Discrete Differential Geometry Planar Curves 2D/3D Shape Manipulation, 3D Printing March 13, 2013 Slides from Olga Sorkine, Eitan Grinspun

# Differential Geometry – Motivation ● Describe and analyze geometric characteristics of shapes  e.g. how smooth? March 13, 2013Olga Sorkine-Hornung2

# Differential Geometry – Motivation ● Describe and analyze geometric characteristics of shapes  e.g. how smooth?  how shapes deform March 13, 2013Olga Sorkine-Hornung3

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood March 13, 2013Olga Sorkine-Hornung4

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood manifold point March 13, 2013Olga Sorkine-Hornung5

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood manifold point March 13, 2013Olga Sorkine-Hornung6

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood manifold point continuous 1-1 mapping March 13, 2013Olga Sorkine-Hornung7

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood manifold point continuous 1-1 mapping non-manifold point March 13, 2013Olga Sorkine-Hornung8

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood manifold point continuous 1-1 mapping non-manifold point x March 13, 2013Olga Sorkine-Hornung9

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood March 13, 2013Olga Sorkine-Hornung10

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood continuous 1-1 mapping March 13, 2013Olga Sorkine-Hornung11

# Differential Geometry Basics ● Geometry of manifolds ● Things that can be discovered by local observation: point + neighborhood continuous 1-1 mapping u v If a sufficiently smooth mapping can be constructed, we can look at its first and second derivatives Tangents, normals, curvatures Distances, curve angles, topology March 13, 2013Olga Sorkine-Hornung12

# Planar Curves March 13, 2013Olga Sorkine-Hornung13

# Curves ● 2D: ● must be continuous March 13, 2013Olga Sorkine-Hornung14

# ● Equal pace of the parameter along the curve ● len (p(t 1 ), p(t 2 )) = |t 1 – t 2 | Arc Length Parameterization March 13, 2013Olga Sorkine-Hornung15

# Secant ● A line through two points on the curve. March 13, 2013Olga Sorkine-Hornung16

# Secant ● A line through two points on the curve. March 13, 2013Olga Sorkine-Hornung17

# Tangent March 13, 2013Olga Sorkine-Hornung18 ● The limiting secant as the two points come together.

# Secant and Tangent – Parametric Form ● Secant: p(t) – p(s) ● Tangent: p(t) = (x(t), y(t), …) T ● If t is arc-length: ||p(t)|| = 1 March 13, 2013Olga Sorkine-Hornung19

# Tangent, normal, radius of curvature p r Osculating circle “best fitting circle” March 13, 2013Olga Sorkine-Hornung20

# Circle of Curvature ● Consider the circle passing through three points on the curve… March 13, 2013Olga Sorkine-Hornung21

# Circle of Curvature ● …the limiting circle as three points come together. March 13, 2013Olga Sorkine-Hornung22

# Radius of Curvature, r March 13, 2013Olga Sorkine-Hornung23

# Radius of Curvature, r = 1/  Curvature March 13, 2013Olga Sorkine-Hornung24

# Signed Curvature ++ –– March 13, 2013Olga Sorkine-Hornung25 ● Clockwise vs counterclockwise traversal along curve.

# Gauss map ● Point on curve maps to point on unit circle.

# Curvature = change in normal direction curveGauss mapcurveGauss map ● Absolute curvature (assuming arc length t ) ● Parameter-free view: via the Gauss map March 13, 2013Olga Sorkine-Hornung27

# Curvature Normal ● Assume t is arc-length parameter [Kobbelt and Schröder] p(t)p(t) p(t)p(t) March 13, 2013Olga Sorkine-Hornung28

# Curvature Normal – Examples March 13, 2013Olga Sorkine-Hornung29

# Turning Number, k ● Number of orbits in Gaussian image. March 13, 2013Olga Sorkine-Hornung30

# ● For a closed curve, the integral of curvature is an integer multiple of 2 . ● Question: How to find curvature of circle using this formula? Turning Number Theorem +2  –2  +4  0 March 13, 2013Olga Sorkine-Hornung31

# Discrete Planar Curves March 13, 2013Olga Sorkine-Hornung32

# Discrete Planar Curves ● Piecewise linear curves ● Not smooth at vertices ● Can’t take derivatives ● Generalize notions from the smooth world for the discrete case! March 13, 2013Olga Sorkine-Hornung33

# Tangents, Normals ● For any point on the edge, the tangent is simply the unit vector along the edge and the normal is the perpendicular vector March 13, 2013Olga Sorkine-Hornung34

# Tangents, Normals ● For vertices, we have many options March 13, 2013Olga Sorkine-Hornung35

# ● Can choose to average the adjacent edge normals Tangents, Normals March 13, 2013Olga Sorkine-Hornung36

# Tangents, Normals ● Weight by edge lengths March 13, 2013Olga Sorkine-Hornung37

# Inscribed Polygon, p ● Connection between discrete and smooth ● Finite number of vertices each lying on the curve, connected by straight edges. March 13, 2013Olga Sorkine-Hornung38

# p1p1 p2p2 p3p3 p4p4 The Length of a Discrete Curve ● Sum of edge lengths March 13, 2013Olga Sorkine-Hornung39

# The Length of a Continuous Curve ● Length of longest of all inscribed polygons. March 13, 2013Olga Sorkine-Hornung40 sup = “supremum”. Equivalent to maximum if maximum exists.

# ● …or take limit over a refinement sequence h = max edge length The Length of a Continuous Curve March 13, 2013Olga Sorkine-Hornung41

# Curvature of a Discrete Curve ● Curvature is the change in normal direction as we travel along the curve no change along each edge – curvature is zero along edges March 13, 2013Olga Sorkine-Hornung42

# Curvature of a Discrete Curve ● Curvature is the change in normal direction as we travel along the curve normal changes at vertices – record the turning angle! March 13, 2013Olga Sorkine-Hornung43

# Curvature of a Discrete Curve ● Curvature is the change in normal direction as we travel along the curve normal changes at vertices – record the turning angle! March 13, 2013Olga Sorkine-Hornung44

# ● Curvature is the change in normal direction as we travel along the curve same as the turning angle between the edges Curvature of a Discrete Curve March 13, 2013Olga Sorkine-Hornung45

# ● Zero along the edges ● Turning angle at the vertices = the change in normal direction  1,  2 > 0,  3 < 0 11 22 33 Curvature of a Discrete Curve March 13, 2013Olga Sorkine-Hornung46

# Total Signed Curvature ● Sum of turning angles 11 22 33 March 13, 2013Olga Sorkine-Hornung47

# Discrete Gauss Map ● Edges map to points, vertices map to arcs.

# Discrete Gauss Map ● Turning number well-defined for discrete curves.

# Discrete Turning Number Theorem ● For a closed curve, the total signed curvature is an integer multiple of 2 .  proof: sum of exterior angles March 13, 2013Olga Sorkine-Hornung50

# Discrete Curvature – Integrated Quantity! ● Integrated over a local area associated with a vertex March 13, 2013 11 22 Olga Sorkine-Hornung51

# Discrete Curvature – Integrated Quantity! ● Integrated over a local area associated with a vertex March 13, 2013 11 22 A1A1 Olga Sorkine-Hornung52

# Discrete Curvature – Integrated Quantity! ● Integrated over a local area associated with a vertex March 13, 2013 11 22 A1A1 A2A2 Olga Sorkine-Hornung53

# Discrete Curvature – Integrated Quantity! ● Integrated over a local area associated with a vertex March 13, 2013 11 22 A1A1 A2A2 The vertex areas A i form a covering of the curve. They are pairwise disjoint (except endpoints). Olga Sorkine-Hornung54

# Structure Preservation ● Arbitrary discrete curve  total signed curvature obeys discrete turning number theorem  even coarse mesh (curve)  which continuous theorems to preserve? that depends on the application… discrete analogue of continuous theorem March 13, 2013Olga Sorkine-Hornung55

# Convergence ● Consider refinement sequence  length of inscribed polygon approaches length of smooth curve  in general, discrete measure approaches continuous analogue  which refinement sequence? depends on discrete operator pathological sequences may exist  in what sense does the operator converge? (point-wise, L 2 ; linear, quadratic) March 13, 2013Olga Sorkine-Hornung56

# Recap Convergence Structure- preservation In the limit of a refinement sequence, discrete measures of length and curvature agree with continuous measures. For an arbitrary (even coarse) discrete curve, the discrete measure of curvature obeys the discrete turning number theorem. March 13, 2013Olga Sorkine-Hornung57

Thank You March 13, 2013