Lessons about Virtual-Environment Software Systems from 20 years of VE building

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? 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.