Methods, systems, and apparatus, including medium-encoded computer program products, for computer aided design of structures include, in one aspect, a method for producing continuous (G2) NURBS surfaces from arbitrary subdivision surface meshes, the method including: obtaining a control mesh of a subdivision surface; obtaining limit surface patches subdivided from the control mesh using at least two levels around any extraordinary regions of the control mesh; discarding discontinuous surface patches, which include a star point; producing replacement surface patches for the discarded patches using adjacent surface patches, wherein the replacement surface patches are curvature continuous with each other and with the adjacent surface patches; combining each of the replacement surface patches with at least one non-discarded surface patch generated from a same polygon of the control mesh as the replacement surface patch; and processing the combined surface patches for output by a physical device.
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