Provided by: netpbm_11.05.02-1.1build1_amd64 
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 brightest: 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 constraints: 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 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 ways to define brightness:
• luminosity
• color value
• 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 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
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common
Options ), pnmnorm recognizes the following command line 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 different 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.
-bpercent
-wpercent
-bvalue
-wvalue
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 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. (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 pnmnorm,
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, respectively.
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, -bpercent=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.
-bsingle
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).
-maxexpand
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 tolerate, 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. pnmnorm 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 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).
-keephues
This 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.
-midvalue=N
-middle=N
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 number 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. -midvalue 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 overall 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
-saturation
-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), pamlevels(1), ppmhist(1), pgmhist(1), pambrighten(1), ppmdim(1), pnmgamma(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(1)