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NAME

       pnmgamma - perform gamma correction on a portable anymap

SYNOPSIS

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

DESCRIPTION

       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 used.

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

       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 sample 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 images 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 display 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 multiplicative inverse of whatever gamma
       value the display program uses) has the effect of darkening the image.

PARAMETERS

       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.

OPTIONS

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

       -cieramp
              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.

       -srgbramp
              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 different
              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.

WHAT IS GAMMA?

       A   good   explanation   of   gamma    is    in    Charles    Poynton's    GammaFAQ     at
       <http://www.inforamp.net/~poynton/ColorFAQ.html>          and          ColorFAQ         at
       <http://www.inforamp.net/~poynton/GammaFAQ.html>

       In brief: The simplest way to code an image is by using sample values  that  are  directly
       proportional  to  the intensity of the color components.  But that wastes the sample space
       because the human eye can't discern differences between low-intensity colors as well as it
       can between 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 anywhere in the sample range.  We store those resulting
       values in the image 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 apply 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  applied  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.

SEE ALSO

       pnm(5)

AUTHOR

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

                                           11 June 2001                               pnmgamma(1)