Path: utzoo!attcan!uunet!snorkelwacker!mintaka!ogicse!milton!wex@dali.pws.bull.com
From: wex@dali.pws.bull.com (Buckaroo Banzai)
Newsgroups: sci.virtual-worlds
Subject: Re: Who says what to whom (was Re: VR Protocols.)
Message-ID: <7801@milton.u.washington.edu>
Date: 17 Sep 90 22:03:14 GMT
References: <31304@unix.cis.pitt.edu> <7507@milton.u.washington.edu>
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In article <1990Sep14.190247.10645@watserv1.waterloo.edu> broehl@watserv1.waterl
oo.edu (Bernie Roehl) writes:
   My idea is that the attributes the building sends to the user *must* be
   defined by the user.  If I'm on a monochrome display, I tell the building
   not to send color.  I do this my not including 'color' is the list of
   attributes I want the building to send.

The problem is not one of implementation - of course you can construct
special cases for buildings, chairs, planes, balls, bats, etc. ad nauseum.
But wouldn't it be better to have a model of objects where you could define
some general rules (for what attributes you want to ask about) and just
specialize a few of them?

The other problem is that you're more or less violating the "object-ness" of
the model.  The more one object knows about the internal implementation of
another object, the less modular your implementation, etc.

   If I list an attribute  *it* doesn't know about, that's
   fine; it doesn't send it and I leave the attribute at its default value.

So what's the default value for something like "has a door I can walk
through"?  Do you see how much knowledge is presupposed simply in asking
that question?  This is what led me to want to separate out the real-world-
modeling aspects from the objects-that-implement-stuff aspects.

   [re gravity:] a Newtonian model is not necessarily what we want.  If I
   were designing reality, I might well choose not to implement gravity
   (even if doing so were easy, which it's not).  "Collisions" are often a
   *bad* thing. 

True, we might choose a relativistic model where gravity is a property of
the space in which the objects interact.  We did try one design in which
there was a special object known as Space, which contained a naive-physics
model and interacted with the objects in it.  Unfortunately, the machine
immediately bogged down in zillions of message passes, as everyone wanted to
talk to Space almost all the time.  I always thought this model was on the
right track, but people moved on before we could explore the possibilities
more fully.

   I think we're entering into what may be one of the great ongoing debates
   in the realm of VR: are we trying to model existing physical reality, or
   are we trying to define new worlds with new sets of properties and
   physical laws?

The problem is that the people paying the bills were airlines; they wanted a
system that more or less accurately modeled the real world.  Sure, it would
be nice if we could drop annoying things like gravity and collisions.
They're *hard*.  But that doesn't mean we can dodge them forever.  And I
don't favor building up a protocol/system which is going to collapse the
first time you throw a real problem at it.

--
--Alan Wexelblat                        phone: (508)294-7485
Bull Worldwide Information Systems      internet: wex@pws.bull.com
"Politics is Comedy plus Pretense."