Setting Gamma For A Specific Monitor

Gamma correction is a crude way of compensating for the nonlinear response function of a display device. Most CRT monitors roughly follow a power law in their response function, ie:

	output luminance = signal ^ gamma

where gamma is typically between 1.5 and 3. Some system software attempts to partially compensate for this natural response function, but usually does not completely eliminate it (which would have other undesirable effects).

Most of the Radiance display drivers look at an environment variable called "DISPLAY_GAMMA" to determine how to set the gamma correction for the current monitor. Many programs also have a -g option for setting the gamma value explicitly, but the environment variable is the only way to control gamma correction for rview, for example.

To determine the approximate gamma for your monitor, display of the distributed picture file ray/lib/lib/gamma.pic like so:

	% ximage -g 1 -b ray/lib/lib/gamma.pic &

Note that the gamma here has been explicitly set to 1, effectively turning gamma correction off for this test.

Now, set the contrast and brightness controls on the monitor to what you would consider your normal settings. Stand back from the monitor a bit and try to match up the average brightness of the stripes on the left of the image with a patch on the right of the image. This will be the gamma value for this monitor.

All that's left to do is to put a line of the form:

	setenv DISPLAY_GAMMA 2.2

in your .login or .cshrc file for C-shell users, or:

	DISPLAY_GAMMA=2.2
	export DISPLAY_GAMMA

in your .profile for Bourne shell users. You may also use a more complicated setting for different monitors if you use many displays, eg:

	if ( $?DISPLAY && ! $?DISPLAY_GAMMA ) then
		switch ($DISPLAY)
		case pink*:
		case floyd*:
			setenv DISPLAY_GAMMA 1.8
			breaksw
		case hobbes*:
		case demo460*:
			setenv DISPLAY_GAMMA 2.6
			breaksw
		case wimsey*:
			setenv DISPLAY_GAMMA 1.7
			breaksw
		endsw
	endif