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 (*alt*ernative) 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:

where sat is the saturated sample, orig the original sample, and Iorig the total intensity of the original color.

sat = orig * strength - Iorig * (strength - 1)

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¶

# 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¶

# DOCUMENT SOURCE¶

This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at

14 September 2018 | netpbm documentation |