lecture 2 - techniontosca.cs.technion.ac.il/.../stanford09_geometry3.pdf · 2009. 1. 16. ·...
TRANSCRIPT
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Differential geometry II
Lecture 2
© Alexander & Michael Bronstein
tosca.cs.technion.ac.il/book
Numerical geometry of non-rigid shapes
Stanford University, Winter 2009
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2Numerical geometry of non-rigid shapes Differential geometry
Intrinsic & extrinsic geometry
� First fundamental form describes completely the intrinsic geometry.
� Second fundamental form describes completely the extrinsic
geometry – the “layout” of the shape in ambient space.
� First fundamental form is invariant to isometry.
� Second fundamental form is invariant to rigid motion (congruence).
� If and are congruent (i.e., ), then
they have identical intrinsic and extrinsic geometries.
� Fundamental theorem: a map preserving the first and the second
fundamental forms is a congruence.
Said differently: an isometry preserving second fundamental form is a
restriction of Euclidean isometry.
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3Numerical geometry of non-rigid shapes Differential geometry
An intrinsic view
� Our definition of intrinsic geometry (first fundamental form) relied so far
on ambient space.
� Can we think of our surface as of an abstract manifold immersed
nowhere?
� What ingredients do we really need?
� Smooth two-dimensional manifold
� Tangent space at each point.
� Inner product
� These ingredients do not require any ambient space!
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4Numerical geometry of non-rigid shapes Differential geometry
Riemannian geometry
� Riemannian metric: bilinear symmetric
positive definite smooth map
� Abstract inner product on tangent space
of an abstract manifold.
� Coordinate-free.
� In parametrization coordinates is
expressed as first fundamental form.
� A farewell to extrinsic geometry!
Bernhard Riemann
(1826-1866)
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5Numerical geometry of non-rigid shapes Differential geometry
An intrinsic view
� We have two alternatives to define the intrinsic metric using the path
length.
� Extrinsic definition:
� Intrinsic definition:
� The second definition appears more general.
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6Numerical geometry of non-rigid shapes Differential geometry
Nash’s embedding theorem
� Embedding theorem (Nash, 1956): any
Riemannian metric can be realized as an
embedded surface in Euclidean space of
sufficiently high yet finite dimension.
� Technical conditions:
� Manifold is
� For an -dimensional manifold,
embedding space dimension is
� Practically: intrinsic and extrinsic views are equivalent!
John Forbes Nash
(born 1928)
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7Numerical geometry of non-rigid shapes Differential geometry
Uniqueness of the embedding
� Nash’s theorem guarantees existence of embedding.
� It does not guarantee uniqueness.
� Embedding is clearly defined up to a congruence.
� Are there cases of non-trivial non-uniqueness?
Formally:
� Given an abstract Riemannian manifold , and an embedding
, does there exist another embedding
such that and are incongruent?
Said differently:
Do isometric yet incongruent shapes exist?
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8Numerical geometry of non-rigid shapes Differential geometry
Bending
� Shapes admitting incongruent isometries are called bendable.
� Plane is the simplest example of a bendable surface.
� Bending: an isometric deformation transforming into .
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9Numerical geometry of non-rigid shapes Differential geometry
Bending and rigidity
� Existence of two incongruent isometries does not
guarantee that can be physically folded into without
the need to cut or glue.
� If there exists a family of bendings continuous
w.r.t. such that and , the
shapes are called continuously bendable or applicable.
� Shapes that do not have incongruent isometries are rigid.
� Extrinsic geometry of a rigid shape is fully determined by
the intrinsic one.
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10Numerical geometry of non-rigid shapes Differential geometry
Alice’s wonders in the Flatland
� Subsets of the plane:
� Second fundamental form vanishes
everywhere
� Isometric shapes and have identical
first and second fundamental forms
� Fundamental theorem: and are
congruent.
Flatland is rigid!
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11Numerical geometry of non-rigid shapes Differential geometry
Rigidity conjecture
Leonhard Euler
(1707-1783)
In practical applications shapes
are represented as polyhedra
(triangular meshes), so…
If the faces of a polyhedron were made of
metal plates and the polyhedron edges
were replaced by hinges, the polyhedron
would be rigid.
Do non-rigid shapes really exist?
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12Numerical geometry of non-rigid shapes Differential geometry
Rigidity conjecture timeline
Euler’s Rigidity Conjecture: every polyhedron is rigid1766
1813
1927
1974
1977
Cauchy: every convex polyhedron is rigid
Connelly finally disproves Euler’s conjecture
Cohn-Vossen: all surfaces with positive Gaussian
curvature are rigid
Gluck: almost all simply connected surfaces are rigid
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13Numerical geometry of non-rigid shapes Differential geometry
Connelly sphere
Isocahedron
Rigid polyhedron
Connelly sphere
Non-rigid polyhedron
Connelly, 1978
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14Numerical geometry of non-rigid shapes Differential geometry
“Almost rigidity”
� Most of the shapes (especially, polyhedra) are rigid.
� This may give the impression that the world is more rigid than non-rigid.
� This is probably true, if isometry is considered in the strict sense
� Many objects have some elasticity and therefore can bend almost
Isometrically
� No known results about “almost rigidity” of shapes.
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15Numerical geometry of non-rigid shapes Differential geometry
Gaussian curvature – a second look
� Gaussian curvature measures how a shape is different from a plane.
� We have seen two definitions so far:
� Product of principal curvatures:
� Determinant of shape operator:
� Both definitions are extrinsic.
Here is another one:
� For a sufficiently small , perimeter
of a metric ball of radius is given by
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16Numerical geometry of non-rigid shapes Differential geometry
Gaussian curvature – a second look
� Riemannian metric is locally Euclidean up to second order.
� Third order error is controlled by Gaussian curvature.
� Gaussian curvature
� measures the defect of the perimeter, i.e., how
is different from the Euclidean .
� positively-curved surface – perimeter smaller than Euclidean.
� negatively-curved surface – perimeter larger than Euclidean.
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17Numerical geometry of non-rigid shapes Differential geometry
Theorema egregium
� Our new definition of Gaussian curvature is
intrinsic!
� Gauss’ Remarkable Theorem
In modern words:
� Gaussian curvature is invariant to isometry.
Karl Friedrich Gauss
(1777-1855)
…formula itaque sponte perducit ad
egregium theorema: si superficies curva
in quamcunque aliam superficiem
explicatur, mensura curvaturae in singulis
punctis invariata manet.
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18Numerical geometry of non-rigid shapes Differential geometry
An Italian connection…
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19Numerical geometry of non-rigid shapes Differential geometry
Intrinsic invariants
� Gaussian curvature is a local invariant.
� Isometry invariant descriptor of shapes.
� Problems:
� Second-order quantity – sensitive
to noise.
� Local quantity – requires
correspondence between shapes.
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20Numerical geometry of non-rigid shapes Differential geometry
Gauss-Bonnet formula
� Solution: integrate Gaussian curvature over
the whole shape
� is Euler characteristic.
� Related genus by
� Stronger topological rather than
geometric invariance.
� Result known as Gauss-Bonnet formula.
Pierre Ossian Bonnet
(1819-1892)
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21Numerical geometry of non-rigid shapes Differential geometry
Intrinsic invariants
We all have the same Euler characteristic .
� Too crude a descriptor to discriminate between shapes.
� We need more powerful tools.