Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!dali.cs.montana.edu!milton!hlab
From: hlab@milton.u.washington.edu (Human Int. Technology Lab)
Newsgroups: sci.virtual-worlds
Subject: Re: Who says what to whom (was Re: VR Protocols.)
Message-ID: <8128@milton.u.washington.edu>
Date: 25 Sep 90 18:02:10 GMT
References: <31304@unix.cis.pitt.edu> <7507@milton.u.washington.edu> <8077@milto
Organization: Tektronix Inc.
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Approved: hitl@hardy.u.washington.edu




In article <8077@milton.u.washington.edu> wex@dali.pws.bull.com (Buckaroo Banzai
) writes:
> 
> I can see two problems - one theoretical, and one implementational.  Theory
> first: The subdivision of space carries with it the implicit assumption that
> object have no effects outside their volume.  This will break down if you
> get to forces that act over a significant distance (say, the effect of solar
> wind and gravity on an earth-moon flight).

Ah, but that's only a problem in Newtonian universes where there is action
at a distance :-).  So let's make all our universes Einsteinian, and let
all forces be the result of local effects.  Global results will occur as
the result of communication between Space objects.  The cost for this is a
large volume of message traffic when the system starts up, or when
different parts of the universe first get connected, but that should reside
rapidly as the system relaxes to its steady state.

> 
> The implementational problem comes the first time you have an adhesive
> collision (say, ball into catcher's mitt) occurring at the boundary of two
> volumes.  There will be *lots* of message-passing going on.

I'm not sure that's necessarily true.  A lot of graphics and text layout
software (TeX and InterViews spring to mind) use a similar concept, in
which the individual components don't care about their positions, but are
all connected to special "glue" components which maintain local
relationships between non-glue components.  Global relationships are
maintained by modifying the parameters of the glue (say by adding "cost"
functions, in terms of physics, this is adding energy functions and
conservaton laws).

>  The more I think about it, the more I think I want a
> parallel language to do any kind of reasonable modeling.  Unfortunately, I
> don't know squat about parallel programming :-( That's why I was hoping to
> sneak by with a parallel implementation of an object-oriented language I did
> know.
> 

I can't see how we can do it without parallel programming.  But I think
your instinct was right: it's possible to hide the parallelism behind an
object-oriented model in which objects can run in parallel.  Only the
systems programmers need to face the synchronization problems directly.
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Bruce Cohen, Computer Research Lab        email: brucec@tekcrl.labs.tek.com
Tektronix Laboratories, Tektronix, Inc.                phone: (503)627-5241
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