Combining parametrically represented geometrical objects using constraints

The paper describes an experimental constraint based 2D geometric modeling system. As a result of constraint solving, an acyclic constraint description graph (ACDG) is obtained. It stores the sequence of applying inserted constraints and can be observed as a parametric description of a geometric object. The main point of the paper is the explanation of joining ACDGs, which represent individual geometric objects, into a new common ACDG. In this way a new parametric representation of a resulting geometrical object is obtained. To preserve a clear hierarchical structure of the common ACDG, constraint interfaces are introduced among individual ACDGs. The user is not extinguished by the mechanism of constraints, constraint solving, and constraint description graphs. Instead of this, a geometrical object is offered to him/her as a black box. By geometrical constraints these black boxes are joined in a natural way using expressions close to the engineer vocabulary.