A(n) allows one computer to simulate the hardware and software of another.

The computer technology that allows us to develop three-dimensional virtual environments (VEs) consists of both hardware and software. The current popular, technical, and scientific interest in VEs is inspired, in large part, by the advent and availability of increasingly powerful and affordable visually oriented, interactive, graphical display systems and techniques. Graphical image generation and display capabilities that were not previously widely available are now found on the desktops of many professionals and are finding their way into the home. The greater affordability and availability of these systems, coupled with more capable, single-person-oriented viewing and control devices (e.g., head-mounted displays and hand-controllers) and an increased orientation toward real-time interaction, have made these systems both more capable of being individualized and more appealing to individuals.

Limiting VE technology to primarily visual interactions, however, simply defines the technology as a more personal and affordable variant of classical military and commercial graphical simulation technology. A much more interesting, and potentially useful, way to view VEs is as a significant subset of multimodal user interfaces. Multimodal user interfaces are simply human-machine interfaces that actively or purposefully use interaction and display techniques in multiple sensory modalities (e.g., visual, haptic, and auditory). In this sense, VEs can be viewed as multimodal user interfaces that are interactive and spatially oriented. The human-machine interface hardware that includes visual and auditory displays as well as tracking and haptic interface devices is covered in Chapters

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