An Introduction to Geometry Design Algorithms Finitos/Diapositivas/2- Geomet… · Instituto...

Geometry Design Tutorial 1Instituto Tecnológico de Veracruz 21-25 January 2008

Fathi El-YafiSenior Product Engineer

Product Department of EXA Corporation

An Introduction to Geometry Design An Introduction to Geometry Design AlgorithmsAlgorithms

Geometry: Overview

• Geometry BasicsDefinitionsDataSemantic

• TopologyMathematicsHierarchy

• CSG Approach• BREP Approach

CurvesSurfaces

vertices:x,y,z location

Geometry: Concept Basics

curves: bounded by two vertices

surfaces: closed set of curves

volumes: closed set of surfaces

body: collection of volumes

loops: ordered set of curves on surface

coedges: orientation of curve w.r.t. loop

surfaces: closed set of curves (loops)

shell: oriented set of surfaces comprising a volume

volumes: closed set of surfaces (shells)

coface: oriented surface w.r.t. shell

Volume 1

Surface 1 Surface 2 Surface 3 Surface 4 Surface 5 Surface 6

Volume 2

Surface 8 Surface 9 Surface 10 Surface 11

Surface 7

Volume 1

Volume 2

Surface 11

Surface 7

Manifold Geometry: Each volume maintains its own set of unique surfaces

Volume 1

Surface 1 Surface 2 Surface 3 Surface 4 Surface 5 Surface 6

Volume 2

Surface 8 Surface 9 Surface 10

Surface 7

Volume 1

Volume 2

Surface 7

Non-Manifold Geometry: Volumes share matching surfaces

Geometry: Data

• Model– Wireframe– Surface– Volume

• Geometrical Data Separated from Attributes• Attributes

– Colors– Parameters– Etc.

• Graphical Objects• Visible Parts

Semantic: Surface-Volume-Deflection-Defects

Semantic: Surfaces and Features

Semantic: Detail

Semantic: Detail-Mesh–Bounding Box

Semantic: Decomposition–CurvatureMesh STL-Mesh FEM

Wireframe Model: Limits

Basics of Topology

Topology??

•Concept of interior and exterior

•Orientation

•Reliable calculation of basic parameters:volume, center of gravity, axis of inertia …

•Contour = oriented surface and limited area

•Tree construction

Relations: (Geometry, Topology)

Volume

Surface

Vertex

Contour

TopologyGeometry

EdgePoint

A contour “grips" to its interior

Either S is a Set of R3

Adhesion : A(S)

P ∈ A( S) : any neighborhood of P contains a point of S

Interior : I(S)

P ∈ I( S) if ∃ V(P) ⊂ S

Boundary : B(S)

P ∈ B( S) if P ∈ A( S) and P ∈ A( C(S)) where C(S) refers to the complementary of S in R3

Topology: Mathematics

Open : S = A(S)

Closed : S = I(S)

Regular SolidS = A(I(S)) : adhesion of its interior = R(S)

A ∩ B = R(A∩B)A ∪ B = R(A∪B)A - B = R(A-B)C(A) = R(C(A))

A ∩ B

R(A ∩ B)A

R(A – B)

Operator of Regularization

A ∪ B

C(A)/B

R(C(A)/B)

Operator of Regularization

Euler Formula: V+F = E+2

F = number of faces, V = number of vertices E = number of edges

Polyhedron Type of Faces F V E

Tetrahedron Equilateral Triangles 4 4 6

Octahedron Equilateral Triangles 8 6 12

Cube Squares 6 8 12

Dodecahedron Pentagons 12 20 30

Icosahedrons Equilateral Triangles 20 12 30

Cauchy Proof (1789-1857)

V+F=E+2 V+F=E+1 V+F+1=(E+1)+1 V+F=E+1

V+F=E+1 V+F=E+1 (V-2)+F=(E-2)+1 V+F=E+1

3+1=3+1!!

Mesh Examples

F = 3844V = 1924E = V + F –2 = 5766

F = 47566V = 23793E = S + F –2 = 71357

Topology : Hierarchy

Constructive RepresentationParametric Volume PrimitivesTransformationsBoolean Operators:

Union, common, subtract

Concept of GraphAdvantage :

Simple DescriptionSimulation of Object « Manufacturing »

CSG : Constructive Solid Geometry

CSG: Primitive Components

CSG: Boolean Operators

Subtract

Common

Fillet

Geometry: Curves

Lap back point

Multiple point

Arc length, abscissa curvilinear

2 2 2 2

ds = dx + dy + dz

dx dy dzs u dudu du du

= + + ∫

τ(s) τ(s+ds)

x, y, z function of u, continuous first order

Geometry: Curvature

Ray ofOsculator Circle

=Curvature Ray

Parametric curve:

For the curve with the equation y = f(x):

Geometry: Vector and TangentialFrenet Reference

( ) et n( )

d xdxdsds

dOM dy d OM d yu uds ds ds ds

dz d zds ds

n = Principal normal

OM(s+dh1)

OM(s+dh2)

Geometry: Torsion

M(s+ds)

Osculator Plane at M(s)

Osculator Plane at M(s+ds)Torsion

Geometry: Propeller Circular

2 2 2 2 2 2 2 2

= -R c o s = -R s inx = R c o sy = R s in , = R c o s , = -R s inz = p

= p = 0

a in s i e t d o n c s =

R = - s in

d xd xdd

d y d yd dd z d zd d

d s d x d y d z R p R p

d xd s R pd yd s R

θθ θθθθ θ θ

θ θθ

= + + = + + ×

2 2 2 2 22 2

2 22 2

R = - c o s

R Rc o s e t = - s in a v e c =

p = 0 =

p s in

-p p c o s e t d o n c

d xd s R p

d d yd s d s R p R pp

d zd zd sd s R p

R pd bb n T Td s R pR p

τθ θ ρ

+ ++ +

= × = =++

Geometry: Propeller Circular

-1 -0.5 0 0.5 1

-1-0.5

-0.500.5

1 -1 -0.5 0 0.5 1

-1-0.5

Discretizing

Geometry: Frenet - SerretReference, Equations, Curvature, Torsion

Frenet Equations:

Frenet Reference:

For any parametric function f(t), the expression of the curvature and the torsion are the following:

The Frenet - Serret equations are a convenient framework for analyzing curvature. T(s) is the unit tangent to the curve as a function of path length s.N(s) is the unit normal to the curve B(s) is the unit binormal; the vector cross product of T(s) and N(s).

Geometry: CurvatureGaussian, Average

Gaussian Curvature

Average Curvature

Curvature Cmap

Gaussian Curvature = 21

Average Curvature = 1_R

Sphere

Gaussian Average

Geometry: Curvature

Geometry: Surfaces of Revolution

-1 -0.5 0 0.5 1

-1-0.5

4-1-0.5

-1-0.500.51

-1-0.5

Geometry: Particular Surfaces

Mobius Strip: 'Endless Ribbon'

The Klein Bottle

The Klein Bottle: Curvature

The Kuen Surfaces

x=2*(cos(u)+u*sin(u))*sin(v)/(1+u*u*sin(v)*sin(v))

y=2*(sin(u)-u*cos(u))*sin(v)/(1+u*u*sin(v)*sin(v))

z=log(tan(v/2))+2*cos(v)/(1+u*u*sin(v)*sin(v))

The Dini Surfaces

x=a*cos(u)*sin(v)y=a*sin(u)*sin(v) z=a*(cos(v)+log(tan((v/2))))+b*ua=1,b=0.2,u={ 0,4*pi},v={0.001,2}

Asteroid

x= pow (a*cos(u)*cos(v),3) y= pow (b*sin(u)*cos(v),3) z= pow (c*sin(v),3)

The «Derviche»

Geometry: Curves

Lagrange Interpolating Polynomial

The Lagrange interpolating polynomial is the polynomial P(x) of degree <= (n-1)that passes through the n points (x1,y1= f(x1)), x2,y2= f(x2)), ..., xn,yn= f(xn)),and is given by:

Geometry: Curves

Lagrange Interpolating Polynomial

Where:

Written explicitly:

Geometry: Curves

Cubic Spline Interpolating Polynomial

A cubic spline is a spline constructed of piecewisethird-order polynomials which pass through a set of m control points. The second derivative of each polynomial is commonly set to zero at the endpoints, since this provides a boundary condition that completes the system of m -2 equations.This produces a so-called "natural" cubic spline and leads to a simple tridiagonal systemwhich can be solved easily to give the coefficients of the polynomials.However, this choice is not the only one possible, and other boundary conditions can be used instead.

Geometry: Curves

Consider 1-dimensional spline for a set of n+1 points (y1, y2, .., yn),let the ith piece of the spline be represented by:

Where t is a parameter and i = 0, …, n-1 then

Rearranging all these equations,leads to the following beautifullysymmetric tridiagonal system:

If the curve is instead closed, the system becomes

Cubic Spline Interpolating Polynomial

Geometry: Curves

Cubic Spline/Lagrange

1 2 3 4 5 6 7

Geometry: Curves

Bézier

Given a set of n + 1 control points P0, P1, .., Pn, the corresponding Bèzier curve (or Bernstein- Bèzier curve) is given by:

Where Bi,n(t) is a Bernstein polynomial and .

A "rational" Bézier curve is defined by:

where p is the order, Bi,p are the Bernstein polynomials,Pi are control points, and the weight Wi of Pi is the last ordinate of the homogeneousPoint Pi

w. These curves are closed under perspective transformations, and can representconic sections exactly.

Geometry: Curves

Bézier:Properties

The Bézier curve always passes through the first and last control points.

The curve is tangent to P1–P0 and Pn–Pn-1 at the endpoints.

The curve lies within the convex hull of the control points.

Geometry: Curves

Bézier:Properties

A desirable property is that the curve can be translated and rotated by performing these Operations on the control points.

Undesirable properties of Bézier curves are their numerical instability for large numbers of control points, and the fact that moving a single control point changes the global shape of the curve .

Geometry: Curves

Bézier: Bernstein Polynomials

Bin(t) = Cn

i (t-1)n-iti , Cni = n! / i!(n-i)!

B0 1(t) = (1-t)B0

0(t) + t B-10(t)

B1 1(t) = (1-t)B1

0(t) + t B00(t)

B0 2(t) = (1-t)B0

1(t) + t B-11(t)

Bi j(t) = (1-t)Bi

j-1(t) + t Bi-1j-1(t)

Geometry: Curves

Bézier: Bernstein Polynomials

•Unit Partition: Σi=0,n Bin(t) = 1

•0<=Bin(t)<= 1

•Bin(0) = 0 et Bi

n(1) = 0

•B0n(0) = 1

•Bnn(1) = 1

Bi j(t) = (1-t)Bi

j-1(t) + t Bi-1j-1(t)

Geometry: Curves

B-Spline

A B-Spline is a generalization of the Bézier curve.Let a vector known as the knot vector be defined

T = {t0, t1, …, tm},

where T is a no decreasing sequence with ,and define control points P0, ..., Pn.Define the degree as: p = m-n-1

The "knots“ tp+1, ..., tm-p-1 are called internal knots.

Geometry: Curves

B-Spline

Define the basis functions as:

Then the curve defined by: is a B-spline.

Specific types include the non periodic B-spline(first p+1 knots equal 0 and last p+1 equal to 1; illustrated above)and uniform B-spline (internal knots are equally spaced).

A curve is p - k times differentiable at a point where k duplicate knot values occur.

A B-spline with no internal knots is a Bézier curve.

Geometry: Curves

NURBS-Curve

A non uniform rational B-spline curve defined by:

where p is the order, Ni,p are the B-Spline basis functions, Pi are control points,and the weight Wi of Pi is the last ordinate of the homogeneous point Pi

w.These curves are closed under perspective transformations, and can representconic sections exactly.

Geometry: Curves

Conics

Parabola w=1

Hyperbole w=4 Ellipse w=1/4

P(t) = w N t P t

( ) ( )

(0,0,0,1,1,1)

Geometry: Curves

•NURBS of degree 2•Control points (Isosceles triangle)•Knot vector (0,0,0,1,1,1)

Geometry: Curves

Circles

(0, 0, 0, 1/3, 1/3, 2/3, 2/3, 1, 1, 1)

(0, 0, 0, 1/4, 1/4, 1/2, 1/2, 3/4, 3/4, 1, 1, 1)

Geometry: Surfaces

A given Bézier surface of order (n, m) is defined bya set of (n + 1)(m + 1) control points ki,j.

Bézier

evaluated over the unit square, where:

is a Bernstein polynomial, and

is the binomial coefficient.

Geometry: Surfaces

S(u,v) = Σi=0,nΣj=0,m Bin(u) Bj

m(v) Pij

(n+1)(n+1) points Pij

Pi(v) = Σj=0,m Bjm(v)Pij

Bézier

Geometry: Surfaces

NURBS are nearly ubiquitous for computer-aided design (CAD), manufacturing (CAM), and engineering (CAE)and are part of numerous industry wide used standards,such as IGES, STEP, ACIS, Parasolid.

Geometry: Surfaces

NURBS: Properties

They are generalizations of non-rational B-Splines and non-rational and rational Béziercurves and surfaces.

NURBS curves and surfaces are useful for a number of reasons:

They are invariant under affine as well as perspective transformations.

They offer one common mathematical form for both standard analytical shapes(e.g., conics) and free-form shapes.

They provide the flexibility to design a large variety of shapes.

They reduce the memory consumption when storing shapes (compared to simpler methods).

They can be evaluated reasonably quickly by numerically stable and accurate algorithms.

An Introduction to Geometry Design Algorithms Finitos/Diapositivas/2- Geomet… · Instituto...

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