as described
above, and then raises the former and lowers the latter as needed to
limit the expansion to the amount you specified.
When pnmnorm limits the expansion due to -maxexpand, it tells you about
it with a message like this:
limiting expansion of 150% to 50%
In any case, pnmnorm tells you exactly what expansion it’s doing, like
this:
remapping 25..75 to 0..100
Before Netpbm 10.26 (December 2004), it was not valid to specify both
-bvalue and -bpercent or -wvalue and -wpercent.
-maxexpand was new in Netpbm 10.32 (February 2006).
The -keephues option says to keep each pixel the same hue as it is in
the input; just adjust its brightness. You normally want this; the
only reason it is not the default behavior is backward compatibility
with a design mistake.
By default, pnmnorm normalizes contrast in each component independently
(except that the meaning of the -wpercent and -bpercent options are
based on the overall brightnesses 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 brightness 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 maximum representable intensity for
the saturated component and the pixel ends up with less overall inten-
sity, and a different hue, than it is supposed to have.
This option is meaningless on grayscale images.
When you don’t specify -keephues, the -luminosity, -colorvalue, and
-saturation options affect the transfer function (which is the same for
all three RGB components), but are meaningless when it comes to apply-
ing the transfer function (since it is applied to each individual RGB
component).
Before Netpbm 9.25 (March 2002), there was no -keephues option.
-luminosity, -colorvalue, and -saturation determine what property of
the pixels pnmnorm normalizes. I.e., what kind of brightness. You
cannot specify more than one of these.
The -luminosity option says to use the luminosity (i.e. the ’Y’ in the
YUV or YCbCr color space) as the pixel’s brightness. The luminosity is
a measure of how bright a human eye would find the color, taking into
account the fact that the human eye is more sensitive to some RGB com-
ponents than others.
This option is default.
This option is meaningless on grayscale images.
Before Netpbm 10.28 (August 2005), there was no -luminosity option, but
its meaning was still the default.
Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
The -colorvalue option says to use the color value (i.e. the ’V’ in the
HSV color space) as the pixel’s brightness. The color value is the
gamma-adjusted intensity of the most intense RGB component.
This option is meaningless on grayscale images.
Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
The -saturation option says to use the saturation (i.e. the ’S’ in the
HSV color space) as the pixel’s brightness. The saturation is the
ratio of the intensity of the most intense RGB component to the differ-
ence between the intensities of the most and least intense RGB compo-
nent (all intensities gamma-adjusted).
In this case, ’brightness’ is more of a metaphor than anything.
’bright’ means saturated and ’dark’ means unsaturated.
This option is meaningless on grayscale images.
Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
SEE ALSO
pnmhisteq(1), ppmhist(1), pgmhist(1), pnmgamma(1), ppmbrighten(1),
ppmdim(1), pnm(1)
netpbm documentation 6 January 2006 Pnmnorm User Manual(0)
