This section describes in detail the two systems for two dimensional shape representation which developed from the approaches to solid modelling discussed above. I have used both of these systems to construct descriptions of example shapes and scenes. Chapter 5 describes programs which operate on both representations, and discusses the relative advantages of each one.
For each of the two approaches, I will discuss the methods used to represent four aspects of qualitative shape: primitive shape, relative size, object-interior position (the relative position of subshapes within an object), and object-exterior position (The relative positions of complete objects, or of parts on different objects). These four are sufficient to describe any combination of objects in a scene, and any combination of subshapes within an object.