Manual: pamaltsat
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) meth‐
ods.
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 rais‐
ing 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 to‐
tal 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 in‐
creased. When the total intensity (or brightness) of a color is too high for the desired saturation, pamaltsat ap‐
plies 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 bal‐
ance, 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 ), pa‐
maltsat 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 in‐
crease 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 con‐
tribute. The only requirement is that every new method depend on a single strength parameter with the semantics de‐
scribed 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)