# Design of Curves and Surfaces by Multi Objective Optimization

AI and Robotics

Oct 23, 2013 (4 years and 8 months ago)

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Design of Curves and Surfaces by
Multi Objective Optimization

Rony Goldenthal

Michel Bercovier

School of Computer Science and Engineering

The Hebrew University of Jerusalem

Introduction

This work is about
curves

and
surfaces
:

Fitting

finding surfaces (curves) as close as
possible to a given set of points.

Design/Fairing

generate a surface
achieving certain quality measures

design
objective (minimal length, minimal
curvature,…)
.

Introduction

cont.

Fitting alone is not sufficient, a certain
quality of the resulting surface must be
ensured.

Design alone is not sufficient, the surface
must relate to some real world geometry

fitting is required.

How to optimize several functions at once ?

Multiple Objective Functions

Suppose we want to incorporate several
design objectives into a single optimization
process:

Approximation error under L
2
or L
inf

Elastic energy

Length/Area

Class A criterions

How to optimize several functions at once ?

Previous work

M. Alhanaty, M. Bercovier; R. Goldenthal, M.
Bercovier:

Optimal Control in CAGD:

Decouple fitting from design

For each knot/parametrization configuration:

Solve the state equation (fitting)

Evaluate the cost function

Minimize the cost function (design objective)

Standard Approach :Weighted Sum
of Objective Functions

Limitations:

Result depends on weights.

Some solutions cannot be reached.

Multiple runs of the algorithm are required
in order to get the “whole picture”.

Difficult to select weights

cost functions
may be in different scales.

Optimization

Multi objective solution does not have a
single optimal

solution, but an
optimal
solution set
.

Possibility to implement a decision tool.

Natural set up for Genetic Algorithms.

Genetic algorithms for the
single objective problem

The control variables: knot vector, parametrization
and NURBS weights modeled as chromosomes.

Motivation for choosing single objective GA:

Highly non
-
linear behavior of knot vector modification

Support splines of any order (degree)

Support highly non linear cost functions

Support non
-
derivable cost functions

Elegant handling of constrains and singular conditions

Single Objective Genetic
Algorithm (SOGA)
-

outline

1.
[Start]

Generate random population of
n

individuals.

2.
[Fitness]

Evaluate the
f(x)
of all
x

in the population.

3.
[New population]

Create a new population from the old
population.

4.
[Replace]

the old population with the new one.

5.
[Test]

for end condition,
stop
, and return the best
solution in current population.

6.
[Loop]

Go to step
2
.

Single Objective Genetic
Algorithm (SOGA)
-

outline

1.
[Start]
Generate random population of
n

individuals.

2.
[Fitness]

Evaluate the
f(x)
of all
x

in the population.

3.
[New population]

Create a new population by these steps (n times):

1.
[Selection]

Select two parents from the population according to
their fitness.

2.
[Crossover]

performed with a crossover probability.

3.
[Mutation]

performed with a mutation probability.

4.
[Accepting]

Place new offspring in the new population.

4.
[Replace]

the old population with the new one.

5.
[Test]

for end condition,
stop
, and return the best solution in current population

6.
[Loop]

Go to step
2
.

GA
-

Example

The parents:

t0 = {0,0,0,0,0.14,0.29,0.43,0.57,0.71,0.86,1,1,1,1}

t1 = {0,0,0,0,0.22,0.34,0.45,0.55,0.66,0.78,1,1,1,1}

Their encoding:

et0 = {0.14,0.14,0.14,0.14,0.14,0.14,0.14}

et1 = {0.22,0.12,0.11,0.10,0.11,0.12,0.22}

One Point crossover with split point = 4:

ec0=
{0.14,0.14,0.14,0.14,0.14
,
0.12,0.22}

Normalization:

ec0=
{0.14,0.14,0.14,0.14,0.14
,
0.11,0.21}

GA

Example

cont.

Mutation:

Before:

ec0 = {0.14,0.14,
0.14
,
0.14
,0.14,0.11,0.21}

After:

ec0 = {0.14,0.13,
0.13
,
0.15
,0.14,0.11,0.21}

The Parents

Crossover

Mutation

Multi Objective Genetic
Algorithm

Multi objective optimization has an
optimal
solution set
.

The main advantage of GA for MOO is that
GA keeps a population and not a single
solution.

The main difference between SOGA and
MOGA is in the selection process.

MOGA
-

Selection

Better than

and
worse than

relationships no
longer hold.

Relationship among individuals is defined as

domination
” relationship.

Domination

For any two solutions x
1

and x
2

x
1
is said to
dominate

x
2

if these conditions
hold:

x
1
is
not worse

than x
2

in all objectives.

x
1
is
strictly better

than x
2

in at least one
objective.

If one of the above conditions does not hold x
1

does
not dominate

x
2
.

Pareto Optimal Set

Non
-
dominated set
: the set of all solutions
which are not dominated by any other
solution in the
sampled search space
.

Global Pareto optimal set
: there exist no
other solution in the
entire search space

which dominates any member of the set.

Multi Objective Genetic
Algorithm

cont.

In each generation the non
-
dominated set is
maintained, fitness is adjusted according to
the domination of each individual.

In order to encourage diversity in the
population fitness is reduced for similar
solutions.

NURBS Curve

Input points: 28

Control points: 20

Order: 6

Cost Functions:

L
2
approximation error

Curve Length

Curvature

NURBS Curve

Approximation Error: 1.12

Curve Length: 3.322

Curvature: 2,082

NURBS Curve

Approximation Error: 0.36

Curve Length: 3.46

Curvature: 191.48

NURBS Curve

Approximation Error: 0.075

Curve Length: 3.69

Curvature: 419.32

NURBS Curve

Approximation Error: 0.0258

Curve Length: 3.707

Curvature: 2467.4

NURBS Surface I

Order: 4,4

Input points: 8,8

Control Points: 8,8

Cost functions:

Surface Area

Surface Curvature

NURBS Surface

Surface Area: 174

Surface Curvature: 4.9e
-
29

NURBS Surface

Surface Area: 155.77

Surface Curvature: 0.099

NURBS Surface

Surface Area: 155.16

Surface Curvature: 0.04

NURBS Surface

Surface Area: 154.72

Surface Curvature: 0.12

NURBS Surface II

Order: 4,4

Input points: 16,16

Control Points: 5,5

Cost functions:

Approximation Error

Surface Curvature

NURBS Surface

Approximation Error: 5.807

Surface Curvature: 3.39

NURBS Surface

Approximation Error: 5.19

Surface Curvature: 3.56

NURBS Surface

Approximation Error: 2.46

Surface Curvature: 5.23

NURBS Surface

Approximation Error: 1.348

Surface Curvature: 9.95

NURBS Surface

Approximation Error: 1.256

Surface Curvature: 11.51

NURBS Surface

Approximation Error: 0.937

Surface Curvature: 20.84

Summary

Decision tool

for approximation and design.

Use of Multi Objective Genetic algorithm.

Result is a set of non
-
dominated solutions.

Implementation supports: NURBS curves and
surfaces of arbitrary order.

Optimization variables:

Knot vector

Parameterization

NURBS weights

Summary

Support for various design objective:

Curves:

Length

Curvature

Approximation Error L
2
/L
inf

Surfaces:

Curvature

Wilmore surfaces

Surface Area

Approximation error L
2
,L
inf

Thank You!

ronygold@cs.huji.ac.il

http://www.cs.huji.ac.il/~ronygold

Extra slides

Applications

Surface fitting and design has numerous
applications mainly in theses areas:

Industrial design.

Computer graphics.

Statistics.

Genetic Algorithms in CAD

Genetic algorithms in CAD,

G. Renner and A. Ekárt

Data fitting with a spline using a real
-
coded
genetic algorithm,

F. Yoshimoto, T. Harada and Y. Yoshimoto

Genetic algorithms in free form curve
design,

A. Márkus, G. Renner and A.J. Váncza

GA
-

Encoding

Each individual contains 3 chromosomes:

One for Parametrization
s
.

One for Knot vector
t
.

One for the NURBS weights
w
.

Each chromosome is encoded by real valued
numbers.

Intervals between two adjacent vector
entries are actually stored (for
s

and
t
).

GA

Initial Population

The initial population was generated randomly
while respecting validity constrains:

Positive NURBS weights.

Monotonically increasing parametrization and knot
vector.

Sum of all intervals in parametrization and

knot vector equals curve’s length.

Population size proportional to #(d.o.f).

GA

Fitness Evaluation

Interpolation/approximation must be
performed prior to fitness evaluation.

For each individual in the population the
fitness is evaluated.

Violation of
Schoenberg
-
Whitney

condition is
penalized by the fitness evaluation.

GA
-

Crossover

Modified one point crossover used

for each
chromosome with the following
modification:

The sum of all the intervals that make the knot
vector and the parametrization must remain
fixed.

GA
-

Mutation

The mutation must respect these constrains:

All intervals must be positive.

Sum of all intervals must remain constant.

The mutation process (identical for all
chromosomes):

Randomly select 2 intervals: i,j.

Randomly select x s.t. x < min(v(i),v(j)).

update:
v(i) = v(i) + x;

v(j) = v(j)

x;

GA
-

Selection

In
SOGA

selecting two parents is done is
done randomly, when the probability of each
individual to be a parent is proportional to
its fitness.

Tournament, based on the above principle
was used.

NURBS Curve

Approximation Error: 1.08

Curve Length: 3.328

Curvature: 926.1

NURBS Curve

Approximation Error: 0.4

Curve Length: 3.42

Curvature: 206.53

NURBS Curve

Approximation Error: 0.004961

Curve Length: 3.61

Curvature: 11,733.6

NURBS Curve

Approximation Error: 0.07

Curve Length: 62,101

Curvature: 144.42

Previous works

P. Laurent
-
Gengoux, M. Mekhilef,
“Optimization of a NURBS representation”

J. Loos, G. Greiner, H
-
P Seidel,
“Modeling of
surfaces with fair reflection line pattern”

G. Brunnet, J. Keifer,
“Inerpolation with
minimal
-
energy splines”