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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-stan‐
dard (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 ]8;;https://en.wikipedia.org/wiki/Luminosity_function\luminosity function]8;;\ ,
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 min‐
imum 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 conve‐
nient 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 max‐
iumum 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 in‐
tensity 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 uni‐
form spectrum of the same total intensity. The spectral method uses one of the simplest mea‐
sures of such a difference: the difference between the highest and lowest component intensi‐
ties, 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 equa‐
tion:

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 opera‐
tion 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 sam‐
ples, 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 ]8;;index.html#commonoptions\ Common Options]8;;\ ), 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 de‐
crease 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 im‐
age 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 program‐
mers 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 op‐
tion.