Virtual reality displays introduce spatial distortions that are very hard to correct because of the difficulty of precisely modelling the camera from the nodal point of each eye. How significant are these distortions for spatial perception in virtual reality? In this study, we used a helmet-mounted display and a mechanical head tracker to investigate the tolerance to errors between head motions and the resulting visual display. The relationship between the head movement and the associated updating of the visual display was adjusted by subjects until the image was judged as stable relative to the world. Both rotational and translational movements were tested, and the relationship between the movements and the direction of gravity was varied systematically. Typically, for the display to be judged as stable, subjects needed the visual world to be moved in the opposite direction to the head movement by an amount greater than the head movement itself, during both rotational and translational head movements, although a large range of movement was tolerated and judged as appearing stable. These results suggest that it not necessary to model the visual geometry accurately and suggest circumstances when tracker drift can be corrected by jumps in the display which will pass unnoticed by the user.

To enhance presence, facilitate sensory motor performance, and avoid disorientation or nausea, virtual-reality applications require the perception of a stable environment. End-end tracking latency (display lag) degrades this illusion of stability and has been identified as a major fault of existing virtual-environment systems. Oscillopsia refers to the perception that the visual world appears to swim about or oscillate in space and is a manifestation of this loss of perceptual stability of the environment. The effects of end-end latency and head velocity on perceptual stability in a virtual environment were investigated psychophysically. Subjects became significantly more likely to report oscillopsia during head movements when end-end latency or head velocity were increased. It is concluded that perceptual instability of the world arises with increased head motion and increased display lag. Oscillopsia is expected to be more apparent in tasks requiring real locomotion or rapid head movement.

Although technologies such as head-mounted displays and CAVEs can be used to provide large immersive visual displays within small physical spaces, it is difficult to provide virtual environments which are as large physically as they are visually. A fundamental problem is that tracking technologies which work well in a small enclosed environment do not function well over longer distances. In this paper, we describe Trike-a 'rideable' computer system which can be used to generate and explore large virtual spaces both visually and physically. This paper describes the hardware and software components of the system and a set of experiments which have been performed to investigate how the different perceptual cues that can be provided with Trike interact within an immersive environment.

The design of virtual environments usually concentrates on constructing a realistic visual simulation and ignores the non-visual cues normally associated with moving through an environment. The lack of the normal complement of cues may contribute to cybersickness and may affect operator performance. Previously (1998) we described the effect of adding vestibular cues during passive linear motion and showed an unexpected dominance of the vestibular cue in determining the magnitude of the perceived motion. Here we vary the relative magnitude of the visual and vestibular cues and describe a simple linear summation model that predicts the resulting perceived magnitude of motion. The model suggests that designers of virtual reality displays should add vestibular information in a ratio of one to four with the visual motion to obtain convincing and accurate performance.