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Published byAnis Park Modified over 9 years ago
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Subdivision Surfaces Ref: Subdivision Surfaces in Character Animation, DeRose et. al, SIGGRAPH98
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Introduction Complex shapes used in character animation –head, hand, clothing, etc. Traditionally, (trimmed) NURBS –Alias/Wavefront, SoftImage –severe topological restrictions Pixar introduces subdivision surfaces in production environment –Geri’s game
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Bad things about trimmed NURBS topologically equivalent to a sheet trimming is expensive; prone to numerical error difficult to maintain smoothness across boundary –manual labor to hide the seams –even more so when model animates
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On the contrary, subdivision surface… do not require trimming smoothness of model is automatically guaranteed support arbitrary topology –construct a surface from an arbitrary polyhedron by repeatedly subdividing each of the faces
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Catmull-Clark Surface one of the first subdivision schemes –based on quadrilateral –(triangle-based Loop surface) control mesh M(0) M(1), M(2), … limit surface can be shown to be tangent plane smooth
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Subdivision Rules valence (of a vertex) n : # of incident edges vertex pt, edge pt, face pt: –each vertex of M(i+1) can be associated with v/e/f of M(i) –fpt: centroid of face –ept: –vpt:
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Implementation Require basic topological data structure –winged edge is sufficient Each subdivision step –geometric computation (as in previous slide) –topological update new vertices (fpt, ept) new edges new faces
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Creating Sharp Features (creases) subdivision rules need to be modified –ept –vpt w/ 1 crease: smooth formula –vpt w/ 2 creases –vpt w/ 3+ creases
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Semi-Sharp Creases
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Task implement subdivision surface in OpenGL –design your own data structure –design a file format for inputting M0 –use picking and keyboard callback function to edit control mesh M0 –slider to control subdivision steps; able to modify mesh in each step –mark (infinitely) sharp edges
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