Non-planar locomotion

Now consider more complicated locomotion cases. If the user is walking over a terrain, then the $y$ component can be simply increased or decreased to reflect the change in altitude. This may seem realistic, but keep in mind that it increases the amount of mismatch between the real and virtual worlds because vertical vection is combined with forward vection.

In the case of moving through a 3D medium, all six forms of vection from Section 8.4 become enabled. Common settings include a virtual spacecraft, aircraft, or scuba diver. Yaw, pitch, and roll vection can be easily generated. For example, imagine flying a virtual spacecraft. By rolling the craft, roll vection can be caused as the stars spin around in a circular pattern. If a developer must make a craft move in these ways, then the prior suggestions for reducing vection intensity should be followed. Furthermore, careful experimentation with human subjects should be performed to determine which forms of vection are worse in the particular application; see Chapter 12. To avoid singularities, for systems in which all 3 DOFs of rotational motion are possible, the virtual vehicle transformations are best maintained in terms of quaternions (recall the issues from Section 3.3).

Adding special effects that move the viewpoint will cause further difficulty with vection. For example, making an avatar jump up and down will cause vertical vection. It is also a bad idea to account for swaying head motions while walking because of the increased mismatch. Imagine a far worse case of looking out through the eyes of an avatar that performs gymnastics. The view of the world may become unbearable during multiple flips.