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SETTING GAMMA FOR A SPECIFIC MONITOR
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Gamma correction is a crude way of compensating for the nonlinear
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response function of a display device. Most CRT monitors roughly follow
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a power law in their response function, ie:
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output luminance = signal ^ gamma
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where gamma is typically between 1.5 and 3. Some system software
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attempts to partially compensate for this natural response function, but
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usually does not completely eliminate it (which would have other
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undesirable effects).
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Most of the Radiance display drivers look at an environment variable
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called "DISPLAY_GAMMA" to determine how to set the gamma correction for the
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current monitor. Many programs also have a -g option for setting the
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gamma value explicitly, but the environment variable is the only way to
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control gamma correction for rvu, for example.
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To determine the approximate gamma for your monitor, display of the
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distributed picture file ray/lib/lib/gamma.pic like so:
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% ximage -g 1 -b ray/lib/lib/gamma.pic &
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Note that the gamma here has been explicitly set to 1, effectively
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turning gamma correction off for this test.
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Now, set the contrast and brightness controls on the monitor to what you
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would consider your normal settings. Stand back from the monitor a bit
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and try to match up the average brightness of the stripes on the left of
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the image with a patch on the right of the image. This will be the
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gamma value for this monitor.
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All that's left to do is to put a line of the form:
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setenv DISPLAY_GAMMA 2.2
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in your .login or .cshrc file for C-shell users, or:
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DISPLAY_GAMMA=2.2
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export DISPLAY_GAMMA
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in your .profile for Bourne shell users. You may also use a more
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complicated setting for different monitors if you use many displays, eg:
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if ( $?DISPLAY && ! $?DISPLAY_GAMMA ) then
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switch ($DISPLAY)
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case pink*:
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case floyd*:
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setenv DISPLAY_GAMMA 1.8
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breaksw
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case hobbes*:
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case demo460*:
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setenv DISPLAY_GAMMA 2.6
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breaksw
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case wimsey*:
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setenv DISPLAY_GAMMA 1.7
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breaksw
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endsw
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endif
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