pamaltsat(1) - Man pages

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 ]8;;https://en.wikipedia.org/wiki/Luminosity_function\luminosity function]8;;\ , rather than brightness, or the nu-
merical 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 inten-
sity 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 satura-
tion, 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 spec-
trum of the same total intensity. The spectral method uses one of the
simplest measures of such a difference: the difference between the high-
est 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, pa-
maltsat 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 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 inten-
sities. For such samples, pamaltsat decreases the strength to the high-
est 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 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 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 vari-
ation of a Netpbm image forat, in which the samples are propor-
tional 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 require-
ment is that every new method depend on a single strength parameter with
the semantics described under the -strength command-line option.