A Scheme for Single Instance Representation in Hierarchical Assembly Graphs

The Hierarchical Assembly Graph (HAG) is a common representation for geometric models. The HAG is a directed acyclic graph. Nodes in the graph represent objects, and arcs denote the sub-part relation between objects. Affine transformations and other instantiation parameters are attached to the arcs. An instance of an object in a HAG is defined as a path ending at its node. Information common to all instances whose paths end at a given node can be attached to this node. Data associated with a single instance cannot be attached to any single node or arc in the graph. Such private data can be stored in an external list, hash table, or a partial expansion of the graph into a tree, but all of these schemes have severe drawbacks in terms of storage, access efficiency, or update efficiency. In this paper we present a scheme for representing single instances in the assembly graph itself, by identifying an instance with the last node in its path when the only way of reaching the last node is through a unique path starting at the first node of the path. We give an algorithm for singling an instance in the graph, i.e. transforming the graph into an equivalent one in which the instance can be identified with a node. We also show how to undo an instance's singling when its private data is no longer needed.