Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!mips!apple!radius!pierce
From: pierce@radius.com (Pierce T. Wetter III)
Newsgroups: comp.graphics
Subject: Re: Gamma correction (was: Radiosity Image Correction)
Message-ID: <1466@radius.com>
Date: 29 May 91 23:30:30 GMT
References: <1991May27.135349.5072@vax5.cit.cornell.edu> <14070@exodus.Eng.Sun.COM>
Lines: 55

>When is the graphics community going to catch on to the need for gamma
>correction in ALL shaded images?

>The broadcast television industry has settled on a standard gamma value of
>2.222222 (1.0/0.45).  This has been build into television sets for
>decades.  This value happens to look correct on all properly adjusted
>monitors that I have seen.

 What does properly adjusted mean?


SOAPBOX ON

  Television "as broadcast" is broadcast for viewing on a screen with a 
 gamma of 2.22. Your average run of the mill TV screens have a gamma of 
 2.6 to 2.8. 

  The value of 2.22 is chosen for its similarities to human vision as well
 as the added benifit of only correcting for gamma once at the broadcast
 station. By choosing 2.22, when quantizing to 8 bits, or 6 Mhz, 
 the maximum real information bandwidth is maintained. (Humans can
 see 1% differences, so a perfectly perceptual space would be logorithmic,
 but I digress.).

 If you really want to adjust your image for best display, you should adjust
 it for a gamma correction of 2.6 to 2.8 depending on your monitor. If you
 are sending this out to video tape, then by all means use 2.222, since you'll
 at least be conistent with everyone else.

  If you want the best possible display, you will have to take samples of
 every possible value of red,green and blue, and make a conversion table.
 Gamma is only the first order effect, in actual monitors all sorts of other
 non-linearities creep in (like the brightness and contrast knobs) that
 require seperate treatement.

>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.


SOAPBOX OFF


  Pierce

-- 
My postings are my opinions, and my opinions are my own not that of my employer.
You can get me at radius!pierce@apple.com.
(Wha'ja want? Some cute signature file? Hah! I have real work to do.