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In the REVE Worlds system, a virtual object's
functionality is exposed as a set of functions which
can be executed, in potentially rapid succession, by a
virtual agent's effectors as part of the execution of
complex actions, such as animated motion and other
kinds of virtual object manipulation. This introduces
the requirement for accurately-timed read- and
write-access to the virtual environment's scene graph
which, in X3D's case, turns out to be problematic
because of its asynchronous execution model. In
addition, it is necessary that the scene graph is
evaluated and updated on a per-frame basis so that
the virtual world's state is at all times consistent with
all laws in effect, which is also problematic because
of X3D's lack of inherent per-frame processing
support. We were able to address those requirements
effectively and elegantly by incorporating the
proposed patter into the REVE Worlds system's
architecture. We have also found modifications of
relevant application code to be straightforward and
transparent, thus increasing the maintainability,
extendibility and life-span of our implementation.
6 CONCLUSIONS
In this paper, we present specific elements of the
X3D specification that may pose problems to virtual
environment developers. We also propose solutions
to those problems and describe an object-oriented
architectural pattern for the transparent incorporation
of those solutions into concrete virtual environment
designs. In particular, we demonstrate how to
address field output timing issues that may arise due
to X3D's asynchronous event model, as well as how
to implement per-frame processing according to an
event delegation mechanism. We have been unable,
however, to overcome the inability to access
primitive geometry node size-related fields (for
instance, for the purposes of automatic bounding
box calculation), as that is a restriction that arises
from the X3D specification itself.
ACKNOWLEDGEMENTS
We would like to thank all members of the X3D
Public mailing list, the X3D developer message
boards and the Web3D Consortium for all the
exciting discussions we had and the invaluable
insights they provided us with over the past few
years of our involvement with X3D and Xj3D.
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AN ARCHITECTURAL PATTERN FOR X3D-BASED VIRTUAL ENVIRONMENTS - An Object-oriented Approach
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