Traditionally, the development of virtual environments has been limited to particular technologies and the associated constraints on environment de"nition and interaction. However, with advances in both graphics hardware/software and the availability of new input/output devices, many of the restrictions on virtual environment development have been removed (Smith, Duke & Massink, 1999). Rendering rates are now reaching acceptable levels on desktop machines, new interaction technologies are being enabled both through physical devices e.g. Spaceballs and 3D position trackers, and logical metaphors e.g. cursor-based #ying (Massink, Duke & Smith, 1999) and high-level application programming interfaces i.e. via toolkits like Manchester University's MAVERIK (GNU MAVERIK, 1999) are being used in the virtual environment development process. As these technologies mature and become adopted in a wider range of applications, there is a need to understand how this technology can be accommodated in software engineering practice.
Unfortunately, it is di$cult to "nd reports that detail the process used to develop virtual environments, but given the maturity of the technology it would seem reasonable to suggest that prototyping and exploratory development play a signi"cant role. Smith and Duke (2000) observe that it is common for designers to move directly to working prototypes and use toolkit based development environments to test-bed ideas and customize the design and implementation of the virtual environment &&on-the-#y''. However, if or when the technology of virtual environments becomes adopted in mainstream software systems and products, exploratory approaches become rather less attractive. Software developers must be concerned with making use of the most appropriate technology in a way that meets the requirements of the client, including quality criteria such as usability, robustness, maintainability, error-tolerance, etc (Sommerville, 1996). The designer's concern is not so much with the physical devices such as headsets and data gloves that have come to characterize virtual environments, but rather with addressing the highly interactive and dynamic nature of user}system interaction that this technology supports.
One ideal of virtual environment applications is to provide an environment where the user can interact freely with a 3D space and the entities within it. Virtual environments attempt to build on latent human knowledge by providing cues that reuse appropriate