If the user is tracked through a very large space, such as a square region of at least meters on each side, then it is possible to make her think she is walking in straight lines for kilometers while she is in fact walking in circles. This technique is called redirected walking . Walking along a straight line over long distances without visual cues is virtually impossible for humans (and robots!) because in the real world it is impossible to achieve perfect symmetry. One direction will tend to dominate through an imbalance in motor strength and sensory signals, causing people to travel in circles.
Imagine a VR experience in which a virtual city contains long, straight streets. As the user walks down the street, the yaw direction of the viewpoint can be gradually varied. This represents a small amount of mismatch between the real and virtual worlds, and it causes the user to walk along circular arcs. The main trouble with this technique is that the user has free will and might decide to walk to the edge of the matched zone in the real world, even if he cannot directly perceive it. In this case, an unfortunate, disruptive warning might appear, suggesting that he must rotate to reset the yaw orientation.
Steven M LaValle 2016-12-31