They are represented by graph nodes, and therefore influence the graph structure. The decomposition approach is based on the idea that shape recognition is a hierarchical process. Shape primitives are defined at the lower level, primitives being the simplest elements which form the region. A graph is constructed at the higher level - nodes result from primitives, arcs describe the mutual primitive relations. Convex sets of pixels are one example of simple shape primitives.
Primitives are simple enough to be successfully described using simple scalar shape properties.
Number of vertices of the subregion represented by the node. Area of the subregion represented by the node. Main axis direction of the subregion represented by the node. Center of gravity of the subregion represented by the node. If a graph is derived using attributes 1-4, the final description is translation invariant.