The VWG handles the geometric aspects of motion by applying the appropriate mathematical transformations. In addition, the VWG usually implements some physics so that as time progresses, the virtual world behaves like the real world. In most cases, the basic laws of mechanics should govern how objects move in the virtual world. For example, if you drop an object, then it should accelerate to the ground due to gravitational force acting on it. One important component is a collision detection algorithm, which determines whether two or more bodies are intersecting in the virtual world. If a new collision occurs, then an appropriate response is needed. For example, suppose the user pokes his head through a wall in the virtual world. Should the head in the virtual world be stopped, even though it continues to move in the real world? To make it more complex, what should happen if you unload a dump truck full of basketballs into a busy street in the virtual world? Simulated physics can become quite challenging, and is a discipline in itself. There is no limit to the complexity. See Section 8.3 for more about virtual-world physics.

In addition to handling the motions of moving objects, the physics must also take into account how potential stimuli for the displays are created and propagate through the virtual world. How does light propagate through the environment? How does light interact with the surfaces in the virtual world? What are the sources of light? How do sound and smells propagate? These correspond to rendering problems, which are covered in Chapters 7 and 11 for visual and audio cases, respectively.

Steven M LaValle 2020-01-06