Virtual Reality: Vision

Visual Displays

Ideally, a virtual reality system would provide synthesized stimuli for all of the senses, and there is debate about whether or not effective VR is possible without providing a full range of sensory modalities. In practice, however, the majority of existing VR systems are limited to visual displays, using computer graphics to create views of the synthetic world. Two properties of these VR visual displays seem essential to approximating a perceptual sense of being in a simulated world. Visual immersion excludes visual sensation of anything except the simulation itself. This avoids [Page 1039]sensory and perceptual conflicts between the real and virtual worlds, increasing the impact of even low fidelity virtual displays. Head tracking involves generating the computer graphics presented to the user from a viewpoint that changes appropriately as the user moves. This provides motion parallax cues allowing perception of the spatial structure of the virtual environment based on the changing view a result of changing viewpoint, but more importantly provides a compelling sense of being “in” the simulation.

Figure 1 Head-Mounted Display

Two classes of visual displays are used in virtual reality systems, each with distinct advantages and limitations. Head-mounted displays (HMDs) are devices worn on the head and feature two small display screens, one in front of each eye (Figure 1). Computer graphics are used to generate the video fed to each of these screens. Lenses (collimated optics) are used to cause the screens to appear farther away than they really are. This allows a user's eyes to focus (accommodate) to a distance consistent with a large space and makes the viewing of displayed images less sensitive to the precise positioning of the HMD on the user. The field of view of HMDs is usually quite limited, ranging horizontally from about 20° to about 45°, though a few HMD systems have fields of view approaching normal human vision. When used in virtual reality simulations, head-mounted displays incorporate a tracking device that measures the position and orientation of the HMD and feeds this information to the computer graphics system. The computer graphics system generates imagery that would be seen in the virtual world from the corresponding position and direction of view.

In screen-based systems, views of the simulated world are displayed on large, fixed, back-projected screens rather than on minidisplays worn by the user. The most effective screen-based VR systems surround the user with screens, sometimes including the ceiling and floor. VR systems involving a single screen are also used, though it is difficult with such systems to achieve a high level of visual immersion. Head tracking, usually involving the user wearing some sort of simple target on her or his head, allows the computer graphics projected onto the display screens to change in response to the user's movements.

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