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)