The most basic operation of the VWG is to maintain a correspondence between user motions in the real world and the virtual world; see Figure 2.15. In the real world, the user's motions are confined to a safe region, which we will call the matched zone. Imagine the matched zone as a place where the real and virtual worlds perfectly align. One of the greatest challenges is the mismatch of obstacles: What if the user is blocked in the virtual world but not in the real world? The reverse is also possible. In a seated experience, the user sits in a chair while wearing a headset. The matched zone in this case is a small region, such as one cubic meter, in which users can move their heads. Head motions should be matched between the two worlds. If the user is not constrained to a seat, then the matched zone could be an entire room or an outdoor field. Note that safety becomes an issue because the user might spill a drink, hit walls, or fall into pits that exist only in the real world, but are not visible in the virtual world. Larger matched zones tend to lead to greater safety issues. Users must make sure that the matched zone is cleared of dangers in the real world, or the developer should make them visible in the virtual world.
Which motions from the real world should be reflected in the virtual world? This varies among VR experiences. In a VR headset that displays images to the eyes, head motions must be matched so that the visual renderer uses the correct viewpoint in the virtual world. Other parts of the body are less critical, but may become important if the user needs to perform hand-eye coordination or looks at other parts of her body and expects them to move naturally.
Steven M LaValle 2019-03-14