This paper focuses on the distributed architecture of the collaborative threedimensional user interface management system, Studierstube. The system allows multiple users to experience a shared 3D workspace populated by multiple applications using see-through head-mounted displays or other presentation media such as projection systems. Building large, ubiquitous, or mobile workspaces requires distribution of applications over several hosts in varying and dynamic configurations. The system design is based on a distributed shared scene graph that alleviates the application programmer from explicitly considering distribution and that avoids a separation of graphical and application data. The idea of unifying all system data in the scene graph is taken to its logical consequence by implementing application instances as nodes in the scene graph. Through the distributed shared scene graph mechanism, consistency of scene graph replicas and the contained application nodes is assured. Dynamic configuration management is based on application migration between participating hosts and a spatial model of locales allowing dynamic workgroup management. We describe a number of experimental workspaces that demonstrate the use of these configuration management techniques.

Our starting point for developing the Studierstube system was the belief that augmented reality, the less obtrusive cousin of virtual reality, has a better chance of becoming a viable user interface for applications requiring manipulation of complex three-dimensional information as a daily routine. In essence, we are searching for a 3-D user interface metaphor as powerful as the desktop metaphor for 2-D. At the heart of the Studierstube system, collaborative augmented reality is used to embed computer-generated images into the real work environment. In the first part of this paper, we review the user interface of the initial Studierstube system, in particular the implementation of collaborative augmented reality, and the Personal Interaction Panel, a two-handed interface for interaction with the system. In the second part, an extended Studierstube system based on a heterogeneous distributed architecture is presented. This system allows the user to combine multiple approaches— augmented reality, projection displays, and ubiquitous computing—to the interface as needed. The environment is controlled by the Personal Interaction Panel, a twohanded, pen-and-pad interface that has versatile uses for interacting with the virtual environment. Studierstube also borrows elements from the desktop, such as multitasking and multi-windowing. The resulting software architecture is a user interface management system for complex augmented reality applications. The presentation is complemented by selected application examples.