Pamaltsat User Manual(1) General Commands Manual Pamaltsat User Manual(1)
NAME
pamaltsat - increase or decrease the saturation of an image using one
of several alternative methods.
SYNOPSIS
pamaltsat [-method name] [-strength number] [-linear] [infile]
DESCRIPTION
This program is part of Netpbm(1).
pamaltsat decreases or increases the saturation of a Netpbm image by
one of various non-standard (alternative) methods.
The input is a Netpbm image from Standard Input or a file named by the
arguments. The output is a Netpbm image in the same format written to
Standard Output.
The most conventional way to change the saturation of an image is what
pambrighten does.
EXAMPLES
To increase saturation by a factor of 2.1 using the logarithmic method:
pamaltsat -method log -strength 2.1 test.ppm
To convert a color image to grayscale:
pamaltsat -strength 0 test.ppm
SATURATION METHODS
The following saturation methods are available.
Logarithmic Method
This saturation model is inspired by the concept of color integrity(1),
which works with color in terms of intensity ratios, where intensity is
a value of the luminosity function
⟨https://en.wikipedia.org/wiki/Luminosity_function⟩ , rather than
brightness, or the numerical value of the sample in the image file.
From this viewpoint, it is natural to define the saturation of a color
as the ratio of maximum and minimum intensities of its primary compo-
nents. In order, however, to make saturation an additive value and to
endow the -strength parameter with the semantics of a multiplier, it is
convenient to operate on the logarithm of that ratio. The addition of
such saturations acquires physical sense, and multiplication corre-
sponds to the raising of intensity to the power of the multiplier.
With this method, pamaltsat raises the intensity of each component to
the power of the strength value. Since the total intensity of the re-
sulting color will differ from that of the original, it is necessary to
restore the intensity by multiplying the component intensities of the
saturated color by the ratio of the intensity of the original color to
that of the saturated color.
Although it is always possible to decrease saturation by any given fac-
tor, there are two cases where it cannot be increased. When the total
intensity (or brightness) of a color is too high for the desired satu-
ration, pamaltsat applies the maximum possible saturation that keeps
the original intensity. For example, any color with at least one com-
ponent at the maxiumum is already fully saturated. When one of the
primary components is zero, the definition of saturation given above no
longer works because of a zero in the denominator. pamaltsat offers no
special treatment of this situation because it does not create discon-
tinuities and therefore produces no visible defects at reasonable
strength levels. When, however, strength approaches infinity, each
color tends to its primary component with the highest intensity.
This method was invented by Anton Shepelev.
Spectral Method
This is the default method. It treats color as a spectrum with three
bands: one for the intensity of each primary component. Since neutral
gray has a uniform (constant) spectrum, saturation can be measured as
the difference of the spectrum of the given color from the uniform
spectrum of the same total intensity. The spectral method uses one of
the simplest measures of such a difference: the difference between the
highest and lowest component intensities, which is an additive value
and therefore amenable to multiplication with good physical sense. Al-
though a complete hue-saturation model can be dervied from this ap-
proach, pamaltsat need not concern itself with it because it always
preserves both hue and total intensity.
In order to change saturation, pamaltsat first multiplies the intensity
of each component by the desired strength. The saturation of the re-
sult is the strength times the saturation of the original, and likewise
the total intensity, but it is then restored by subtraction of the neu-
tral gray with a suitable intensity.
The effect of this method on each component intensity may be expressed
in the following equation:
sat = orig * strength - Iorig * (strength - 1)
where sat is the saturated sample, orig the original sample, and Iorig
the total intensity of the original color.
The method is also related to color integrity because both its opera-
tions are part of that concept: multiplication of component intensities
by the same quotient is an important operation because changes bright-
ness but keeps color balance, and subtraction of a constant from all
component intensities is described by the inventor of color integrity
as 'subtraction of white.'
This procedure may produce both negative and over-unity component in-
tensities. For such samples, pamaltsat decreases the strength to the
highest value that keeps the resulting color in range.
This method was invented by Anton Shepelev.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pamaltsat recognizes the
following command line options:
-method method
specifies the saturation method to use:
The default is spectrum
-strength strength
This specifies a real nonnegative factor whereby to modify satu-
ration. A value greater than unity will increase saturation,
whereas a value less than unity will decrease it. Unity will
leave the image unchanged, and zero will produce greyscale out-
put according to Rec 709.
pamaltsat preserves the total intensity of each pixel and never
affects neutral gray pixels.
This option is mandatory.
-linear
This tells pamaltsat that the input is the intensity-linear
variation of a Netpbm image forat, in which the samples are pro-
portional to light intensity rather than to brightness, as they
are in true-or gamma-adjusted- Netpbm image formats.
USAGE NOTES
Since pamaltsat does not affect neutral colors, you should adjust the
color balance before saturation. You can do this with pamlevels.
EXTENSIBILITY
pamaltsat is written with an eye to extending it with new saturation
methods, which programmers are welcome to contribute. The only re-
quirement is that every new method depend on a single strength parame-
ter with the semantics described under the -strength command-line op-
tion.
SEE ALSO
pambrighten(1), ppmflash(1),
AUTHOR
This program was first submitted by Anton Shepelev (an-
ton.txt@gmail.com).
HISTORY
pamaltsat was new in Netpbm 10.84 (September 2018).
Table Of Contents
•
SYNOPSIS ⟨#synopsis⟩
•
DESCRIPTION ⟨#description⟩
•
EXAMPLES ⟨#examples⟩
•
SATURATION METHODS ⟨#saturation_methods⟩
•
OPTIONS ⟨#options⟩
•
USAGE NOTES ⟨#usage_notes⟩
•
EXTENSIBILITY ⟨#extensibility⟩
•
SEE ALSO ⟨#seealso⟩
•
AUTHOR ⟨#author⟩
•
HISTORY ⟨#history⟩
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/pamaltsat.html
netpbm documentation 14 September 2018 Pamaltsat User Manual(1)
Generated by dwww version 1.15 on Sat Jun 29 02:33:16 CEST 2024.