Form feature modelling in a hybrid CSG/BRep scheme

Abstract

A new hybrid pseudo-CSG/BRep schema for product modelling is described. In this schema, there exists a clear distinction between morphology and the geometry of an object. At the highest level of abstraction, the object morphology or shape is hierarchically defined by means of a graph, called Feature Adjacency Graph (FAG), in which the nodes are either positive or negative volumetric cells (form features) and the arcs are adjacency/interaction relationships between those cells. The hierarchical morphological characterization of an object made by the FAG allows to one decrease or even suppress the semantic gap between the product model and the underlying solid model since each cell always embodies a form feature. In this manner it becomes possible to capture the designer intent, as well as to help the CAD/CAM integration. At the low level of abstraction, i.e., the BRep level, two associated data structures are maintained. The first, called Feature Topological Structure (FTS), holds the family of volumetric cells, each one of them represented explicitly by its surface boundary (BRep). The second constitutes the evaluated BRep model itself for the object solid. It should be noted that any constructive or destructive compositional action in the object definition occurs simultaneously in both data structures. In a way, the FTS retains at a low level, the construction history of the object model. Besides, the availability of the topological entities belonging to the BRep allows a graphical dialog with the model and also facilitates the generation of instructions for NC milling machines. In short, this hybrid pseudo-CSG/BRep intends to capture the virtues of both representation schemas and eliminate as much as possible as their underlying drawbacks.