II.

Interface Devices

Virtual reality simulations differ from other computer simulations in that they require special interface devices that transmit the sights, sounds, and sensations of the simulated world to the user. These devices also record and send the speech and movements of the participants to the simulation program.

To see in the virtual world, the user wears a head-mounted display (HMD) with screens directed at each eye. The HMD also contains a position tracker to monitor the location of the user's head and the direction in which the user is looking. Using this information, a computer recalculates images of the virtual world—a slightly different view for each eye—to match the direction in which the user is looking, and displays these images on the HMD. The computer must generate these new views at least ten times a second in order to prevent the user's view from appearing halting and jerky and from lagging behind the user's movements. Virtual-world scenes must be kept relatively simple so that the computer can update the visual imagery quickly enough. Because of these simplifications and other shortcomings of current visual displays and computer graphics, VR participants can easily distinguish a simulation from physical reality.

Users hear sounds in the virtual world through earphones in the HMD. The information reported by the position tracker on the HMD can also be used to update audio signals. When a sound source in virtual space is not directly in front of or behind the user, the computer transmits sounds to arrive at one ear a little earlier or later than at the other and to be a little louder or softer and slightly different in pitch. However, as with visual imagery, there are currently scientific and engineering challenges that must be overcome in order to simulate accurately all the sounds heard in the physical world.

The haptic interface, which relays the sense of touch and other physical sensations in the virtual world, is the least developed and perhaps the most challenging to create. Currently, with the use of a glove and position tracker, the computer locates the user's hand and measures finger movements. The user can reach into the virtual world and handle objects but cannot actually feel them. It is particularly difficult to generate the sensations that are felt when a person taps a hard surface, picks up an object, or runs a finger across a textured surface. To simulate these sensations, a set of computer-controlled motors faster and more accurate than any presently available would have to generate force feedback by physically pushing against the user. Another problem is determining how a user would wear these motors and the wiring needed to control them. Touch sensations would also have to be synchronized with the sights and sounds users experienced in their HMDs. A current solution to the haptics challenge is the use of desktop devices that can apply small forces, through a mechanical linkage, to a stylus held in the user's hand. Users can feel when the point of the stylus encounters a virtual object, and they can drag the stylus across the surface to feel its texture and surface geometry.