CDCSG-A New Geometrie Modelling Method

This paper presents a new geometrie modelling method-CELL DECOM POSED CONSTRUCTIVE SOLID GEOMETRY(CDCSG). Like most solid modelling systems, CDCSG uses a binary tree to represent the construtive inf ormation of prim-: itives (CDCSG tree) plus boundary representation (B-rep) to describe a solid model. There are two main differences between this new method and existing ones.First, it does not give a global boundary representation for its described model as in CSG plus B-rep method. B-rep in CDCSG is used to describe individual primitives. Sec ond, unlike CSG trees, only two quasi-Boolean operators("GLUE" and"DRILL") are used for building up the CDCSG tree. Comparing with the CSG plus B-rep method, CDCSG has following advantages. It is easy to modify a model (adding, deleting and/or transforming a primitive with respect to the model) without rebuilding the whole tree which could be very difficult and costly (if not impossible) when B-rep is used to describe the entire model. Since each part of a final model could be stored and retrieved independently in CDCSG, we could store only the most fre quently used primitives in fast memory, and keep most of the data in slow memory so that the problem emerging from CSG plus B-rep method that the whole model must stay in fast memory for Boolean operation could be solved. By a little human interaction, CDCSG could speed up Boolean operation significantly and make mass calculation (such as volume, center of weight etc.) much easier. This new method has already be successfully implemented in a new solid modelling system-EASY-CAD at South CAD Center, Ministry of Machinery and Electronics Industry, PRC.