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Pnmnorm User Manual(0) Pnmnorm User Manual(0)
NAME
pnmnorm - normalize the contrast in a Netpbm image
SYNOPSIS
pnmnorm
[-bpercent=percent | -bvalue=N | -bsingle]
[-wpercent=percent | -wvalue=N | -wsingle]
[-midvalue=N]
[-middle=N]
[-maxexpand=percent]
[-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 contrast by forcing the
brightest pixels to white, the darkest pixels to black, and spreading out the ones in
between. It produces the same kind of file as output. This is pretty useless for a PBM
image.
The program offers two ways of spreading out the pixels in between the darkest and bright-
est: linear and quadratic. In the quadratic case, you specify some in between brightness
and specify what brightness that should become in the output. With those three con-
straints: the brightness that becomes black, the brightness that becomes white, and the
brightness that becomes that middle value, pnmnorm computes a quadratic equation that maps
all the other brightnesses from input values to output values.
The program first determines a mapping of old brightness to new brightness. For each pos-
sible 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 out-
put 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 ways to define brightness:
o luminosity
o color value
o saturation
In the case of saturation, "brightness" is pretty much a misnomer, 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 com-
ponents.
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 pnm-
norm 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 this behavior and specify either a differ-
ent percentage, or specific brightness values to map to black and to white, or just have
the single greatest brightness map to white and the least brightness map to black.
To specify a percentage, use 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 con-
trast, 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. (In Netpbm 10.26 (January 2005), the
-bvalue/-wvalue takes precedence, and before that, it's a syntax error to specify both).
If you want to maximize the change instead of minimizing it, just cascade two runs of pnm-
norm, specifying values for the first and percentages for the second.
-bpercent and -wpercent values are floating point decimal. Zero is valid and is the same
as -bvalue=0 or -wvalue=maxval, respectively.
Because there are whole numbers of pixels at each brightness, pnmnorm obviously can't
guarantee the exact percentage, so it arranges that at least the percentage of pixels you
specify get remapped as promised.
It is possible for your -bpercent or -wpercent to overlap your -wvalue or -bvalue, respec-
tively. For example, you say -bpercent=20 and -wvalue=100 for an image in which only 10
percent of the pixels are darker than 100. In that case, pnmnorm adjusts the percentile
value as required. In the example, it uses 99 as the black value (like -bvalue=99).
It is also possible for your -bpercent and -wpercent options to select the same brightness
value for the stretch-to-white and stretch-to-black value because of the fact that pnmnorm
can't subdivide a histogram cell. E.g. if an image is all brightness 100, then no matter
what -bpercent and -wpercent values you choose, it's the same as saying -bvalue=100
-wvalue=100. In that case, pnmnorm changes one of the values by 1 to make it legal. In
the example, pnmnorm would either make the black value 99 or the white value 101.
Before Netpbm 10.43 (June 2008), pnmnorm fails if the -wpercent and/or -bpercent values
specify an overlap.
The stretch points are further constrained by the -maxexpand option. Sometimes, too much
contrast is a bad thing. If your intensities are all concentrated in the middle, -bper-
cent=2 and -wpercent=1 might mean that an intensity of 60 gets stretched up to 100 and
intensity of 20 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.
To specify that the single least brightness in the image should stretch to black in the
output, specify -bsingle. To specify that the single greatest brightness in the image
should stretch to white in the output, specify -wsingle. -bsingle and -wsingle were new
in Netpbm 10.69 (December 2014).
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 specifies the maximum expansion you will toler-
ate, as an additional percentage. 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. pnm-
norm figures out what intensity to stretch to full intensity 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 because of -maxexpand, it tells you about it with a mes-
sage 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 -bper-
cent 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.
When you specify -midvalue=N, pnmnorm uses a quadratic function to map old brightnesses to
new ones, making sure that an old brightness of N becomes 50% bright in the output. You
can override that 50% default with -middle. The value of -middle is a floating point num-
ber in the range 0 through 1 with 0 being full darkness and 1 full brightness. If your
-midvalue and -middle indicate an ambiguous or impossible quadratic function (e.g. -mid-
value is the same as -bvalue, so an infinite number of quadratic functions fit), pnmnorm
just ignores your -midvalue and maps linearly.
-midvalue and -middle were new in Netpbm 10.57 (December 2011).
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 over-
all intensity, 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 applying 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 components 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 difference between the intensities of the most and least intense RGB
component (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)
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master
documentation is at
http://netpbm.sourceforge.net/doc/pnmnorm.html
netpbm documentation 19 December 2014 Pnmnorm User Manual(0)
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