Path: utzoo!utgpu!watmath!clyde!att!pacbell!ames!sgi!bam@sbc2.SGI.COM
From: bam@sbc2.SGI.COM (Brian A. McClendon)
Newsgroups: comp.graphics
Subject: Re: Color lookup tables
Summary: more on color tables: SGI and Intergraph
Message-ID: <22812@sgi.SGI.COM>
Date: 2 Dec 88 23:18:48 GMT
References: <3048@cs.Buffalo.EDU>
Sender: daemon@sgi.SGI.COM
Distribution: na
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 45

article <3048@cs.Buffalo.EDU>, lively@sunybcs.uucp (Richard S. Lively) writes:
> (deleted)
> When I asked how different windowing environments handle this, I received
> the following responses:
> 
>            SunView     -     any window except active window is black
>            X           -     does not allow creation of the second window
>            Mac II      -     separate lookup table for each window (but
>                              not exactly perfect)
> 
	     SGI (GL)	 -     When a LUT is loaded by a process, it REALLY
	     		       gets loaded.  Any info about the previous 
	     		       contents is lost.  In the 12 bitplane case,
	     		       16 different 256 entry tables can coexist, but
	     		       it is up to the application to avoid
	     		       others' tables.
	     Intergraph  -     Each process (not each window) has a 'virtual'
	     (Environ V)       LUT that is only loaded when the process
	     		       has the input focus.  New processes adopt
	     		       the current LUT as an initial state to minimize
	     		       ugliness.
	     		       
> Are these answers correct?  If so, what design decisions prompted these
> choices?  Is there any reason that each window can't have a separate
> lookup table which can become active or inactive dynamically in the
> middle of a scan line?

Loading a LUT in mid scanline is usually unrealistic/impossible.  Many of
the workstation-rated(KxK +) output tables can't be loaded anytime except
vertical blank and when the tables hit upwards of 4096 entries, it is 
difficult to load completely in only one VB.

The Atari ST has some programs (Spectrum 512 is the main example) that
produce up to 512 colors on the screen at once using only 4 bits/pixel.
They load up to about 16 entries per horizontal scanline but that trick
is a hack, not a Atari-approved feature.


--

			- brian

Brian McClendon
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