Research Of Constraint-driven Technology For History-independent Shape Modeling

The constraint-driven is the key technology of the3D CAD shape modeling system,which will maintain the engineering definitions of the3D models, for example: sizerequirements, shape requirements. The existing multi-mainstream3D CAD system arehistory-based parametric modeling systems, it will need to reconstruct the3D model as thehistory records at the time to driven the model, which makes it inefficient for modifyingthe large parts, and may lose the features. On the contrary, the history-free shape systemsare more efficient than the history-based ones and will not lose the feature information.The researches of constraint-driven technology in the history-independent shape modelingsystem are as follows:1）As to expressing the constrains, the virtual geometric elements has been introducedin order to determine the direction of constraints. And the inputted constraints areconverted into structure and dimension constraints among existing geometric elements andvirtual geometric elements. Thus, the basic constraints model has been build.2）In the3D designing process, the constraint model is often under-constrained, Andthe solution of under-constrained model always is not unique. Thus, it needs to identifymore implicit constraints based on the input ones to improve the constraint model at thetime to build one. Therefore, the remaining degrees of freedom (DOFs) of such geometricelements are analyzed based on the constraint graph, and the complementary constraintsare appended so that the constraint model is more complete.3）The quality of the solving results depends on the algorithm to a certain extend. Inorder to solve the constraint model, the model-based geometric reasoning method has beenused to solve the constraint model. The constraint solving sequence has been determinedaccording to the constraint propagation direction, and the transforming matrix has beendetermined by the constraint. And the constraint driven has been achieved which meet theengineer design needs.4）The proposed method has been implemented and validated by two examples in a prototype system named InteModel.