Newsgroups: comp.graphics
Path: utzoo!utgpu!watserv1!watcgl!imax!dave
From: dave@imax.com (Dave Martindale)
Subject: Re: Gamma correction (was: Radiosity Image Correction)
Message-ID: <1991Jun6.164951.21349@imax.com>
Organization: Imax Corporation, Oakville Canada
References: <1991May27.135349.5072@vax5.cit.cornell.edu> <14070@exodus.Eng.Sun.COM> <1466@radius.com>
Date: Thu, 6 Jun 1991 16:49:51 GMT

In article <1466@radius.com> pierce@radius.com (Pierce T. Wetter III) writes:
>
>>Also, since gamma correction is attempting to correct for the nonlinearity
>>of the CRT electron guns, and since the three electron guns in a color CRT
>>are of the same design, the gamma values for the three channels should be
>>the same.
>
>  Nope, there three different guns, and the monitor has about 3 pots for each
>  gun that can be tweaked + the brightness and contrast knobs. If you're 
> going to approximate the brightness/voltage curve with one parameter, you'll
> find its different for each gun.

All the discussion about gamma correction assumes that the monitor has
been properly adjusted in the first place.  The black level should be
set so that the guns are just barely cut off with zero input voltage,
and so that very dim greys are neutral in colour.  The individual
colour gains should be set to give white of the correct colour
temperature.  A grey scale that goes from black to white should
remain the same colour temperature throughout the scale.

If you've adjusted your monitor so it meets all these requirements,
I think you'll find that this constrains the adjustment of almost
every control in the monitor except "contrast".  When this is done,
you should find that the brightness/voltage curves for the three
colours track quite accurately (if they didn't, the grey scale
would show colour shifts).  And that a single value for gamma
pretty accurately describes how the monitor responds to voltage.

If the monitor is misadjusted, then the best gamma-correction value
may be different for each colour.  But in that case, the transfer
function of at least one of RGB is likely to look nothing like
the power-law function that gamma correction models, and gamma
correction isn't going to provide good compensation for this.
You need something more powerful than simple gamma correction.
Fixing the monitor's calibration is probably easier than fixing
the model.

If you want to get more accurate, you could take a bunch of brightness
vs. voltage measurements and fit a curve to it - the relationship
of brightness to voltage is not an exact power-law curve, and a bunch
of numbers can describe it better than one.  But still, the three
colours should track each other pretty closely unless the monitor
is misadjusted.

	Dave Martindale