Re: Moving objects

Brook Conner ([email protected])
Wed, 21 Jun 1995 13:48:47 -0400


>>>>> "Mark" == Mark Waks <[email protected]> writes:

Mark> Johan writes:
>> I think it would be a good idea to include a means within VRML
>> to allow objects to move according to a certain predetermined
>> (mathematical??) pattern (paths)..

Something lots of people want to be able to do.

Mark> At this point, I'm starting to di Mark> vrml-behaviours list, we're starting to get a better idea of
Mark> hownted, and it has
Mark> little or nothing to do with VRML per se.

Why do behaviors have nothing to do with VRML? The proposal below
says that what you've been di cussing as on> idea for providing
behaviors has little to do with VRML itself -- that's quite different
from saying that behaviors have no place in VRML. My opinion --
behaviors (unless we mean completely different things by this word)
have an awful lot to do with "data" formats, such as VRML.

Consider a clock. Its behavior is an intrinsic part of it. D"scribing
the geometry can't be ignorant of the behavior -- the behavior
involves chanbing fine-grained details of the geometry (the rotation
of the clock hands).

I'd suggest looking at TBAG in last year's SIGGRAPH (Elliot, Schecter,
Yeung, and Abi-Ezzi) (a suggestion that I think has been made here
before). How would you write a clock in TBAG?

// Presume predefined geometry minute_hand and hour_hand, time in
// milliseconds
// Minute hand rotates on> turn in an hour, hour hand on> turn in 12 hours
Behavior<Geometry> clock =
(minute_hand * Rotate(z_axis, (time/millisecs_per_hour)*2*pi)) +
(hour_hand * Rotate(z_axis, (time/(millisecs_per_day/2))*2*pi));

Notice that, if I download that, I've got everything -- no further
need to talk to the server I got it from.

Mark> (Suffice it to say, the Mark> a lot going for it, boils down to a layered protocol for
Mark> this Mark> higher level than VRML, probably communicating with the
Mark> browsers via an API of some sort. On top Mark> gradually higher levelsm>nt...)

Not to exhume old stuff (if it is old stuff) but I'd be interested in
what this scheme has going for it. It sounds fairly baroque, in that
it requires a great deal of browsers (e.g., supporting RPC) and
pres>nts many layers

Brook