What are desirable and undesirable features of virtual environment (VE) software architectures? What should be present (and absent) from such systems if they are to be optimally useful? How should they be structured? In order to help answer these questions, we present experience from application designers, toolkit designers, and VE system architects along with examples of useful features from existing systems. Topics are organized under the major headings of 3D space management, supporting display hardware, interaction, event management, time management, computation, portability, and the observation that less can be better. Lessons learned are presented as discussion of the issues, field experiences , nuggets of knowledge, and case studies .

A virtual environment (VE) user's avatar may penetrate virtual objects. Some VE designers prevent visual interpenetration, assuming that prevention improves user experience. However, preventing visual avatar interpenetration causes a discrepancy between visual and proprioceptive cues. We investigated users' detection thresh-olds for visual interpenetration and visual-proprioceptive discrepancy and found that users are much less sensitive to visual-proprioceptive discrepancy than to visual interpenetration. We propose using this result to better deal with user penetration of virtual objects.

We designed, developed, deployed, and evaluated the Collaborative nanoManipulator (CnM), a distributed, collaborative virtual environment system supporting remote scientific collaboration between users of the nanoManipulator interface to atomic force microscopes. This paper describes the entire collaboration system, but focuses on the shared nanoManipulator (nM) application. To be readily accepted by users, the shared nM application had to have the same high level of interactivity as the single-user system and include all the functions of the single-user system. In addition the application had to support a user's ability to interleave working privately and working collaboratively. Based on our experience developing the CnM, we present: a method of analyzing applications to characterize the concurrency requirements for sharing data between collaborating sites, examples of data structures that support distributed collaboration and interleaved private and collaborative work, and guidelines for selecting appropriate synchronization and concurrency control schemes.