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Curve on a surface

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(1)

Iain Claridge

Surface Curvature

Siddhartha Chaudhuri http://www.cse.iitb.ac.in/~cs749

(2)

Curves and surfaces in 3D

For our purposes:

A curve is a map α : ℝ  3 (or from some subset I of ℝ)

A surface is a map M : ℝ2  3 (or from some subset Ω of 2)

I

a b α(b)

α(a)

0

M(0)

Wikipedia

(3)

Curves and surfaces in 3D

For our purposes:

A curve is a map α : ℝ  3 (or from some subset I of ℝ)

α(t) = (x, y, z)

A surface is a map M : ℝ2  3 (or from some subset Ω of ℝ2)

M(u, v) = (x, y, z)

We will assume everything is arbitrarily differentiable, regular, etc

(4)

Curve on a surface

A curve C on surface M is defined as a map C(t) = M(u(t), v(t))

where u and v are smooth scalar functions

I

a b

M(u(a), v(a)) M(u(b), v(b))

Wikipedia

(5)

The curve C(v) = (u0, v) for constant u0 is called a u‑curve

The curve C(u) = (u, v0) for constant v0 is called a v‑curve

These are collectively called coordinate curves

Example: coordinate curves (θ-curves and φ-curves)

on a sphere

Special cases

Wikipedia

(6)

Tangent vector

The tangent vector to the surface curve C at t can be found by the chain rule

C

t = M

u

d u

d t + M

v

d v d t

C

t (t1)

C

t (t2)

t1 t2

Wikipedia

(7)

Tangent vector

We will use the following shorthand

Then the tangent vector is

Mu := M

u Mv := M

v

u˙ := d u

d t v˙ := d v

d t C˙ := C

t C˙ = Mu u˙ + Mv v˙

(8)

Regular surface

A surface M is regular if Ċ ≠ 0

… for all curves C : t ↦ M(u(t), v(t)) on the surface

… such that the map t ↦ (u(t), v(t)) is regular

Eqivalently, Mu × Mv ≠ 0 everywhere

(the derivatives are not collinear)

A point where Mu × Mv ≠ 0 is called a regular point

(else, it is a singular point)

(9)

Tangent space

All tangent vectors at a point p are of the form

If the point is regular, the tangent vectors form a 2D space called the tangent space Tp at p

Mu and Mv are basis vectors for the tangent space

The unit normal to the tangent space, also known as the normal to the surface at the point, is

Muu˙ + Mv v˙

N = Mu×Mv

‖Mu×Mv

(10)

Tangent space

N = Mu×Mv

‖Mu×Mv

C˙ = Mu u˙ + Mv v˙

p

C Tp

Wikipedia

(11)

Thought for the Day #1

If we change the parameters of the surface to, e.g.

u := u(r, s), v := v(r, s)

does the normal change, and if so how?

(12)

Arc length on a surface

Consider a curve C on surface M

Its (differential) arc length at point p is

Squaring

or

‖ ˙C = ‖Muu˙ + Mv v‖˙

‖ ˙C2 = (MuMu) ˙u2 + 2(MuMv) ˙uv˙ + (MvMv) ˙v2

‖ ˙C2 = E u˙2 + 2 F u˙ v˙ + G v˙ 2

(13)

First Fundamental Form

The map (x, y) ↦ Ex2 + 2Fxy + Gy2 is called the first fundamental form of the surface at p

For a regular surface, the matrix is positive definite since E (and G) > 0 and EG – F2 > 0

Because of the relation to differential arc length ds, the first fundamental form is often written as

ds2 = E du2 + 2F du dv + G dv2 and called a Riemannian metric

I p( x , y ) =

[

x y

] [

E FF G

] [

xy

]

(14)

Second Fundamental Form

Consider a curve C on surface M parametrized by arc length

Its curvature at point p is

Writing L = N·Muu , M = N·Muv , N = N·Mvv we have

The map (x, y) ↦ Lx2 + 2Mxy + Ny2 is called the second fundamental form of the surface at p

‖ ¨C

‖ ¨C = L u˙2 + 2 M u˙ v˙ + N v˙ 2

II p(x , y ) =

[

x y

] [

ML MN

] [

xy

]

(15)

Caution!

Remember that the fundamental forms depend on the surface point p

The coefficients E, F, G, L, M, N are not in

general constant over the surface (it would be clearer but more cluttered to write them as Ep, Fp etc).

They can take different values at different points.

(16)

Analogies with curves

Curves:

First derivative arc length→ Second derivative curvature→

Surfaces:

First fundamental form distances→

Second fundamental form (extrinsic) curvatures→

(17)

Intrinsic and Extrinsic Properties

Properties of the surface related to the first

fundamental form are called intrinsic properties

Determined only by measuring distances on the surface

Properties of the surface related to the second fundamental form are called extrinsic properties

Determined by looking at the full embedding of the surface in 3

(18)

Gaussian Curvature

The Gaussian curvature at a surface point is an intrinsic property

But this involves L, M, N from the second fundamental form, how is this intrinsic?

K = L NM

2

E GF

2

(19)

Theorem Egregium of Gauss

The Gaussian curvature can be expressed solely as a function of the coefficients of the first

fundamental form and their derivatives

Wikipedia

(20)

Intrinsic classification of surface points

A surface point is

Jean H. Gallier

(21)

Negative

Zero

Positive

Jhausauer@wikipedia

(22)

www.ian-ko.com

(23)

Principal Curvatures

Geodesic curves passing through a point assume maximum and minimum curvatures in orthogonal directions

These curvatures are called the principal

curvatures K1 and K2, and the corresponding directions the principal directions

The principal curvatures are extrinsic properties

(24)

Principal Curvatures

Wikipedia

(25)

Principal Curvatures

The principal curvatures are the eigenvalues of the shape operator, computed from the fundamental form matrices

(and the principal directions are the eigenvectors)

It turns out that K = K1K2

S =

[

F GE F

]

−1

[

ML MN

]

= EG1F2

[

L G−M EM F M G−L F N EM FN F

]

(26)

Bonnet’s Theorem

A surface in 3-space is uniquely determined upto rigid motion by its first and second fundamental forms

(Compare to the Fundamental Theorem of Space Curves:

curvature and torsion uniquely define a curve upto rigid motion.

An even more direct analogue is the Fundamental Theorem of Surface Theory: two fields of 2x2 matrices over a simply

connected open 2D domain, that satisfy certain conditions, are the first and second fundamental form matrices of a surface

uniquely defined upto rigid motion. Do you see why this

theorem and Bonnet’s Theorem are not saying the same thing?)

References

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