[ViewVC] Annotation of: radiance/ray/doc/notes/gamma_corr

                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 rvu, 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
Revision:	1.2
Committed:	Thu Jan 1 19:31:45 2004 UTC (20 years, 4 months ago) by greg
Branch:	MAIN
CVS Tags:	rad5R4, rad5R2, rad4R2P2, rad5R0, rad5R1, rad3R7P2, rad3R7P1, rad4R2, rad4R1, rad4R0, rad3R6, rad3R6P1, rad3R8, rad3R9, rad4R2P1, rad5R3, HEAD
Changes since 1.1:	+1 -1 lines
Log Message:	Renamed rview, lam, calc, and neat to rvu, rlam, icalc, and neaten
#	User	Rev	Content
1	greg	1.1	SETTING GAMMA FOR A SPECIFIC MONITOR
2
3			Gamma correction is a crude way of compensating for the nonlinear
4			response function of a display device. Most CRT monitors roughly follow
5			a power law in their response function, ie:
6
7			output luminance = signal ^ gamma
8
9			where gamma is typically between 1.5 and 3. Some system software
10			attempts to partially compensate for this natural response function, but
11			usually does not completely eliminate it (which would have other
12			undesirable effects).
13
14			Most of the Radiance display drivers look at an environment variable
15			called "DISPLAY_GAMMA" to determine how to set the gamma correction for the
16			current monitor. Many programs also have a -g option for setting the
17			gamma value explicitly, but the environment variable is the only way to
18	greg	1.2	control gamma correction for rvu, for example.
19	greg	1.1
20			To determine the approximate gamma for your monitor, display of the
21			distributed picture file ray/lib/lib/gamma.pic like so:
22
23			% ximage -g 1 -b ray/lib/lib/gamma.pic &
24
25			Note that the gamma here has been explicitly set to 1, effectively
26			turning gamma correction off for this test.
27
28			Now, set the contrast and brightness controls on the monitor to what you
29			would consider your normal settings. Stand back from the monitor a bit
30			and try to match up the average brightness of the stripes on the left of
31			the image with a patch on the right of the image. This will be the
32			gamma value for this monitor.
33
34			All that's left to do is to put a line of the form:
35
36			setenv DISPLAY_GAMMA 2.2
37
38			in your .login or .cshrc file for C-shell users, or:
39
40			DISPLAY_GAMMA=2.2
41			export DISPLAY_GAMMA
42
43			in your .profile for Bourne shell users. You may also use a more
44			complicated setting for different monitors if you use many displays, eg:
45
46			if ( $?DISPLAY && ! $?DISPLAY_GAMMA ) then
47			switch ($DISPLAY)
48			case pink*:
49			case floyd*:
50			setenv DISPLAY_GAMMA 1.8
51			breaksw
52			case hobbes*:
53			case demo460*:
54			setenv DISPLAY_GAMMA 2.6
55			breaksw
56			case wimsey*:
57			setenv DISPLAY_GAMMA 1.7
58			breaksw
59			endsw
60			endif