RelCAD: A Relational CAD System with Constraint-Based Geometric Modelling

Most conventional CAD systems have been proved to be Computer-Aided Drafting systems. They do not offer sufficient support to the design activity because they cannot understand the semantic information and keep the semantic information throughout the whole design process. The semantic information here means the fuctional requirements on the product of the design activity. The desighner is responsible for maintaining the functional requirements in different design phases. This is such a heavy burden for him that he cannot concentrate on the creative part of the design activity. The constraint-based geometric modelling method sees the geometric/numerical relationships, the functional requirements on the geometric level, as constraints. It connects the constraints to the geometric model and satisfies the constraints continually to guarantee the consistence of the geometric model each time when the geometric model is modified. This releases the designer from the burden of maintaining the consistence of the geometric model and therefore offers the designer a great support in the design process. This paper presents the development of a CAD system -- RelCAD with the constraint-basedgeometric modelling facility. In chapter 1, issues of design acitivity and the Computer-AidedDesign(CAD) are discussed, problems of conventional CAD systems are pointed out, and themotivation of the RelCAD system is discussed. In chapter 2, several presentative methods of constraint-based geometric modelling are introduced, their advantages and disadvantages are discussed. The rest of this paper introduces the development of the RelCAD system. Chapter 3 presents the strategy of the RelCAD geometric modelling method. Chapter 4 introduces an important feature of the RelCAD geometric modelling method -- the generalized segment concept. Chapter 5 discusses the constraint statisfaction mechanism of the RelCAD system. In chapter 6, a graphical user interface for the manipulation of constraints is introduced. In chapter 7, some issues of the object-oriented implementation of the RelCAD system are discussed.