From: s047@sand.sics.bu.oz.au (Jeremy Lee)
Newsgroups: sci.virtual-worlds
Subject: Re: TECH: My standard is better than your standard.
Date: 28 Jul 92 04:00:47 GMT
Organization: University of Washington
Originator: hlab@milton.u.washington.edu



In article <Bs08MI.51v@watserv1.waterloo.edu> Bernie wrote:

>In article<1992Jul26.075108.9516@u.washington.edu> dlwood@mailbox.
>syr.edu (David L Wood) writes:
>
>>If the universe is defined by a message router [...]
>
>That's the point of contention, really... but more on this later...
>
>>that the message router should only inform you
>>(the client) of the existence of an object only if it occupies one or more
>>pixels after being projected with perspective shrinkage.
>
>Right.  Of course, this depends on what your display's resolution is; do we
>want the router to have to deal with things like that?  If it does, it isn't
>just a router... it's something else entirely.

Exactly. Routers should just pass the messages.

>The other problem is that this still doesn't reduce the amount of data enough.
>If I look out my window right now, I can see the house across the street.
>Including the house in my rendering database is no problem; however, including
>all its contents (down to the forks and knives in the kitchen drawers) would
>be.  Yet if it weren't for intervening walls, I could certain see a fork
>at that distance.
>
>With just size/distance clipping, most of the objects in the house wouldn't
>get clipped, even though I can't see them; more memory and more workload for
>my poor renderer to have to deal with.

This is the problem that graphics people have been working on for the
better part of 20 years. There is no simple solution.

>>I don't know about you, but objects flickering in and out in my peripheral
>>vision would be a bit distracting! :)  But, clipping is best handled by the
>>renderer, not the message router.
>
>It's all related, though...
>If someone moves around in the hypothetical house across the street, do I need
>to know about it?  Based on size/distance, yes... based on obstruction by
>walls, no.  And even if I can see it, I can't necessarily interact with it
>(we need to reduce the number of object-object collision tests that must be
>done, since otherwise it's an N-squared problem and *very* expensive).

No, It's not really an N squared problem. To begin with, each object
only has to deal with N other objects. Second, you don't have to check
all the collisions all the time, only when an object moves. There are
tricks and tips and neat things to do that will make the problem of
colision detection less hard.

And don't be restricted by a visual model. I may not be able to see it,
but can I hear it? Am I touching it?

>>I really like the idea of a message router controlling the passing
>>of messages between objects in a world.
>
>Which raises all the problems of centralization.  A new object enters the world
,
>the router has to do size/distance testing between it and each of the 10,000
>objects already there.

Where if it is distribueted, then we can go from an N squared worst case
to an N best case.

>>the clients should never be trusted to
>>perform any task that you can do yourself (where "you" is the server).
>
>That's a fundamental philosophical difference between us.  The server should
>never do anything that can be handled by the clients, since there is far more
>computing power in the clients (of which there are many) that there is in the
>server (of which there is but one). The router should do as little as possible,
>and if we can figure out a way to eliminate it altogether, so much the better;
>otherwise, sooner or later, it will become the bottleneck.

Hear hear. In fact, if we make the routers just transparent networking
message passers, then they do fade from the overall view.

>>128 bits is not enough.
>
>Not enough for *what*?  Quantum-level descriptions of the *entire universe* can
>be handled in 128 bits.  It's not like internet addresses or color palettes or
>DOS memory; we're talking the basic physical dimensions of the universe, here.

And the universe is bigger even than any of OUR puny imaginings. I feel
that 128 bit numbers are correct.

>>You could use a system of variable length coordinates.  An id byte would
>>indicate how many bits (or bytes, take your choice) are used to locate a
>>coordinate.  Really, anything to extend the ability of the protocol
>>is better than putting a limit like 16 bytes.
>
>I agree with this wholeheartedly.

I don't. Imagine the extra work you have to do converting it between
formats, even to do simple arithmetic! If it's that much of a problem,
then here....

Char:  8
Tiny:  16
Short: 32
Long:  64
Big:   128

And we'll use them as basic declarations, to use when appropriate. Type
conversion will be transparent, and easier to manage then if we have to
stick another byte-field on to record total bit length. We'll just do it
the way C does it, but with a bigger range.

Now. Everybody happy? Good. Next problem.

>>I'll admit 16 bytes is
>>fairly hefty, but if you want to dump an entire universe into a file [...]
>
>Wow, between your high-capacity disk drives and Jeremy Lee's processor, you'd
>have one heck of a VR station!

Wouldn't we!

>>Honestly, how much CPU time is burned up by sorting out messages and
>>occassionally computing a size over distance function? Not much.
>
>For 10,000 objects at 30 frames/sec?  Using Cray XXII's as routers may
>be a swell idea, but here in Canada our research budgets are kind of limited
>right at the moment...

Sounds all to familiar....

>>You NEED a central control by definition of DOMAIN.
>
>A fundamental centralization vs decentralization debate.
>
>I think we're zooming in on the most important issues here... let's keep at it.

Decentralization. Vote for anarchy!!! 

>        Bernie Roehl, University of Waterloo Electrical Engineering Dept

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*    . Jeremy Lee s047@sand.sics.bu.oz.au    Student of Everything    *
*   /|            "Where the naked spotless intelect is without       *
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* /  |rchimedes      Leland, look to the light" - Dale Cooper         *
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