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Triangle Meshes Notation Validity of Triangle Meshes Topology/Geometry

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Triangle Meshes Notation Validity of Triangle Meshes Topology/Geometry Notation • nT = #tris; nV = #verts; nE = #edges • Euler: nV – nE + nT = 2 for a simple closed surface Triangle meshes – and in general sums to small integer – argument for implication that nT:nE:nV is about 2:3:1 CS 4620 Lecture 11 y et al.] ole [F Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 1 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 2 Validity of triangle meshes Topology/geometry examples • in many cases we care about the mesh being able to • same geometry, different mesh topology: bound a region of space nicely • in other cases we want triangle meshes to fulfill assumptions of algorithms that will operate on them (and may fail on malformed input) • two completely separate issues: • same mesh topology, different geometry: – topology: how the triangles are connected (ignoring the positions entirely) – geometry: where the triangles are in 3D space Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 3 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 4 Topological validity Geometric validity • strongest property, and most simple: be a manifold • generally want non-self-intersecting surface – this means that no points should be "special" • hard to guarantee in general – interior points are fine – because far-apart parts of mesh might intersect – edge points: each edge should have exactly 2 triangles – vertex points: each vertex should have one loop of triangles • not too hard to weaken this to allow boundaries y et al.] ole [F Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 5 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 6 Representation of triangle meshes Representations for triangle meshes • Compactness • Separate triangles • Efficiency for rendering • Indexed triangle set – enumerate all triangles as triples of 3D points – shared vertices • Efficiency of queries • Triangle strips and triangle fans – all vertices of a triangle – compression schemes for transmission to hardware" – all triangles around a vertex • Triangle-neighbor data structure – neighboring triangles of a triangle – supports adjacency queries – (need depends on application) • Winged-edge data structure • finding triangle strips – supports general polygon meshes • computing subdivision surfaces • mesh editing Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 7 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 8 Separate triangles Separate triangles • array of triples of points – float[nT][3][3]: about 72 bytes per vertex • 2 triangles per vertex (on average) • 3 vertices per triangle • 3 coordinates per vertex • 4 bytes per coordinate (float) • various problems – wastes space (each vertex stored 6 times) – cracks due to roundoff – difficulty of finding neighbors at all Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 9 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 10 Indexed triangle set Indexed triangle set • Store each vertex once • Store each vertex once • Each triangle points to its three vertices • Each triangle points to its three vertices Triangle { Triangle { Vertex vertex[3]; Vertex vertex[3]; } } Vertex { Vertex { float position[3]; // or other data float position[3]; // or other data } } // ... or ... // ... or ... Mesh { Mesh { float verts[nv][3]; // vertex positions (or other data) float verts[nv][3]; // vertex positions (or other data) int tInd[nt][3]; // vertex indices int tInd[nt][3]; // vertex indices } } Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 11 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 11 Indexed triangle set Indexed triangle set • array of vertex positions – float[nV][3]: 12 bytes per vertex • (3 coordinates x 4 bytes) per vertex • array of triples of indices (per triangle) – int[nT][3]: about 24 bytes per vertex • 2 triangles per vertex (on average) • (3 indices x 4 bytes) per triangle • total storage: 36 bytes per vertex (factor of 2 savings) • represents topology and geometry separately • finding neighbors is at least well defined Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 12 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 13 Triangle strips Triangle strips • Take advantage of the mesh property – each triangle is usually adjacent to the previous – let every vertex create a triangle by reusing the second and third vertices of the previous triangle 4, 0 – every sequence of three vertices produces a triangle (but not in the same order) – e. g., 0, 1, 2, 3, 4, 5, 6, 7, … leads to # (0 1 2), (2 1 3), (2 3 4), (4 3 5), (4 5 6), (6 5 7), … – for long strips, this requires about one index per triangle Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 14 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 15 Triangle strips Triangle strips • array of vertex positions – float[nV][3]: 12 bytes per vertex • (3 coordinates x 4 bytes) per vertex • array of index lists – int[n ][variable]: 2 + n indices per strip 4, 0 S – on average, (1 + !) indices per triangle (assuming long strips) • 2 triangles per vertex (on average) • about 4 bytes per triangle (on average) • total is 20 bytes per vertex (limiting best case) – factor of 3.6 over separate triangles; 1.8 over indexed mesh Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 15 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 16 Triangle fans Triangle neighbor structure • Same idea as triangle strips, but keep oldest rather than • Extension to indexed newest triangle set – every sequence of three vertices produces a triangle • Triangle points to its three – e. g., 0, 1, 2, 3, 4, 5, … leads to neighboring triangles # (0 1 2), (0 2 3), (0 3 4), (0 3 5), … • Vertex points to a single – for long fans, this requires neighboring triangle about one index per triangle • Can now enumerate • Memory considerations exactly the same as triangle strip triangles around a vertex Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 17 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 18 Triangle neighbor structure Triangle neighbor structure • Extension to indexed • Extension to indexed triangle set triangle set • Triangle points to its three • Triangle points to its three neighboring triangles neighboring triangles • Vertex points to a single • Vertex points to a single neighboring triangle neighboring triangle • Can now enumerate • Can now enumerate triangles around a vertex triangles around a vertex Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 18 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 18 Triangle neighbor structure Triangle neighbor structure Triangle { Triangle neighbor[3]; Vertex vertex[3]; } // t.neighbor[i] is adjacent // across the edge from i to i+1 Vertex { // ... per-vertex data ... Triangle t; // any adjacent tri } // ... or ... Mesh { // ... per-vertex data ... int tInd[nt][3]; // vertex indices int tNbr[nt][3]; // indices of neighbor triangles int vTri[nv]; // index of any adjacent triangle } Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 19 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 20 Triangle neighbor structure Triangle neighbor structure Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 20 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 20 Triangle neighbor structure Triangle neighbor structure Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 20 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 20 Triangle neighbor structure Triangle neighbor structure TrianglesOfVertex(v) { • indexed mesh was 36 bytes per vertex t = v.t; do { • add an array of triples of indices (per triangle) find t.vertex[i] == v; t = t.nbr[pred(i)]; – int[n ][3]: about 24 bytes per vertex } while (t != v.t); T } • 2 triangles per vertex (on average) pred(i) = (i+2) % 3; • (3 indices x 4 bytes) per triangle succ(i) = (i+1) % 3; • add an array of representative triangle per vertex – int[nV]: 4 bytes per vertex • total storage: 64 bytes per vertex – still not as much as separate triangles Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 21 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 22 Triangle neighbor structure—refined Triangle neighbor structure—refined Triangle { Triangle { Edge nbr[3]; Edge nbr[3]; Vertex vertex[3]; Vertex vertex[3]; } } // if t.nbr[i].i == j // if t.nbr[i].i == j // then t.nbr[i].t.nbr[j] == t // then t.nbr[i].t.nbr[j] == t Edge { Edge { // the i-th edge of triangle t // the i-th edge of triangle t Triangle t; Triangle t; int i; // in {0,1,2} int i; // in {0,1,2} // in practice t and i share 32 bits // in practice t and i share 32 bits } } Vertex { Vertex { // ... per-vertex data ... // ... per-vertex data ... Edge e; // any edge leaving vertex Edge e; // any edge leaving vertex } T0.nbr[0] = { T1, 2 } } T0.nbr[0] = { T1, 2 } T1.nbr[2] = { T0, 0 } T1.nbr[2] = { T0, 0 } V0.e = { T1, 0 } V0.e = { T1, 0 } Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 23 Cornell CS4620 Fall 2006 •!Lecture 11 © 2008 Steve Marschner • 23 Triangle neighbor structure Winged-edge mesh TrianglesOfVertex(v) { • Edge-centric rather than {t, i} = v.e; do { face-centric {t, i} = t.nbr[pred(i)]; } while (t != v.t); – therefore also works for } polygon meshes pred(i) = (i+2) % 3; • Each (oriented) edge points to: succ(i) = (i+1) % 3; – left and right forward edges – left and right backward edges – front and back vertices – left and right faces • Each face or vertex points to
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