Up until now, this chapter has described how to use sensors to track the motions of one or more rigid bodies. By contrast, this section describes how sensors are used to build geometric models of rigid bodies. These could be movable or stationary models, as introduced in Section 3.1. A movable model typically corresponds to an object that is being manipulated by the user, such as a sword, hammer, or coffee cup. These models are often built from a 3D scanner, which images the object from many viewpoints in a controlled way. The object may be placed on a surface that is surrounded by cameras and other sensors, or it could be placed on a turntable that rotates the object so that it is observed from numerous viewpoints. Alternatively, the sensors may move around while the object remains stationary; see Figure 9.26(a).