Virtual environments can provide a user-friendly interface to diverse applications such as battlefield simulation, virtual conferencing, and collaborative modeling. Some of these applications including battlefield simulation require large-scale virtual environments. City planning or the emerging electronic commerce can also benefit from the natural interfaces provided by large-scale virtual environments.
Tools for modeling individual object, laying out the modeled objects in the virtual environment, and walking-through the environment are a must for building a large-scale virtual environment. We present the design of a multi-user, large-scale 3-D layout system CIAO (Collaborative Immersive Architectural layOut) in this thesis. Because CIAO supports full functionality for layout and walk-through, one can construct virtual environments with CIAO in conjunction with a conventional modeler. Using CIAO, designers can try out what they think and get realistic feedback instantly. In order to enable the synergy among a group of design participants, CIAO is designed to meet the following requirements: Awareness, Responsiveness and Consistency. We call the virtual environment that satisfies the three requirements listed above a collaborative virtual environment.
One of prime difficulties in designing and implementing a collaborative environment is that the responsiveness and consistency requirements often conflict with each other, and suggest opposite directions in the design space. Although there exist many virtual environments that addressed awareness and consistency issues, none of them achieved an optimal level of responsiveness. In fact, many systems sacrifice their responsiveness in order to keep consistency. In this thesis, we describe how CIAO achieves optimal responsiveness without sacrificing awareness or consistency.
In a collaborative environment, it is often possible for many participants to attempt to manipulate a single shared object almost at the same time. Concurrent actions may result in inconsistent views of an object among a group of users. Without careful coordination, a sequence of concurrent actions would soon confuse the participants by presenting different views of the supposedly same shared world, violating the consistency requirement.
The optimal responsiveness is achieved by a new multicast-based, optimistic concurrency control mechanism. In other words, the users manipulate objects without any waiting, and also notify other participants of their actions immediately in CIAO. If there are conflicting operations on the same object, the token associated to the object is used for the maintenance of consistency. The users can also manipulate a group of related objects as easily and quickly as they manipulate individual object. In fact, the concurrency control in CIAO is optimal in the responsiveness standpoint. The interface of CIAO is designed to provide rich awareness, and to reduce any confusion that might be caused by concurrent actions from multiple users at the same time. We believe the interface, with the concurrency control, enables close and responsive collaboration among the participants in a large-scale layout session.
The main contributions of CIAO are as follows. The first and foremost characteristic of CIAO is that it is very responsive. In fact, most operations in CIAO are completed without being delayed at all by the concurrency control mechanisms. Even operations on a large group of related objects do not entail any latency for concurrency control. This is achieved by combining an efficient implementation of optimistic concurrency control and an elegant way of manipulating a group of related objects. The second contribution is in new interfacing techniques that minimize the user confusion/embarrassment that might be caused by concurrent interactions of multiple users. These techniques bring the sense of isolation to a CIAO user when his current task may perceptually interfered by the concurrent actions of other users. Finally, we present application level network fault-tolerance framework for further improvement of notification time.
