Provided by: netpbm_11.05.02-1.1build1_amd64 
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 , 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 components. 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 corresponds 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 resulting 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 factor, there are two cases where it
cannot be increased. When the total intensity (or brightness) of a color is too high for the desired
saturation, pamaltsat applies the maximum possible saturation that keeps the original intensity. For
example, any color with at least one component 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 discontinuities 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. Although a complete hue-saturation model can be dervied from
this approach, 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 result is the strength times the saturation of the original, and
likewise the total intensity, but it is then restored by subtraction of the neutral 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 operations are part of that concept:
multiplication of component intensities by the same quotient is an important operation because changes
brightness 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 intensities. 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 ), 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 saturation. 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 output 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 proportional 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 requirement is that every new method depend on a single strength
parameter with the semantics described under the -strength command-line option.
SEE ALSO
pambrighten(1), ppmflash(1),
AUTHOR
This program was first submitted by Anton Shepelev (anton.txt@gmail.com).
HISTORY
pamaltsat was new in Netpbm 10.84 (September 2018).
Table Of Contents
•
SYNOPSIS
•
DESCRIPTION
•
EXAMPLES
•
SATURATION METHODS
•
OPTIONS
•
USAGE NOTES
•
EXTENSIBILITY
•
SEE ALSO
•
AUTHOR
•
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)