08.3D Object modeling.ppt

Overview of3D Object modeling Modeling 3D Objects 3D Objects 3D Objects 3D Objects 3D Objects 3D Objects Representations of Geometry Representations of Geometry Point Cloud Range Image Polygon Soup Representations of Geometry Mesh Subdivision Surface Parametric Surface Implicit Surface Representations of Geometry Voxels BSP Tree CSG Sweep Volumes Sweep Octrees An Octree is essentially the same idea as Quadtree, however, each node in an Octree has eight children rather than four. This makes Octrees good candidates for representing 3D objects. Representations of Geometry Scene Graph Skeleton Application Specific Representations of Geometry Taxonomy of 3D Representations Equivalence of Representations Each fundamental representation has enough expressive power to model the shape of any geometric object It is possible to perform all geometric operations with any fundamental representation! Computational Differences Polygonal ModelsIntroduce data structures for building polygonal models Meshes A mesh – is a collection of polygons (often triangles) which describe an object. The faces are arranged in such a way that they form the outside surface of that object. It could be thought of as the skin of the object. The object can be anything, a sphere, a human, a potato, or a spaceship. Representing a Mesh – Edges Consider a mesh, in terms of edges: Representing a Mesh – Vertices Consider a mesh, in terms of vertices: Recall: Normals Every polygon has a vector normal n (perpendicular) to it We can use the cross product (u x v) to find n The normal direction indicates the front face of the polygon Inward and Outward Facing Polygons in OpenGL The order {v1, v2, v7} and {v2, v7, v1} are equivalent in that the same polygon will be rendered by OpenGL but the order {v1, v7, v2} is different The first two describe outwardly facing polygons The second describes an inwardly facing polygon Use the right-hand rule: counter-clockwise encirclement of outward-pointing no