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UW-Madison CS 559 - Subdivision Schemes

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Subdivision SchemesTessellating a SphereSubdivision MethodThe First StageSphere Subdivision AdvantagesFractal SurfacesFractal Terrain (1)Fractal Terrain ExampleFractal Terrain DetailsFractal TerrainsPowerPoint PresentationSlide 12Slide 13Implementing Subdivision (1)Implementing Subdivision (2)Implementing Subdivision (3)Vertex DataEdge DataFace DataMesh Data StructureFractal Terrain AlgorithmGeneral Subdivision SchemesButterfly SchemeButterfly Scheme (1)Modified Butterfly SchemeSlide 26Modified Butterfly ExampleSubdivision Schemes•Basic idea: Start with something coarse, and refine it into smaller pieces for rendering–We have seen how subdivision may be used to render parametric curves and Bezier surfaces–We will see how it can be used for modeling specific objects, and as a modeling scheme in itself•In this lecture:–Subdivision for tessellating a sphere, and implementation details–Subdivision for fractal surfaces–Subdivision for B-spline patches–General subdivision surfacesTessellating a Sphere•Spheres are best parameterized in polar coordinates:–Note the singularity at the poles•Tessellation: The process of approximating a surface with a polygon mesh•One option for tessellating a sphere:–Step around and up the sphere in constant steps of  and –Problem: Polygons are of wildly different sizes, and some vertices have very high degree22 ,20sin ,cossin ,coscos zyxSubdivision Method•Begin with a course approximation to the sphere, that uses only triangles–Two good candidates are platonic solids with triangular faces: Octahedron, Isosahedron–They have uniformly sized faces and uniform vertex degree•Repeat the following process:–Insert a new vertex in the middle of each edge–Push the vertices out to the surface of the sphere–Break each triangular face into 4 triangles using the new verticesOctahedronIsosahedronThe First StageEach face gets split into 4:Each new vertex is degree 6, original vertices are degree 4Sphere Subdivision Advantages•All the triangles at any given level are the same size–Relies on the initial mesh having equal sized faces, and properties of the sphere•The new vertices all have the same degree–Mesh is uniform in newly generated areas–This is a property we will see later in subdivision surfaces–Makes it easier to analyze what happens to the surface•The location and degree of existing vertices does not change–The only extraordinary points lie on the initial meshFractal Surfaces•Fractals are objects that show self similarity–The word is overloaded – it can also mean other things•Landscapes and coastlines are considered fractal in nature–Mountains have hills on them that have rocks on them and so on–Continents have gulfs that have harbors that have bays and so on•Subdivision is the natural way of building fractal surfaces–Start with coarse features, Subdivide to finer features–Different types of fractals come from different subdivision schemes and different parameters to those schemesFractal Terrain (1)•Start with a coarse mesh–Vertices on this mesh won’t move, so they can be used to set mountain peaks and valleys–Also defines the boundary–Mesh must not have dangling edges or vertices•Every edge and every vertex must be part of a face•Also define an “up” direction•Then repeatedly:–Add new vertices at the midpoint of each edge, and randomly push them up or down–Split each face into four, as for the sphereFractal Terrain ExampleA mountainsideFractal Terrain Details•There are options for choosing where to move the new vertices–Uniform random offset–Normally distributed offset – small motions more likely–Procedural rule – eg Perlin noise•Scaling the offset of new points according to the subdivision level is essential–For the subdivision to converge to a smooth surface, the offset must be reduced for each level•Colors are frequently chosen based on “altitude”Fractal Terrainshttp://members.aol.com/maksoy/vistfrac/sunset.htmTerrain, clouds generated using procedural textures and Perlin noisehttp://www.planetside.co.uk/ -- tool is called TerragenTerrain, clouds generated using procedural textures and Perlin noisehttp://www.planetside.co.uk/ -- tool is called TerragenTerrain, clouds generated using procedural textures and Perlin noisehttp://www.planetside.co.uk/ -- tool is called TerragenImplementing Subdivision (1)•We must represent a polygon mesh•Basic operations:–Split an edge, creating a new vertex–Split a face, creating new edges and new faces based on the old edges and the old and new vertices•Questions influencing the data structures:–Should we store edges explicitly?–Should faces know about their edges?–How do we access the required information when performing the basic operations?Implementing Subdivision (2)•Split an edge, create a new vertex and two new edges–Each edge must be split exactly once–Need to know endpoints of edge to create new vertex•Split a face, creating new edges and new faces based on the old edges and the old and new vertices–Require knowledge of which new edges to use–Require knowledge of new vertex locations•Note: Everything works from edges!Implementing Subdivision (3)•Should we explicitly store edges? Yes!–Easy to step through all edges and split them–Each edge can store the edges that will replace it–Each face can point to its edges, so it knows the location of new vertices and edges•What does each vertex need to store?•What does each edge need to store?•What does each face need to store?•Answers are in C, but converting to C++ classes is easyVertex Data•Vertices need to know where they are, and they will be copied as part of the algorithm•For rendering, may want:–Per-vertex normals: Compute at the end by averaging normals from faces that share the vertex–Texture coordinatesstruct Vertex {float x[3];float t[2];float n[3];int num_faces;}Edge Data•Need to know endpoints, new vertex when split, and new edges when split–Store pointers as indexes into a list•Also want to identify boundary edges, which should not have vertices perturbed when splitstruct Edge {int v_start; // Index of start vertexint v_end; // Index of end vertexint v_new; // Index of new vertexint e_start; // Index of one sub-edgeint e_end; // Index of other sub-edgebool boundary;}Face Data•Faces are triangles that need to


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UW-Madison CS 559 - Subdivision Schemes

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