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
       ⟨https://en.wikipedia.org/wiki/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  ⟨index.html#commonoptions⟩ ), 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 ⟨#synopsis⟩

       •

              DESCRIPTION ⟨#description⟩

       •

              EXAMPLES ⟨#examples⟩

       •

              SATURATION METHODS ⟨#saturation_methods⟩

       •

              OPTIONS ⟨#options⟩

       •

              USAGE NOTES ⟨#usage_notes⟩

       •

              EXTENSIBILITY ⟨#extensibility⟩

       •

              SEE ALSO ⟨#seealso⟩

       •

              AUTHOR ⟨#author⟩

       •

              HISTORY ⟨#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