Properties of NURBS Curves

```Curve Modeling
NURBS
Dr. S.M. Malaek
Assistant: M. Younesi
Motivation
Motivation
We need more control
over the curve
Motivation
Design Sketch
Motivation
Rendered
Motivation
Final Physical Product
Motivation

Different weights of control points
Motivation

It is hard to product exact circle with B-spline.

Circles can only be represented with rational function
(i.e., functions that are quotients of two polynomials).
four closed B-spline curves with 8 control points:
Degree 2
Degree 3
Degree 5
Degree 10
Motivation

Circles, ellipses and many other
curves that cannot be represented by
polynomials.

we need an extension to B-spline
curves.
Motivation

Generalize B-splines to rational curves
using homogeneous coordinates:
Non-Uniform Rational B-Splines
(NURBS)
NURBS

Different weights of Pi, to have different
attraction factors.
Wi=1
W1=0.4
W3=3.0
NURBS

Different weights of Pi, to have different
attraction factors.
Wi=1
W0=W2=W3=W5=1, W1=W4=4
NURBS

B-spline Curve: Given n+1 control points P0, P1, ..., Pn
and knot vector U = { u0, u1, ..., um } of m+1 knots, the Bspline curve of degree p:
n
 xi 
y 
i

Pi 
 zi 
 
1
C(u )   N (u )P
i,p
i 0
i
Multiply the coordinates of Pi
with a weight w
 wi xi 
w y 
i i 
w

Pi 
 ww z i 


 wi 
w x 
w y 

C (u )   N (u ) 
 w z  B-Spline Curve in

 homogeneous coordinate
 w 
w
n
i 0
i
i
i
i
w
i
i,p
i
NURBS
w x 
w y 

C (u )   N (u ) 
w z 


w


n
w
i
i
i
i
w
i
i,p
i 0
i
  N (u )w x 
 N (u )w y 

C (u )  
  N (u )w z 


  N (u ) w 
n
i 0
i,p
i
i
i,p
i
i
i,p
i
i
n
w
i 0
n
i 0
n
i 0
i,p
i
NURBS

Convert it back to Cartesian coordinate by dividing Cw(u) with
the fourth coordinate:
x 
N (u ) w  y 
C(u )  
 N (u ) w  z 
 
1
i
n
i 0
C(u ) 
n
j 0
j,p

n
i
 j 0 N j, p (u)w j
i
n
N i , p (u ) wi
N i, p (u ) wi
n
i
i
1
i 0
Ri, p (u ) 
i,p
N
i 0
i, p
C(u ) 
(u ) wi Pi
n
 Ri, p (u)Pi
i 0
Two
Immediate Results
Two Immediate Results
C(u ) 
n
 Ri, p (u)Pi
i 0
Ri, p (u ) 
N i, p (u ) wi
 j 0 N j, p (u)w j
n
1. If all weights are equal to 1, a NURBS
curve reduces to a B-spline curve.
2. NURBS Curves are Rational.
Properties
of
NURBS Basis Function
1.
Properties of NURBS Basis Function
Ri,p(u) is a degree p rational function in u
2.
Nonnegativity -- For all i and p, Ri,p(u) is nonnegative
3.
Local Support -- Ri,p(u) is a non-zero on [ui,ui+p+1)
4.
On any knot span [ui, ui+1) at most p+1 degree p basis
functions are non-zero.
5.
Partition of Unity -- The sum of all non-zero degree p
basis functions on span [ui, ui+1) is 1:
Properties of NURBS Basis Function
6.
If the number of knots is m+1, the degree of the basis
functions is p, and the number of degree p basis functions is
n+1, then m = n + p + 1 :
7.
Basis function Ri,p(u) is a composite curve of degree p
rational functions with joining points at knots in [ui, ui+p+1 )
8.
At a knot of multiplicity k, basis function Ri,p(u) is
continuous.
9.
If wi = c for all i, where c is a non-zero constant, Ri,p(u) =
Ni,p(u)
Cp-k
Properties
of
NURBS Curves
Properties of NURBS Curves
1.
NURBS curve C(u) is a piecewise curve with each
component a degree p rational curve.
2.
Equality m = n + p + 1 must be satisfied .
3.
A clamped NURBS curve C(u) passes through the two
end control points P0 and Pn
4.
Strong Convex Hull Property
5.
Local Modification Scheme
C(u) is Cp-k continuous at a knot of multiplicity k
6.
7.
B-spline Curves and Bézier Curves Are Special Cases
of NURBS Curves
Properties of NURBS Curves
8.
Modifying Weights: increasing the value of wi will
pull the curve toward control point Pi

a NURBS
curve of
degree 6
and its
NURBS
basis
functions.
The
selected
control
point is
P9.
Properties of NURBS Curves

Modifying Weights: decreasing the value of wi will push
the curve away from control point Pi
If a weight becomes zero, the coefficient of Pi is zero and, control point Pi has no
impact on the computation of C(u) for any u (i.e., Pi is "disabled").

Properties of NURBS Curves
A NURBS curve of degree 6
defined by 9 control points (n = 8)
and 16 knots (m = 15) .The
selected control point is P4. Since
the coefficient of P4, N4,6(u), is
non-zero on [u4, u4+6+1) = [0,1),
changing w4 affects the entire
curve!
Rational
Bézier Curves

Rational Bézier Curves
We have learned that projecting a 4-dimensional
B-spline curve to hyperplane w=1 yields a 3dimensional NURBS curve.
What if this B-spline curve is a Bézier curve?
The result is a Rational Bézier curve!
Rational Bézier Curves

Since a rational Bézier curve is a special case of NURBS
curves, rational Bézier curves satisfy all important
properties that NURBS curves have.

since there is no internal knots, rational Bézier curves do
not have the local modification property, which means
modifying a control point or its weight will cause a global
change
n
C(u )   Ri ,n (u )Pi
i 0
Ri ,n (u ) 
Bi ,n (u ) wi

n
j 0
Bn, j (u ) w j
n
C(u )  
i 0
Rational Bézier Curves
Bi ,n (u ) wi
Ri ,n (u )  n
Ri ,n (u )Pi
 j 0 Bn, j (u)w j
Modifying the weight of a control point
will push or pull the curve away from
or toward the control point.
Rational
Bézier Curves:
Conic Sections
Rational Bézier Curves: Conic Sections

we use rational Bézier curve of degree 2, the
coefficients are:
B2,0 (u )  (1  u ) 2
B2,1 (u )  2(1  u )u
B2, 2 (u )  u 2
Ri ,n (u ) 
Bi ,n (u ) wi

n
j 0
n
Bn , j (u ) w j
C(u )   Ri ,n (u )Pi
i 0
Rational Bézier Curves: Conic Sections

The equation of this rational Bézier curve of degree 2 is :

1
2
2
C(u ) 
(
1

u
)
P

2
(
1

u
)
uw
P

u
P2
0
1
2
2
(1  u )  2(1  u )uw  u
r
w
, 0  r 1
1 r
Hyperbola
W>1
r>1/2
Parabola
W=1
r=1/2
Ellipse
W<1
r<1/2
Straight line
W=0
r=0
P0=1
P1=w
P2=1

Rational Bézier Curves: Conic Sections
Hyperbola
W>1
r>1/2
Parabola
W=1
r=1/2
Ellipse
W<1
r<1/2
Straight line
W=0
r=0
P0=1
P1=w
P2=1
Rational Bézier Curves: Conic Sections
Circular Arcs and Circles

Weights w is equal to the sine of the half ngle at control point
P1
w  sin( a)
Rational Bézier Curves: Conic Sections
Quarter Circle:
 P1=90º, a=45º,
w  sin( 45 )  2 2

Rational Bézier Curves: Conic Sections
1/3 Circle:
 P1=60º, a=30º,
w  sin( 30 )  1 2

Rational Bézier Curves: Conic Sections
Full Circle:
1 1 1 2 2 2


U  0,0,0, , , , , , ,1,1,1
3 3 3 3 3 3


1 1 1 1 3 3


U  0,0,0, , , , , , ,1,1,1
4 4 2 2 4 4


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