pnmgamma(1)						      General Commands Manual						       pnmgamma(1)

NAME

       pnmgamma - perform gamma correction on a PNM image

SYNOPSIS

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

DESCRIPTION

       Reads a psuedo-PNM image as input.  Performs gamma correction, and produces a PNM image as output.

       Alternatively, this program can undo gamma correction.

       The  PPM  specification	states	that the sample values in the image are nonlinear, i.e. not directly proportional to light intensity, i.e.
       gamma corrected.  But there exist images that are PPM in every respect except that their sample values are directly proportional  to  light
       intensity.   People may loosely refer to these as PPM, but they are not.  pnmgamma converts these pseudo-PPM images to true PPM by perform-
       ing gamma correction.  To get true PPM, you must specify the -cieramp option and no gamma values.  That causes pnmgamma to  apply  the  CIE
       Rec. 709 gamma transfer function, as specified by the PPM format specification.

       On  the	other  hand, you can use the -ungamma option to convert from true PPM to linear RGB pseudo-PPM.  (Again, if the input is true PPM,
       specify the -cieramp option and no gamma values).

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

       You can also apply a different transfer function (which means you don't end up with a true PPM image) by  selecting  the  gamma	values	as
       arguments  or  omitting	the -cieramp option.  The gamma value is the power to which the input value is raised in the transfer function.  A
       value of 1 means the output is the same as the input.  A value less than one makes the output samples numerically less than the input  sam-
       ples; A value greater than one makes the samples numerically greater.

       Without	the -cieramp option, the transfer function is a simple power function.	With -cieramp, it is a power function modified with a lin-
       ear ramp near black, as described in CIE Rec. 709.

       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.

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 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)