Stationary vs. movable models

There will be two kinds of models in the virtual world, which is embedded in ${\mathbb{R}}^3$:

• Stationary models, which keep the same coordinates forever. Typical examples are streets, floors, and buildings.
• Movable models, which can be transformed into various positions and orientations. Examples include vehicles, avatars, and small furniture.

Motion can be caused in a number of ways. Using a tracking system (Chapter 9), the model might move to match the user's motions. Alternatively, the user might operate a controller to move objects in the virtual world, including a representation of himself. Finally, objects might move on their own according to the laws of physics in the virtual world. Section 3.2 will cover the mathematical operations that move models to the their desired places, and Chapter 8 will describe velocities, accelerations, and other physical aspects of motion.