| Along with the great progress in 3D data measurement device and the popularization of complex surface modeling, recursive subdivision has been a research focus in the world over the past 20 years in computer aided geometric design and computer graphics.Subdivision surface can express free smooth surface through low resolution control mesh and subdivision rules. It has been widely used in recent years, for it not only solves some problems in computer graphics, such as arbitrary topology and compatibility, but also retains several advantages of traditional NURBS, including the properties of localness and affine-invariance. This paper deals with 31/2 -subdivision and its extension, the main work is as follows:(1) This thesis studies the modeling and application of subdivision surfaces. Subdivision method has become an important tool in computer graphics especially in 3D modeling and animation fields. Three important subdivision schemes based on triangular meshes (Loop subdivision, Modified Butterfly subdivision and 3 -subdivision) are listed in the end.(2) This thesis introduces a 3 -subdivision algorithm to rebuild sharp features. First, sharp edges are automatically tagged according to the given thresholdθand the sharp degree of each vertex is computed. Then, based on different sharp degrees, geometric rules of 31/2 -subdivision scheme are modified and subdivision masks are designed. Finally sharp features are reconstructed by keeping sharp edges non-flipping in odd refinement steps and inserting edge points in even refinement steps. Experimental results show that this algorithm keeps sharp features better than original 31/2 -subdivision method does.(3) For the purpose of reducing the computation and holding sharp features, this thesis presents a local 31/2 -subdivision algorithm to rebuild sharp features. Adaptive subdivision is performed only in the areas facing to the viewpoint in the pre-process. And in the each adaptive subdivision, we choose the normal angle of the adjacent faces as the control error to detect whether the approximation is sufficient. Sharp edges are automatically tagged according to the given thresholdθand subdivision masks are designed based on different sharp degrees. |
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