Distributed Virtual Walkthrough
In a distributed walkthrough system, a geometry database server maintains
a huge collection of virtual objects and their positions in the virtual
environment. Human viewers utilizing database clients connected to the
server through networks may view the objects by walking through the virtual
environment. To model a large and complex environment, the database is
often very large.
Several approaches have been proposed to distribute objects from the
server to the clients in distributed virtual reality applications. Most
existing systems, such as DIVE, SIMNET, and VLNET, transmit the whole geometry
database to the clients before the walkthrough starts. To use the systems
in an Internet environment, either the size of the geometry database needs
to be small or the viewer has to wait for a long pre-loading time before
starting the walkthrough. Another approach to distribute the objects in
the geometry database is to send them on demand to the clients during the
walkthrough. This approach employs a standard client-server architecture,
in which a central server maintains the geometry database of the virtual
environment and distributes data to clients when requested. Both approaches,
however, require the models of the visible objects to be completely transmitted
before the corresponding images can be rendered. This might result in uneven
frame rates in a congested network such as the Internet and will cause
a jittered walkthrough. It also limits the scene complexity that the system
In this project, we are developing a distributed walkthrough system
for use in an internet environment. This system has two major features:
We have just completed the implementation of the prototype system. However,
there are still a lot of fine tuning needed in order to get the best performance.
On-demand transmission: The system
only transmits the region of the environment visible to the client to its
machine. We define the user scope and the object. The user
scope indicates how far a user can see and the object scope indicates how
far an object can be seen. We only need to download those objects whose
scopes overlap with the user scope to the client machine.
Progressive transmission and reconstruction:
Sending the whole model of each visible object to the client machine on-demand
may cause a possibly long pause in the walkthrough. Instead, our system
encodes each model in the form of a progressive mesh for transmission.
In most situations, the visibility of each object only changes slightly
from one frame to the next. Hence, we only need to send a small portion
of the model to the client machine, where the object model is progressively
reconstructed. If the network is too busy, we may still use a low resolution
model of the object for rendering.
here for an animation sequence produced by our
prototype system. (The file is gzip'ed. It is very large. Compressed size
is 4.6MB and uncompressed size is 17MB.)
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Last updated on 9th October, 2004.