Pamlevels User Manual(1) General Commands Manual Pamlevels User Manual(1)
NAME
pamlevels - effect a 'levels' transformation
SYNOPSIS
pamlevels -from1 from1 -to1 to1 -from2 from2 -to2 to2 [-from3 from3
-to3 to3] [-linear] [-fitbrightness] [pamfile]
DESCRIPTION
This program is part of Netpbm(1).
pamlevels reads a Netpbm image from pamfile or standard input, applies
a 'levels' transformation, and sends the result to standard output. It
finds a unique transformation that maps color from1 to to1, color from2
to to2, and-if -from3 and -to3 are supplied-color from3 to to3. Trans-
formations based on two mappings are linear in light intensity and
those based on three mappings quadratic. The color-mapping options may
be shortened to -f1...-f3 and -t1...-t3.
Color values have the following format: color[:scale], where color is a
Netpbm color specification ⟨libppm.html#colorname⟩ and scale an op-
tional coefficient that is applied to the intensity (i.e. not gamma-ad-
justed) of each RGB component of color.
When the transformation is linear (i.e. uses two mappings) and pre-
serves zero (i.e. maps black to black), it corresponds to multiplica-
tion of light intensity by a constant and preserves color integrity(1).
EXAMPLES
In the examples below, \ denotes a line continuation:
To brighten an image by setting a darker white point-
pamlevels -f1 black -t1 black \
-f2 white:0.9 -t2 white in.ppm > out.ppm
To adjust the white point-
pamlevels -f1 black -t1 black \
-f2 rgbi:0.9/0.83/0.80 -t2 white in.ppm > out.ppm
To set a lighter black point-
pamlevels -f1 white:0.06 -t1 black \
-f2 white -t2 white in.ppm > out.ppm
To increase brightness by compression-
pamlevels \
-f1 black -t1 black -f2 white -t2 white \
-f3 white:0.5 -t3 white:0.6 in.ppm > out.ppm
The latter tranformation is similar to gamma-correction.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pamlevels recognizes the
following command line options:
-from1
-to1
-from2
-to2
-from3
-to3 These options define the mappings of input colors to output col-
ors that anchor the transformation function. See DESCRIPTION
⟨#description⟩ .
You must specify at least two of these pairs.
-linear
This option tells pamlevels to work with the intensity-linear
variation on PPM where the samples are proportional to light in-
tensity, rather than brightness (gamma-adjusted) as in true PPM.
The input must be of this form and the pamlevels makes the out-
put of this form.
Note that the numbers in a color specification like
rgbi:0.9/0.83/0.80 are brightness levels (gamma-adjusted) re-
gardless of the input and output format.
You cannot use this with -fitbrightness because that function is
not implemented.
-fitbrightness
This option selects a transformation which is not very useful -
it is linear or quadratic in brightness rather than light inten-
sity of the pixels. There is no physical basis for doing it
this way and the result is normally undesirable.
Note that many tools other than Netpbm do the transformation
this way. One use for this option is simply to demonstrate the
poor result of this method.
One advantage of this transformation is that it is faster, be-
cause the input and output image formats use brightness values.
The result is approximately correct.
You cannot use this with -linear because that function is not
implemented.
SEE ALSO
pnmnorm(1), pamrecolor(1), pnmgamma(1), pnm(1)
AUTHOR
This program was first submitted by Anton Shepelev (an-
ton.txt@gmail.com).
HISTORY
pamlevels was new in Netpbm 10.83 (June 2018).
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/pamlevels.html
netpbm documentation 13 June 2018 Pamlevels User Manual(1)
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