As the user moves through the physical world, his sense organs move along with him. Furthermore, some sense organs move relative to the body skeleton, such as our eyes rotating within their sockets. Each sense organ has a configuration space, which corresponds to all possible ways it can be transformed or configured. The most important aspect of this is the number of degrees of freedom or DOFs of the sense organ. Chapter 3 will cover this thoroughly, but for now note that a rigid object that moves through ordinary space has six DOFs. Three DOFs correspond to its changing position in space: 1) side-to-side motion, 2) vertical motion, and 3) closer-further motion. The other three DOFs correspond to possible ways the object could be rotated; in other words, exactly three independent parameters are needed to specify how the object is oriented. These are called yaw, pitch, and roll, and are covered in Section 3.2.
As an example, consider your left ear. As you rotate your head or move your body through space, the position of the ear changes, as well as its orientation. This yields six DOFs. The same is true for your right eye, but it also capable of rotating independently of the head. Keep in mind that our bodies have many more degrees of freedom, which affect the configuration of our sense organs. A tracking system may be necessary to determine the position and orientation of each sense organ that receives artificial stimuli, which will be explained shortly.
Steven M LaValle 2020-01-06