Suppose that the virtual world covers a much larger area than the part of the real world that is tracked. In other words, the matched zone is small relative to the virtual world. In this case, some form of interaction mechanism is needed to move the user in the virtual world while she remains fixed within the tracked area in the real world. An interaction mechanism that moves the user in this way is called locomotion. It is as if the user is riding in a virtual vehicle that is steered through the virtual world.
Figure 10.4 shows a spectrum of common locomotion scenarios. At the left, the user walks around in an open space while wearing a headset. No locomotion is needed unless the virtual world is larger than the open space. This case involves no mismatch between real and virtual motions.
The two center cases correspond to a seated user wearing a headset. In these cases, an interaction mechanism is used to change the position of the matched zone in the virtual world. If the user is seated in a swivel chair, then he could change the direction he is facing (yaw orientation) by rotating the chair. This can be considered as orienting the user's torso in the virtual world. If the user is seated in a fixed chair, then the virtual torso orientation is typically changed using a controller, which results in more mismatch. The limiting case is on the right of Figure 10.4, in which there is not even head tracking. If the user is facing a screen, as in the case of a first-person shooter game on a screen, then a game controller is used to change the position and orientation of the user in the virtual world. This is the largest amount of mismatch because all changes in viewpoint are generated by the controller.