本文在基于约束的参数驱动法的基础上，对图形设计中经常遇到的循环约束现象提出平面刚体的概念，并对平面刚体的成因、条件作了详细的分析，给出平面刚体的数据结构，并将其思想具体应用于CAD系统的开发中。%This paper puts forward the concept of entity on the base of parameter driving method based on constraints to solve the circulating constraints emerged in usually design, and analysis carefully forming reason, shows the data structure, applies this principle into the development of the CAD system.
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