Justin Solomon MIT, Spring Numerical Geometry of Nonrigid Shapes

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1 Justin Solomon MIT, Spring 2017 Numerical Geometry of Nonrigid Shapes

2 Intrinsically far Extrinsically close

3 Geodesic distance [jee-uh-des-ik dis-tuh-ns]: Length of the shortest path, constrained not to leave the manifold.

4 Local minima Straightest Geodesics on Polyhedral Surfaces (Polthier and Schmies)

5 Not OK OK Computationally expensive Extrinsic may suffice for near vs. far

6 Locally OK Single source Multi-source All-pairs

7 Approximate geodesics as paths along edges Meshes are graphs

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12 Asymmetric Anisotropic May not improve under refinement

13 Graph shortest-path does not converge to geodesic distance.

14 Geodesic distances are need special discretization. So, we need to understand the theory! \begin{math}

15 Globally shortest path Local minimizer of length Locally straight path

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17 Equality exactly when parameterized by arc length. Proof on board.

18 Lemma. Let γ t : a, b S be a family of curves with fixed endpoints in surface S; assume γ is parameterized by arc length at t=0. Then, Corollary. γ: a, b S is a geodesic iff

19 The only acceleration is out of the surface No steering wheel!

20 Boundary value problem Given: γ 0, γ(1) Initial value problem (ODE) Given: γ 0, γ (0)

21 γ v 1 where γ v is (unique) geodesic from p with velocity v.

22 Locally minimizing distance is not enough to be a shortest path!

23 Cut point: Point where geodesic ceases to be minimizing Set of cut points from a source p

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25 \end{math}

26 Graph shortest path algorithms are well-understood. Can we use them (carefully) to compute geodesics?

27 Shortest path had to come from somewhere. All pieces of a shortest path are optimal.

28 Initialization:

29 Iteration k: During each iteration, S remains optimal.

30 CS 468, 2009

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32 Dijkstra s algorithm, modified to approximate geodesic distances.

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36 Given: ~x 2 ~n Find: ~x 1 Derivation from Bronstein et al., Numerical Geometry of Nonrigid Shapes

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38 Quadratic equation for p Find:

39 Bronstein et al., Numerical Geometry of Nonrigid Shapes Two orientations for the normal

40 Bronstein et al., Numerical Geometry of Nonrigid Shapes Two orientations for the normal

41 Update should be from a different triangle! Bronstein et al., Numerical Geometry of Nonrigid Shapes Front from outside the triangle

42 Bronstein et al., Numerical Geometry of Nonrigid Shapes Front from outside the triangle

43 Bronstein et al., Numerical Geometry of Nonrigid Shapes Must reach x 3 after x 1 and x 2

44 Alternative edge-based update: Add connections as needed [Kimmel and Sethian 1998]

45 Bronstein, Numerical Geometry of Nonrigid Shapes

46 Modified update step Update all triangles adjacent to a given vertex

47 Greek: Image Solutions are geodesic distance

48 STILL AN APPROXIMATION

49 [Novotni and Klein 2002]: Circular wavefront

50 Raster scan and/or parallelize Bronstein, Numerical Geometry of Nonrigid Shapes Grids and parameterized surfaces

51 Crane, Weischedel, and Wardetzky. Geodesics in Heat. TOG 2013.

52 Trace gradient of distance function

53 Equal left and right angles Polthier and Schmies. Shortest Geodesics on Polyhedral Surfaces. SIGGRAPH course notes Trace a single geodesic exactly

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55 Surazhsky et al. Fast Exact and Approximate Geodesics on Meshes. SIGGRAPH Dijkstra-style front with windows explaining source.

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57 Cut point: Point where geodesic ceases to be minimizing Set of cut points from a source p

58 Function on surface expressing difference in triangle inequality Intersection by pointwise multiplication Sun, Chen, Funkhouser. Fuzzy geodesics and consistent sparse correspondences for deformable shapes. CGF2010. Stable version of geodesic distance

59 Morphological operators to fill holes rather than remeshing Campen and Kobbelt. Walking On Broken Mesh: Defect-Tolerant Geodesic Distances and Parameterizations. Eurographics 2011.

60 Sample points Geodesic field Triangulate (Delaunay) Fix edges Query (planar embedding) Xin, Ying, and He. Constant-time all-pairs geodesic distance query on triangle meshes. I3D 2012.

61 From Geodesic Methods in Computer Vision and Graphics (Peyré et al., FnT 2010)

62 Heeren et al. Time-discrete geodesics in the space of shells. SGP 2012.

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65 Justin Solomon MIT, Spring 2017 Numerical Geometry of Nonrigid Shapes