pnmgamma(1)                                                   General Commands Manual                                                  pnmgamma(1)

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 val-
       ues 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  multi-
       plicative 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 con-
       tains 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  trans-
       fer 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  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
       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)