Most conventional CAD systems have been proved to be Computer-Aided Drafting systems. They do not offer sufficient support for the design activity. The reason is that they cannot understand the semantic information and keep the semantic information throughout the whole design process. The semantic information here means the functional requirements on the design product. The designer 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 desgin activity.
Since the geometric model is still the main platform of a CAD system on which the design activity goes on, bringing the semantic information into the geometric model is of great importance.
On the geometric level, the functional requirements can be represented as geometric/numerical relationships among the design parts and engineering equations. They are called `constraints‘. Constraints should always be satisfied. Through constraints the structure of the geomtric model can be maintained during the whole design activity. 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.
The constraint-based geometric modelling is a way of constructing geometric models with constraints. There are usually three methods for handling constraints: the numerical method, the symbolic method, and the modelling operation method. At the beginning of this book, some approaches using these methods are discussed.
In this book a new constraint-based geometric modelling method is introduced. This method combines the advantages of the three existing methods. Object-oriented concept is used to develop this method. The implementation of a CAD system called `RelCAD‘, based on this method, is also introduced in this book.
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