Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!icdoc!sot-ecs!mrd
From: mrd@ecs.soton.ac.uk (Mark Dobie)
Newsgroups: comp.graphics
Subject: Re: Separate Gamma Values? (was: Re: standard RGB wavelengths?)
Message-ID: <3312@ecs.soton.ac.uk>
Date: 26 Jul 90 09:02:51 GMT
References: <7833@ccncsu.ColoState.EDU> <101880022@hpcvlx.cv.hp.com> <139554@sun.Eng.Sun.COM> <10273@celit.fps.com>
Organization: University of Southampton, UK
Lines: 40

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.

This is my understanding also.

>don't recall Roy Hall's book mentioning this.  He does mention
>correcting for a given color monitor but nothing about separate gamma
>curves.  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.

Now a question:

On a related topic, in Foley and van Dam (the old one, I'm afraid :-()
they mention the logarithmic relationship between human's perception of
brightness and actual brightness. My question is, is this really a
hard and fast logarithmic relationship? or does this just fit the
experimental data best. Are there a variety of models for this? or is
there some (biological?) evidence pointing to the log relationship.

Just curious.

>--Bill Davidson


-- 

Mark Dobie                              M.Dobie@uk.ac.soton.ecs (JANET)
University of Southampton		M.Dobie@ecs.soton.ac.uk (Bitnet)