pnmgamma(1)                 General Commands Manual                pnmgamma(1)

       pnmgamma - perform gamma correction on a portable anymap

       pnmgamma [-ungamma] [-cieramp|-srgbramp] [value [pnmfile]]
       pnmgamma  [-ungamma] [-cieramp|-srgbramp] redgamma greengamma bluegamma

       Performs gamma correction on pseudo-PNM images.

       The PPM format specification specify that certain sample  values  in  a
       file  represent  certain light intensities in an image.  In particular,
       they specify that the sample values are directly proportional to gamma-
       corrected  intensity  values.  The gamma correction they specify is CIE
       Rec. 709.

       However, people sometimes work with approximations of PPM and PGM where
       the  relationship  between  the image intensities and the sample values
       are something else.  For example, the sample value  might  be  directly
       proportional  to  the  intensity with no gamma correction (often called
       "linear intensity").  Or a different gamma  transfer  function  may  be

       pnmgamma  allows  you to manipulate the transfer function, thus working
       with and/or creating pseudo-PPM  files  that  are  useful  for  various

       For  example, if you feed a true PPM to pnmgamma -cieramp -ungamma, you
       get as output a file which is PPM in every respect except that the sam-
       ple  values  are  directly proportional to the light intensities in the
       image.  If you feed such a file to pnmgamma -cieramp,  you  get  out  a
       true PPM.

       The situation for PGM images is analogous.  And pnmgamma treats PBM im-
       ages as PGM images.

       When you feed a linear PPM image to a display program  that  expects  a
       true  PPM,  the  display appears darker than it should, so pnmgamma has
       the effect of lightening the image.  When you feed a true PPM to a dis-
       play  program  that  expects linear sample values, and therefore does a
       gamma correction of its own on them, the display appears  lighter  than
       it  should,  so  pnmgamma  with a gamma value less than one (the multi-
       plicative inverse of whatever gamma value the display program uses) has
       the effect of darkening the image.

       The  only  parameters are the specification of the input image file and
       the gamma values.  Every gamma transfer function pnmgamma uses contains
       an exponent, which is the gamma value, and you can choose that value.

       Furthermore,  you can choose different values for each of the three RGB
       components.  If you specify only one gamma value,  pnmgamma  uses  that
       value for all three RGB components.

       If  you don't specify any gamma parameters, pnmgamma chooses a default.
       For the transfer functions defined by standards,  the  default  is  the
       value  defined by the standard.  If you specify anything else, you will
       be varying from the standard.  For the simple power  function  transfer
       function, the default gamma is 1/.45.

              Apply  the  inverse  of the specified transfer function (i.e. go
              from gamma-corrected nonlinear intensities  to  linear  intensi-

              Use  the  CIE Rec. 709 gamma transfer function.  Note that it is
              true CIE Rec. 709 only if you use the default gamma value  (i.e.
              don't  specify any gamma parameters).  This transfer function is
              a power function modified with a linear ramp near black.

              If you specify neither  -cieramp  nor  -srgbramp,  the  transfer
              function defaults to a simple power function.

              Use the Internation Electrotechnical Commission (IEC) SRGB gamma
              transfer function (as specified in the standard IEC  61966-2-1).
              Note  that  it  is  true  SRGB only if you use the default gamma
              value (i.e. don't specify any gamma parameters).  This  transfer
              function  is like the one selected by -cieramp, but with differ-
              ent constants in it.

              Note that SRGB is often spelled "sRGB".  In  this  document,  we
              use standard English typography, though, which doesn't allow for
              that kind of capitalization.

              If you specify neither  -cieramp  nor  -srgbramp,  the  transfer
              function defaults to a simple power function.

       A  good  explanation  of  gamma  is  in  Charles Poynton's GammaFAQ  at
       <> and  Col-
       orFAQ at <>

       In  brief:  The simplest way to code an image is by using sample values
       that are directly proportional to the intensity  of  the  color  compo-
       nents.   But  that  wastes the sample space because the human eye can't
       discern differences between low-intensity colors as well as it can  be-
       tween  high-intensity  colors.  So instead, we pass the light intensity
       values through a transfer function that makes it  so  that  changing  a
       sample  value by 1 causes the same level of perceived color change any-
       where in the sample range.  We store those resulting values in the  im-
       age file.  That transfer function is called the gamma transfer function
       and the transformation is called gamma correcting.

       Virtually all image formats, either specified or de facto,  use  gamma-
       corrected values for their sample values.

       What's  really  nice  about  gamma  is that by coincidence, the inverse
       function that you have to do to convert the gamma-corrected values back
       to  real light intensities is done automatically by CRTs.  You just ap-
       ply a voltage to the CRT's electron gun that  is  proportional  to  the
       gamma-corrected sample value, and the intensity of light that comes out
       of the screen is close to the intensity value you had  before  you  ap-
       plied the gamma transfer function!

       And  when  you consider that computer video devices usually want you to
       store in video memory a value proportional to the  signal  voltage  you
       want  to go to the monitor, which the monitor turns into a proportional
       drive voltage on the electron gun, it is really convenient to work with
       gamma-corrected sample values.


       Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.

                                 11 June 2001                      pnmgamma(1)
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