Riemann Surface. David Gu. SMI 2012 Course. University of New York at Stony Brook. 1 Department of Computer Science

Transcription

1 Riemann Surface 1 1 Department of Computer Science University of New York at Stony Brook SMI 2012 Course

2 Thanks Thanks for the invitation.

3 Collaborators The work is collaborated with Shing-Tung Yau, Feng Luo, Tony Chan, Paul Thompson, Yalin Wang, Ronald Lok Ming Lui, Hong Qin, Dimitris Samaras, Jie Gao, Arie Kaufman, and many other mathematicians, computer scientists and medical doctors.

4 Klein s Program Klein s Erlangen Program Different geometries study the invariants under different transformation groups. Geometries Topology - homeomorphisms - Conformal Transformations Riemannian Geometry - Isometries Differential Geometry - Rigid Motion

5 Motivation Scientific View Conformal structure is a natural geometric structure; many physics phenomena are governed by conformal geometry, such as electro-magnetic fields, brownian motion, phase transition, percolation... Conformal geometry is part of nature.

6 Motivation Practical View Conformal geometry offers the theoretic frameworks for Shape Space Mapping Space

7 Shape Space A shape embedded in E 3 has the following level of information 1 Topological structure (genus, boundaries) 2 Conformal structure, (conformal module ) 3 Riemannian metric, (conformal factor) 4 Embedding structure, (mean curvature )

8 Shape Space Surfaces can be classified according to their geometric structures Each topological equivalent class has infinite many conformal equivalent classes, which form a finite dimensional Riemannian manifold. Each conformal equivalent class has infinite many isometric classes, which form an infinite dimensional space.

9 Mapping Space Given two metric surfaces, fixing the homotopy type, there are infinite many of diffeomorphisms between them, which form the mapping space. Each mapping uniquely determines a differential - Beltrami differential. Each Beltrami differential essentially uniquely determines a diffeomorphism. The mapping space is converted to a convex functional space. Variational calculus can be carried out in the functional space.

10 Motivation Conformal geometry lays down the theoretic foundation for Surface mapping Shape classification Shape analysis Applied in computer graphics, computer vision, geometric modeling, wireless sensor networking and medical imaging, and many other engineering, medical fields.

11 Sample of Applications 1 Computational Topology: Shortest word problem 2 Geometric Modeling: Manifold Spline 3 Graph Theory: Graph embedding 4 Meshing: Smooth surface meshing that ensures curvature convergence. 5 Shape Analysis: A discrete Heat kernel determines the discrete metric.

12 History History In pure mathematics, conformal geometry is the intersection of complex analysis, algebraic topology, Riemann surface theory, algebraic curves, differential geometry, partial differential equation. In applied mathematics, computational complex function theory has been developed, which focuses on the conformal mapping between planar domains. Recently, computational conformal geometry has been developed, which focuses on the conformal mapping between surfaces.

13 History History Conventional conformal geometric method can only handle the mappings among planar domains. Applied in thin plate deformation (biharmonic equation) Membrane vibration Electro-magnetic field design (Laplace equation) Fluid dynamics Aerospace design

14 Reasons for Booming Data Acquisition 3D scanning technology becomes mature, it is easier to obtain surface data.

15 System Layout

16 3D Scanning Results

17 3D Scanning Results

18 System Layout

19 Reasons for Booming Our group has developed high speed 3D scanner, which can capture dynamic surfaces 180 frames per second. Computational Power Computational power has been increased tremendously. With the incentive in graphics, GPU becomes mature, which makes numerical methods for solving PDE s much easier.

20 Fundamental Problems 1 Given a Riemannian metric on a surface with an arbitrary topology, determine the corresponding conformal structure. 2 Compute the complete conformal invariants (conformal modules), which are the coordinates of the surface in the Teichmuller shape space. 3 Fix the conformal structure, find the simplest Riemannian metric among all possible Riemannian metrics 4 Given desired Gaussian curvature, compute the corresponding Riemannian metric. 5 Given the distortion between two conformal structures, compute the quasi-conformal mapping. 6 Compute the extremal quasi-conformal maps. 7 Conformal welding, glue surfaces with various conformal modules, compute the conformal module of the glued surface.

21 Complete Tools Computational Library 1 Compute conformal mappings for surfaces with arbitrary topologies 2 Compute conformal modules for surfaces with arbitrary topologies 3 Compute Riemannian metrics with prescribed curvatures 4 Compute quasi-conformal mappings by solving Beltrami equation

22 Books The theory, algorithms and sample code can be found in the following books. You can find them in the book store.

23 Source Code Library Please me for updated code library on computational conformal geometry.

24 Conformal Mapping

25 Holomorphic Function Definition (Holomorphic Function) Suppose f :C C is a complex function, f : x+ iy u(x,y)+iv(x,y), if f satisfies Riemann-Cauchy equation u x = v y, u y = v x, then f is a holomorphic function. Denote dz= dx+idy,d z= dx idy, z = 1 2 ( x i y ), z = 1 2 ( x +i y ) then if f z = 0, then f is holomorphic.

26 biholomorphic Function Definition (biholomorphic Function) Suppose f :C C is invertible, both f and f 1 are holomorphic, then then f is a biholomorphic function. γ0 D 0 γ1 γ2 D 1

27 Conformal Map S 1 {(U α,φ α )} S 2 {(V β,τ β )} U α f V β φ α τ β z τ β f φ 1 α w The restriction of the mapping on each local chart is biholomorphic, then the mapping is conformal.

28 Conformal Mapping

29 Definition (Conformal Map) Let φ :(S 1,g 1 ) (S 2,g 2 ) is a homeomorphism, φ is conformal if and only if φ g 2 = e 2u g 1. Conformal Mapping preserves angles. θ θ

30 Conformal Mapping Conformal maps Properties Map a circle field on the surface to a circle field on the plane.

31 Quasi-Conformal Map Diffeomorphisms: maps ellipse field to circle field.

32 Conformal Structure

33 Riemann Surface Intuition 1 A conformal structure is a equipment on a surface, such that the angles can be measured, but the lengths, areas can not. 2 A Riemannian metric naturally induces a conformal structure. 3 Different Riemannian metrics can induce a same conformal structure.

34 Manifold Definition (Manifold) M is a topological space, {U α } α I is an open covering of M, M α U α. For each U α, φ α : U α R n is a homeomorphism. The pair (U α,φ α ) is a chart. Suppose U α U β = /0, the transition function φ αβ : φ α (U α U β ) φ β (U α U β ) is smooth φ αβ = φ β φ 1 α then M is called a smooth manifold, {(U α,φ α )} is called an atlas.

35 Manifold U α U β S φ α φ β φ αβ φ α (U α ) φ β (U β )

36 Conformal Atlas Definition (Conformal Atlas) Suppose S is a topological surface, (2 dimensional manifold), A is an atlas, such that all the chart transition functions φ αβ :C C are bi-holomorphic, then A is called a conformal atlas. Definition (Compatible Conformal Atlas) Suppose S is a topological surface, (2 dimensional manifold), A 1 and A 2 are two conformal atlases. If their union A 1 A 2 is still a conformal atlas, we say A 1 and A 2 are compatible.

37 Conformal Structure The compatible relation among conformal atlases is an equivalence relation. Definition (Conformal Structure) Suppose S is a topological surface, consider all the conformal atlases on M, classified by the compatible relation {all conformal atlas}/ each equivalence class is called a conformal structure. In other words, each maximal conformal atlas is a conformal structure.

38 Conformal Structure Definition (Conformal equivalent metrics) Suppose g 1,g 2 are two Riemannian metrics on a manifold M, if g 1 = e 2u g 2,u : M R then g 1 and g 2 are conformal equivalent. Definition (Conformal Structure) Consider all Riemannian metrics on a topological surface S, which are classified by the conformal equivalence relation, {Riemannian metrics on S}/, each equivalence class is called a conformal structure.

39 Relation between Riemannian metric and Conformal Structure Definition (Isothermal coordinates) Suppose (S,g) is a metric surface, (U α,φ α ) is a coordinate chart, (x, y) are local parameters, if g = e 2u (dx 2 + dy 2 ), then we say (x, y) are isothermal coordinates. Theorem Suppose S is a compact metric surface, for each point p, there exits a local coordinate chart (U,φ), such that p U, and the local coordinates are isothermal.

40 Riemannian metric and Conformal Structure Corollary For any compact metric surface, there exists a natural conformal structure. Definition (Riemann surface) A topological surface with a conformal structure is called a Riemann surface.

41 Riemannian metric and Conformal Structure Theorem All compact orientable metric surfaces are Riemann surfaces.

42 Uniformization

43 Conformal Canonical Representations Theorem (Poincaré Uniformization Theorem) Let (Σ, g) be a compact 2-dimensional Riemannian manifold. Then there is a metric g=e 2λ g conformal to g which has constant Gauss curvature. Spherical David Euclidean Gu Hyperbolic

44 Uniformization of Open Surfaces Definition (Circle Domain) A domain in the Riemann sphere Ĉ is called a circle domain if every connected component of its boundary is either a circle or a point. Theorem Any domain Ω in Ĉ, whose boundary Ω has at most countably many components, is conformally homeomorphic to a circle domain Ω in Ĉ. Moreover Ω is unique upto Möbius transformations, and every conformal automorphism of Ω is the restriction of a Möbius transformation.

45 Uniformization of Open Surfaces Spherical Euclidean Hyperbolic

46 Conformal Canonical Representation Simply Connected Domains

47 Conformal Canonical Forms Topological Quadrilateral p 4 p 3 p 1 p 2 p 4 p 3 p 1 p 2

48 Conformal Canonical Forms Multiply Connected Domains

49 Conformal Canonical Forms Multiply Connected Domains

50 Conformal Canonical Forms Multiply Connected Domains

51 Conformal Canonical Forms Multiply Connected Domains

52 Conformal Canonical Representations Definition (Circle Domain in a Riemann Surface) A circle domain in a Riemann surface is a domain, whose complement s connected components are all closed geometric disks and points. Here a geometric disk means a topological disk, whose lifts in the universal cover or the Riemann surface (which is H 2, R 2 or S 2 are round. Theorem Let Ω be an open Riemann surface with finite genus and at most countably many ends. Then there is a closed Riemann surface R such that Ω is conformally homeomorphic to a circle domain Ω in R. More over, the pair (R,Ω ) is unique up to conformal homeomorphism.

53 Conformal Canonical Form Tori with holes

54 Conformal Canonical Form High Genus Surface with holes

55 Teichmüller Space

56 Teichmüller Space Shape Space - Intuition 1 All metric surfaces are classified by conformal equivalence relation. 2 All conformal equivalences form a finite dimensional manifold. 3 The manifold has a natural Riemannian metric, which measures the distortions between two conformal structures.

57 Teichmüller Theory: Conformal Mapping Definition (Conformal Mapping) Suppose (S 1,g 1 ) and (S 2,g 2 ) are two metric surfaces, φ : S 1 S 2 is conformal, if on S 1 g 1 = e 2λ φ g 2, where φ g 2 is the pull-back metric induced by φ. Definition (Conformal Equivalence) Suppose two surfaces S 1,S 2 with marked homotopy group generators, {a i,b i } and {α i,β i }. If there exists a conformal map φ : S 1 S 2, such that φ [a i ]=[α i ],φ [b i ]=[β i ], then we say two marked surfaces are conformal equivalent.

58 Teichmüller Space Definition (Teichmüller Space) Fix the topology of a marked surface S, all conformal equivalence classes sharing the same topology of S, form a manifold, which is called the Teichmüller space of S. Denoted as T S. Each point represents a class of surfaces. A path represents a deformation process from one shape to the other. The Riemannian metric of Teichmüller space is well defined.

59 Teichmüller Space Topological Quadrilateral p 4 p 3 p 1 p 2 Conformal module: h w. The Teichmüller space is 1 dimensional.

60 Teichmüller Space Multiply Connected Domains Conformal Module : centers and radii, with Möbius ambiguity. The Teichmüller space is 3n 3 dimensional, n is the number of holes.

61 Topological Pants Genus 0 surface with 3 boundaries is conformally mapped to the hyperbolic plane, such that all boundaries become geodesics.

62 Teichmüller Space Topological Pants γ 1 γ 2 γ 0 γ 0 τ 2 γ 0 2 τ0 γ1 2 γ1 τ0 γ 1 2 τ 1 τ 22 γ2 2 τ1 τ2 2 γ2 2 τ1 τ 0 τ 22 γ0 2 γ0 2 γ 1 2 τ 1 τ 2 γ 0 2

63 Teichmüller Space Topological Pants Decomposition - 2g 2 pairs of Pants P1 γ p1 p2 P2 θ

64 Compute Teichmüller coordinates Step 1. Compute the hyperbolic uniformization metric. Step 2. Compute the Fuchsian group generators.

65 Compute Teichmüller Coordinates Step 3. Pants decomposition using geodesics and compute the twisting angle.

66 Teichmüller Coordinates Compute Teichmüller coordinates Compute the pants decomposition using geodesics and compute the twisting angle.

67 Quasi-Conformal Maps

68 Quasi-Conformal Mappings Intuition 1 All diffeomorphisms between two compact Riemann surfaces are quasi-conformal. 2 Each quasi-conformal mapping corresponds to a unique Beltrami differential. 3 The space of diffeomorphisms equals to the space of all Beltrami differentials. 4 Variational calculus can be carried out on the space of diffeomorphisms.

69 Quasi-Conformal Map Most homeomorphisms are quasi-conformal, which maps infinitesimal circles to ellipses.

70 Beltrami-Equation Beltrami Coefficient Let φ : S 1 S 2 be the map, z,w are isothermal coordinates of S 1, S 2, Beltrami equation is defined as µ < 1 φ z = µ(z) φ z

71 Diffeomorphism Space Theorem Given two genus zero metric surface with a single boundary, {Diffeomorphisms} = {Beltrami Coefficient}. {Mobius}

72 Solving Beltrami Equation The problem of computing Quasi-conformal map is converted to compute a conformal map. Solveing Beltrami Equation Given metric surfaces (S 1,g 1 ) and (S 2,g 2 ), let z,w be isothermal coordinates of S 1,S 2,w = φ(z). Then g 1 = e 2u 1 dzd z (1) g 2 = e 2u 2 dwd w, (2) φ :(S 1,g 1 ) (S 2,g 2 ), quasi-conformal with Beltrami coefficient µ. φ :(S 1,φ g 2 ) (S 2,g 2 ) is isometric φ g 2 = e u 2 dw 2 = e u 2 dz+ µddz 2. φ :(S 1, dz+ µddz 2 ) (S 2,g 2 ) is conformal.

73 Quasi-Conformal Map Examples D3 D2 D0 p q p q D1 D3 D2 D0 p q D1

74 Quasi-Conformal Map Examples

75 Variational Calculus on Diffeomorphism Space Let {µ(t)} be a family of Beltrami coefficients depends on a parameter t, µ(t)(z) = µ(z)+tν(z)+tε(t)(z) for z C, with suitable µ, ν,ε(t) L (C), where as t 0, f µ(t) (w)=f µ (w)+tv(f µ,ν)(w)+o( t ), V(f µ,ν)(w)= f µ (w)(f µ (w) 1) ν(z)((f µ ) z (z)) 2 π C f µ (z)(f µ (z) 1)(f µ (z) f µ (w)) dx

76 Solving Beltrami Equation

77 Summary Conformal structure is more flexible than Riemannian metric Conformal structure is more rigid than topology Conformal geometry can be used for a broad range of engineering applications.

78 Thanks For more information, please to Thank you!