the generation of auditory and haptic images has been ignored. Accordingly, our discussion in this section is focused primarily on the visual channel. Information related to the computer generation of the auditory or haptic components of VEs is found primarily in discussions of human-machine interfaces.

It is possible to imagine a VE system that can create photorealistic images, that can be fully interactive in real time, and that has graphics, computation, and communication capabilities to handle all possible environments of interest with equal ease—that is, a general-purpose system that can generate environments relevant to manufacturing, health care, military training, etc. Such a system is beyond the current technology, and it is anticipated that for a long time to come, trade-offs between realism and interactive capacity will be required. Furthermore, due to these limitations, effective VE implementation will depend on targeted application domains. Some applications, such as architectural visualization, may require photorealistic rendering, whereas others, such as training, may not. Many manufacturing and medical applications may require a much higher level of real-time interaction than an architectural walkthrough. Although there are many applications in which a realistic visual environment is unnecessary (and maybe even undesirable), the ability to generate such an environment is clearly an important target for the development of VE technology.

One requirement for creating a realistic visual environment concerns the frame rate, that is, the number of still images that must be presented per second to provide the illusion of continuous motion. It has been demonstrated that frame rates must be greater than 8 to 10 per s to maintain this illusion. A second requirement concerns the response time a VE system must exhibit to preserve an illusion of instantaneous interactive control. Research shows that such delays must be less than 0.1 s. A third requirement concerns the picture resolution needed for realism. According to some VE technologists, a scene can be rendered in all of the detail resolvable to the human eye with 80 million polygons (Catmull et al., 1984). However, using today's hardware, a system that used 80 million polygons per picture would be far too slow to be truly interactive—thus the current major trade-off between realistic images and realistic interactivity. These requirements, of course, can be highly application dependent. Applications with rapidly moving objects may require significantly higher frame rates and shorter delays, whereas highly abstract or stylized applications may require fewer polygons or lower resolution.

Hardware Maintaining an adequate graphic frame rate is so computationally demanding that special-purpose hardware is often necessary. The main purpose of this graphics hardware is to provide rapid geometric



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