摘要
本文提出了一种构造实体模型的新方法,CELL DECOMPOSED CONSTRUCTIVE SOLID GEOMETRY,简称为CDCSG。这种新方法较之常用的CSG加边界表达的方法有两点主要区别:第一,在CDCSG中边界表示是用来描述各基本体素的几何信息和拓扑信息而不是用来描述整体模型的。因此它没有给出显式的模型总体边界表示。第二,与CSG树不同,在CDCSG系统中建立结构二叉树(CDCSG树)时,只使用了“粘合”和“钻孔”两种简化了的布尔算子。这一造型方法和目前通用的CSG加边界表达的造型方法相比有下列明显的优点:在基于CSG加边界表达的造型系统中,有关体素间的联系信息是用显式描述的,这时要修改一个模型(增加,删除或改变与模型有关的基本体素)是很困难且代价相当高的(如果不是不可能的话),而在CDCSG造型系统中这种修改却非常容易实现;在基于边界表达的造型系统中,为了执行构造CSG树时的布尔运算,必须整个模型同时存在于计算机内存。这样就造成了复杂模型数据量大和计算机内存有限的矛盾。CDCSG方法的另一个优点是可以设计出一种动态存储管理系统。在这个系统中,每个基本体素和修改后的体素均可独立存取和调用;因而,我们能够只将使用最频繁的基本体素存于高速存储区而将大多数的数据存于低速存储区。这样就可以克服上述矛盾。
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.
出处
《计算机辅助设计与图形学学报》
EI
CSCD
1990年第2期14-23,共10页
Journal of Computer-Aided Design & Computer Graphics
