Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ucsd!network.ucsd.edu!celit!billd
From: billd@fps.com (Bill Davidson)
Newsgroups: comp.graphics
Subject: Re: Separate Gamma Values? (was: Re: standard RGB wavelengths?)
Message-ID: <10311@celit.fps.com>
Date: 27 Jul 90 02:09:47 GMT
References: <139554@sun.Eng.Sun.COM> <10273@celit.fps.com> <3312@ecs.soton.ac.uk>
Organization: FPS Computing Inc., San Diego, CA
Lines: 59

In article <3312@ecs.soton.ac.uk> mrd@ecs.soton.ac.uk (Mark Dobie) writes:
>In <10273@celit.fps.com> billd@fps.com (Bill Davidson) writes:
>>This makes me worry about possible extra problems for gamma
>>correction.  My understanding of gamma problems is that linear changes
>>in voltage do no give linear changes in brightness.  A gamma curve
>>plots the changes of voltage vs. brightness.  If one color of phosphor
>>is more efficient than another color, might this also change the gamma
>>curve?  Should we really have three separate gamma values (one each for
>>red, green and blue)?  This might help bring the color balance back a
>>bit closer to the proper value.
>>[more babbling deleted]
>>Am I just out in left field?
>
>I think this is a reasonable conclusion (but I'm no expert). I have a
>program (posted here a while ago) called ppmgamma which gamma corrects
>a ppm file. It allows you to specify separate gamma values for r,g,b
>so I think you're right.

Well, that's a poor way to judge it because *I* wrote that program so
any confusion I may have gets carried over into what I write.  I'm glad
to hear that somebody at least noticed it.  I wasn't sure if no news
was good news or if no one cared about gamma correction.  I've wondered
about this question of separate gamma values for some time but had
never bothered to ask about it before.  I figured I'd at least build
the capability into the program.

In any case, I received some mail from someone who at least sounds like
he knows what he's talking about.  Why he chose not to post I don't
know.  My confusion could cause damage if people start thinking along
the same lines so I'm including it here (name deleted since I didn't
ask permission but he works for a company which is well known for,
among other things, working with many types of CRT's at a hardware
level).

    light_output = K * volts**gamma.

    This power-law nonlinearity is in the grid-cathode-voltage-to-beam-
    current transfer characteristic, which in turn comes from the geometry
    of the CRT grids, cathodes, and Maxwell's equations.  Beam current can
    be measured in electrons/sec.  The phosphor efficiency is just a
    scaling constant (photons per electron) which happens after the
    nonlinearity (and so won't affect the exponent -- gamma -- of the
    power-law nonlinearity).  The geometry of the guns and grids in the
    CRT might have asymmetries that could result in differences in gamma
    among the three channels.

So, the need for three separate gamma curves can exist but it's
apparently not caused by differences in phosphor efficiency.

Ideally, a monitor should have a gamma value of 1.0.  I'm a little
suprised that I've never heard of gamma values being a marketing issue
(as in Monitor Company A sez: "Our monitors have a lower gamma than
Company B's monitors" and Company C sez: "....").  Certainly if I were
in the market for a high res color monitor and all other things were
equal, I'd buy the one with the lowest gamma value.  Gamma correction
may make up for losses in intensity but in doing so it sacrafices a
number of intermediate intesities.

--Bill Davidson