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From: webber@KLINZHAI.RUTGERS.EDU.UUCP
Newsgroups: mod.computers.workstations
Subject: Re: Graphics on SUN's;  SUN tape drive operations
Message-ID: <132@brandx.klinzhai.RUTGERS.EDU>
Date: Thu, 26-Mar-87 02:46:40 EST
Article-I.D.: brandx.132
Posted: Thu Mar 26 02:46:40 1987
Date-Received: Sat, 28-Mar-87 07:04:21 EST
References: <14@auscso.UUCP> <8703252224.AA09472@tcgould.TN.CORNELL.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 39
Keywords: Choosing 256 colors, speeding up tape drives, bug in tape driver
Approved: works@red.rutgers.edu
Summary: Gamma correction is not exponential (something else is though)

In article <8703252224.AA09472@tcgould.TN.CORNELL.EDU>,
cheryl@TCGOULD.TN.CORNELL.EDU (cheryl) writes: 
> The 255 different levels are 255 settings on the VOLTAGES of each
> of the 3 color guns.  It is a well-known principle of computer
> graphics that there is a nonlinear relationship between these these
> voltage settings and the actual intensities that are displayed on
> the screen.  In fact, it is an exponential relationship.  Standard
> nomenclature refers to the factor in this exponential relationship
> as GAMMA, hence the phrase "GAMMA correction."  Since the gamma
> correction necessary varies from monitor to monitor, most
> manufacturers of graphics hardware leave it up to the on-site
> computer graphics wiz to do the gamma correction herself -- or
> himself as the case may be.

The nonlinear relationship between the voltage and light energy
emitted by the phosphors is indeed GAMMA, but this is not an
exponential relation.  It is a low order polynomial, roughly x to the
2.2 (on average maxing around x to the 2.8 for some monitors).

There is an exponential relationship between light energy and the
neuropsychological perception of brightness, but that is another
matter (i.e., you would percieve x, 1.02 * x, 1.02 * 1.02 * x, 1.02 *
1.02 * 1.02 * x, etc. as a linear progression in brightness although
x is light energy).

Thus GAMMA correction is a monitor hardware internal problem whereas
the exponential intensity perception is a psychoneurological
phenomenon.

Although there are doubtless many technical references on this
material, a good introduction to this sort of thing is:
      Raster Graphics Handbook (Second Edition), by Conrac Division,
      Conrac Corporation, published by Van Nostrand Reinhold Company,
      in 1985.


- ------------------------------------ BOB (webber@aramis.rutgers.edu
                                            webber@red.rutgers.edu
                                            topaz!webber)