will generally be dependent on the application domain for which the VE is being built. Real-world simulation applications will be highly bound by the graphics and network protocols and by consistency issues; information visualization and scientific visualization applications will be bound by the computational performance and will involve issues of massive data management (Bryson and Levit, 1992; Ellis et al., 1991). Some applications, such as architectural visualization, will require photorealistic rendering; others, such as information display, will not. Thus the particular hardware and software required for VE implementation will depend on the application domain targeted. There are some commonalities of hardware and software requirements, and it is those commonalities on which we focus in our examination of the state of the art of computer hardware and software for the construction of real-time, three-dimensional virtual environments.

HARDWARE FOR COMPUTER GRAPHICS

The ubiquity of computer graphics workstations capable of real-time, three-dimensional display at high frame rates is probably the key development behind the current push for VEs today. We have had flight simulators with significant graphics capability for years, but they have been expensive and not widely available. Even worse, they have not been readily programmable. Flight simulators are generally constructed with a specific purpose in mind, such as providing training for a particular military plane. Such simulators are microcoded and programmed in assembly language to reduce the total number of graphics and central processing unit cycles required. Systems programmed in this manner are difficult to change and maintain. Hardware upgrades for such systems are usually major undertakings with a small customer base. An even larger problem is that the software and hardware developed for such systems are generally proprietary, thus limiting the availability of the technology. The graphics workstation in the last 5 years has begun to supplant the special-purpose hardware of the flight simulator, and it has provided an entry pathway to the large numbers of people interested in developing three-dimensional VEs. The following section is a survey of computer graphics workstations and graphics hardware that are part of the VE development effort.

Notable Graphics Workstations and Graphics Hardware

Graphics performance is difficult to measure because of the widely varying complexity of visual scenes and the different hardware and software approaches to computing and displaying visual imagery. The most



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