دانلود رایگان مقاله تبدیل یک مدل CAD به یک سطح زیربخش غیر یکنواخت

Abstract

CAD models generally consist of multiple NURBS patches, both trimmed and untrimmed. There is a long-standing challenge that trimmed NURBS patches cause unavoidable gaps in the model. We address this by converting multiple NURBS patches to a single untrimmed NURBS-compatible subdivision surface in a three stage process. First, for each patch, we generate in domain space a quadrangulation that follows boundary edges of the patch and respects the knot spacings along edges. Second, the control points of the corresponding subdivision patch are computed in model space. Third, we merge the subdivision patches across their common boundaries to create a single subdivision surface. The converted model is gap-free and can maintain inter-patch continuity up to C2.

8. Conclusion

We present a novel framework to convert a trimmed NURBS model, which consists of several patches with inter-patch gaps, to a single gap-free non-uniform subdivision surface. The connectivity of the target subdivision base mesh is computed via quad partition in domain space. The control point positions are then obtained by solving a fitting problem based on the limit stencils of the subdivision scheme and the boundary conditions. We further introduce the Pixar sharp edge rules to the non-uniform subdivision to handle concave corners in trimming. The inter-patch merging (C0 or G1) is automatic. For some cases, C1 and C2 can be achieved. The approximation error is controllable via further refinements in the connectivity construction. Although this paper deals with cubic degree, the framework can be extended to support higher degrees, using Cashman’s subdivision scheme.