Up to this point, the discussion of movement has been confined to specialized topics. Section 5.3 covered eye movements and Section 6.2 covered the perception of motion. The transformations from Chapter 3 indicate how to place bodies and change viewpoints, but precise mathematical descriptions of motions have not yet been necessary. We now want to model motions more accurately because the physics of both real and virtual worlds impact VR experiences. The accelerations and velocities of moving bodies impact simulations in the VWG and tracking methods used to capture user motions in the physical world. Therefore, this chapter provides foundations that will become useful for reading Chapter 9 on tracking, and Chapter 10 on interfaces.
Section 8.1 introduces fundamental concepts from math and physics, including velocities, accelerations, and the movement of rigid bodies. Section 8.2 presents the physiology and perceptual issues from the human vestibular system, which senses velocities and accelerations. Section 8.3 then describes how motions are described and produced in a VWG. This includes numerical integration and collision detection. Section 8.4 focuses on vection, which is a source of VR sickness that arises due to sensory conflict between the visual and vestibular systems: The eyes may perceive motion while the vestibular system is not fooled. This can be considered as competition between the physics of the real and virtual worlds.