This section covers tracking of all DOFs for a moving rigid body, with the most important case being head tracking. For convenience, we will refer to the position and orientation of a body as its pose. Six-DOF tracking enables from 3.4 to be fully derived from sensor data, rather than inventing positions from a plausible head model, as in (9.22). By estimating the position, the powerful depth cue of parallax becomes much stronger as the user moves her head from side to side. She could even approach a small object and look at it from any viewpoint, such as from above, below, or the sides. The methods in this section are also useful for tracking hands in space or objects that are manipulated during a VR experience.