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From: cheryl@TCGOULD.TN.CORNELL.EDU.UUCP
Newsgroups: mod.computers.workstations
Subject: Re: Graphics on SUN's;  SUN tape drive operations
Message-ID: <8703252224.AA09472@tcgould.TN.CORNELL.EDU>
Date: Wed, 25-Mar-87 17:24:41 EST
Article-I.D.: tcgould.8703252224.AA09472
Posted: Wed Mar 25 17:24:41 1987
Date-Received: Fri, 27-Mar-87 04:04:45 EST
References: <14@auscso.UUCP>
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: cheryl%batcomputer.UUCP@tcgould.tn.cornell.edu (cheryl)
Distribution: world
Organization: Theory Center, Cornell University, Ithaca NY
Lines: 25
Keywords: Choosing 256 colors, speeding up tape drives, bug in tape driver
Approved: works@red.rutgers.edu

In article <14@auscso.UUCP> johnl@auscso.UUCP (John Lange) writes:
>I have recently begun to play with the graphics capabilities of our
>SUN3/160.  It has a color display that is capable of displaying a
>total of 256 colors on the screen at one time.  This is done by
>choosing from a pallette of 2^24 colors.  To set the colormap of 256
>colors, it is necessary to specify red, green, and blue indeces of
>[0] to [255].  In my endeavors, I have discovered that approx. the
>first 100 levels of indeces in the red, green, and blue come out
>looking virtually identical - they are all VERY dark.  This may be a
>hardware problem with our particular SUN.  It seems a bit wasteful
>to have 256 indeces when only the last 150 or so look different.  It

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.

Cheryl Stewart
MSI Computer Support