Exploiting perceptual limitations and illusions to support walking through virtual environments in confined physical spaces

Head-mounted displays (HMDs) allow users to immerse in a virtual environment (VE) in which the user’s viewpoint can be changed according to the tracked movements in real space. Because the size of the virtual world often differs from the size of the tracked lab space, a straightforward implementation of omnidirectional and unlimited walking is not generally possible. In this article we review and discuss a set of techniques that use known perceptual limitations and illusions to support seemingly natural walking through a large virtual environment in a confined lab space. The concept behind these techniques is called redirected walking. With redirected walking, users are guided unnoticeably on a physical path that differs from the path the user perceives in the virtual world by manipulating the transformations from real to virtual movements. For example, virtually rotating the view in the HMD to one side with every step causes the user to unknowingly compensate by walking a circular arc in the opposite direction, while having the illusion of walking on a straight trajectory. We describe a number of perceptual illusions that exploit perceptual limitations of motion detectors to manipulate the user’s perception of the speed and direction of his motion. We describe how gains of locomotor speed, rotation, and curvature can gradually alter the physical trajectory without the users observing any discrepancy, and discuss studies that investigated perceptual thresholds for these manipulations. We discuss the potential of self-motion illusions to shift or widen the applicable ranges for gain manipulations and to compensate for overor underestimations of speed or travel distance in VEs. Finally, we identify a number of key issues for future research on this topic. ! 2012 Elsevier B.V. All rights reserved. 1. Locomotion in virtual environments In the real world we navigate with ease by walking, running, driving, etc. Sensory information such as vestibular, proprioceptive, and efferent copy signals as well as visual information create consistent multi-sensory cues that indicate one’s own acceleration, speed and direction of travel. Since walking is the most basic and intuitive way of moving within the real world, keeping such an active and dynamic ability to navigate through large-scale virtual environments (VEs) is highly desirable for many 3D applications, such as urban planning, tourism, 3D entertainment, serious games, and robotics. Although these application domains are inherently three-dimensional, usually virtual reality (VR)-based user interfaces do not support real full-scale walking [1]. Immersive VEs were initially restricted to visual displays, combined with interaction devices for providing (often unnatural) inputs (e.g., a joystick or mouse) to generate self-motion. More and more research groups are investigating natural, multimodal methods of generating self-motion. Typically, immersive VEs are characterized, for example, by head-mounted displays (HMDs) and a tracking system for measuring position and orientation data. An obvious approach to implement real walking in such a setup is to map the user’s head movements or gaits to changes of the virtual camera by means of a one-to-one mapping. This technique has the drawback that the user’s movements are restricted by the limited range of the tracking sensors and a rather small tracked lab space in the real word. Therefore, the first challenge for virtual locomotion interfaces is that they enable walking over large distances in the virtual world while physically remaining within a reasonably small space. To address unlimited walking in immersive VEs, various prototypes of interface devices have been developed to prevent a displacement in the real world. These devices include torus-shaped omni-directional treadmills [2–4], motion foot pads, robot tiles [5,6] and motion carpets [7]. All these systems are costly and support only a single user. For multi-walker scenarios it would 0141-9382/$ see front matter ! 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.displa.2012.10.007 ⇑ Corresponding author. Tel.: +49 931 31 85868; fax: +49 931 31 87364. E-mail address: [email protected] (G. Bruder). URL: http://img.uni-wuerzburg.de/personen/dr_gerd_bruder/ (G. Bruder). Displays 34 (2013) 132–141