Post on 15-Jan-2016
Curves with chord length parameterization
Reporter: Hongguang Zhou
Oct. 15th, 2008
What is chord length parameterization? Given a curve p(t), t ∈[a, b]
If t =chord (t) =
So p(t) is chord-length parameterized.
( )
( ) ( )
p t A
p t A p t B
p(t) -A
p(t) -A + P(t) -B
Chord length
Chord-length vs arc-length parametrization
Motivation: Straight line segments is chord length
parameterized.
What other types of parametric curve is chord length parameterized?
Find
Motivation: Chord length parameterization has many usefu
l properties: Geometric parameter, unique;
No self-intersection;
Ease of point-curve testing;
Simplification of curve-curve/curve-surface intersecting.
Outline Interpreting chord-length parametrization
Defining bipolar coordinates
Circles and chord-length parametrization
Plane chord-length parameterization
Plane rational chord-length parameterization
Chord-length parameterization in higher-dimensional space
References Rational quadratic circles are parametrized by chord len
gth
Gerald Farin (CAGD 06)
Curves with rational chord-length parametrization J. Sánchez-Reyes, L. Fernández-Jambrina (CAGD 08)
Curves with chord length parameterization Wei Lü (CAGD In Press)
Rational quadratic circles are parametrized by chord lengt
h
Gerald Farin CAGD.(2006) 722–724
About the author Gerald Farin : Professor of Com
puter Science and Engineering at Arizona State University (ASU) since 1987.
His three main areas of research interest:
Curve and surface modelling; NURBS; Industrial curve and surface
applications.
About the author
Editor in Chief in (CAGD) since 1994.
Editor of the SIAM book series on Geometric Design for 10 years
An editorial board member of the Springer Verlag series on Mathematics and Visualization. A member of a number of interdisciplinary project committees
Rational quadratic circles
An arc of a circle the standard form of a rationalquadratic curve
c0, c1, c2 form an isosceles triangle with base c0, c2v1 = cos
Chord length parametrization
Rational quadratic circle segments in standard form:
Its parametrization is not the arc length
It is the chord length parametrization.
It is the only one which is parametrized by chord length.
Chord length parametrization
chord(t) = t, 0,1t
Chord length parametrization
The complementary segment of the full circle is obtained by replacing v1 by −v1.
It is the chord length parametrization,too.
Curves with chord length parameterization
Wei Lü CAGD In press
About the author Siemens PLM Software, 2000 Eastman D
rive, Milford, OH 45150, USA
Plane chord-length parameterization Using complex analysis;
Parametric plane curve in P(t) = (x(t), y(t))
Complex function Z(t) = x(t)+iy(t), t R
Plane chord-length parameterization
( )t : The signed angle from Z1 −Z(t) to Z(t)−Z0
(1) -scheme of plane chord length parameterization.
Plane chord-length parameterization
(2) rational quadratic Bézier curve
Plane chord-length parameterizationThe following properties of plane chord-length parameteri
zation: Z0, Z (t)∗ and Z1 always form an isosceles triangle with the base angle
, or , if
is constant other than 0 orπ, it becomes a circular arc. = 0 (or π), it is a (unbounded) straight line segment through points Z
0 and Z1.
, it is well defined and bounded.
End conditions
( )t ( )t ( )2
t
( )t
( )t
12( )
Plane chord-length parameterization
A curve Z = Z(t) admits a pre-specified chord length function Chord(t) =ξ(t) (0 ξ(t) 1)
Rational chord-length parameterization In complex field, a rational function is
The degree of a complex function, regarded as a curve, is at most twice of that in complex
field.
t in complex field
Complex function regarded as a curve
Rational chord-length parameterizationLemma:
A complex function U = U(t) with |U(t)| = 1 is rational
There is a complex polynomial H = H(t) such thatRemark:
arg(U) = 2arg(H); arg(U) = 2arg(H)+2π; arg(U) = 2arg(H)−2π. deg(U(t)) = 2deg(H(t))
Rational curves
Z(t) can be converted into a rational Bézier form with its Bézier control points in Euclidean space
Rational curves Each chord length parameterization is entirely determined b
y a unit-circular parameterization.
Bounded straight line segments are the only curves with the polynomial (linear) chord-length parameterization ( ≡ 0).
Unbounded straight line segments are the only curves having a rational linear chord-length parameterization ( ≡ π).
Circular arcs are the only curves admitting a rational quadratic chord-length parameterization with constant other than 0 and π.
( )t
( )t
( )t
Rational cubics and G1 Hermite interpolation H(t) =h0(1−t)+h1t +i, h0 , h1 R
The form of rational cubic Bézier
If h0=h1, Z(t) a circular arc
Rational cubics and G1 Hermite interpolationGiven: the end points Z0,Z1 and tangent directions T0,T1
Find: a rational curve Z(t) to interpolate Z0,Z1, T0,T1.
Rational cubics and G1 Hermite interpolation
0 00 170 , 60
0 00 1130 , 30
G1 Hermite interpolation with higher degree curves To Hermite interpolant the S-shaped curve data Use the higher-degree chord-length parameterizati
on.
α0: the signed angle from Z1 − Z0 to T0, α1: the signed angle from T1 to Z1 − Z0.
s: shape parameter
Examples:
Examples:
Chord-length parameterization in higher-dimensional space
to be the curve with chord-length parameterization
P(0)=p0, P(1)=p1 , Q(t) :a unit-vector valued function being always perpendicular to L;
(x(t), y(t)): a plane chord-length parameterization curve interpolating two end points P0 = (− l/2,0) (at t = 0) and P1 = (l/2, 0) (at t = 1)
Chord-length parameterization in higher-dimensional space
Construct: a chord-length parameterized curve in higher-
dimensional space.
(1) Create a unit vector-valued function Q(t) perpendicular to L.
(2) Build up a planar chord-length parameterization (x(t), y(t)).
If Q(t) : a fixed unit vector.
Represents a planar chord-length parameterized curve.
(α,β)-scheme for three-dimensional curves
L,M and N constitute an orthogonal system.
3-dimensional chord-length parameterized curve is essentially decided by two angular functions α(t) , β(t).
α(t) : its chord-length parameterization , β(t) : its rotation around the axis through two end points.
Curves with rational chord-length parametrization
J. Sánchez-Reyes, L. Fernández-Jambrina CAGD 08
About the author J. Sánchez-Reyes: Instituto de Matemática Aplicada a la Ciencia e Ingeniería, ETS
Ingenieros Industriales, Universidad de Castilla-La Mancha, Campus Universitario, 13071-Ciudad Real, Spain
L. Fernández-Jambrina: ETSI Navales, Universidad Politécnica de Madrid, Arco de la Vi
ctoria s/n, 28040-Madrid, Spain
Interpreting chord-length parametrization
Interpreted as a geometric coordinate Consider two fixed points A,B, and define the coordinate u of a gene
ric point p with respect to A,B as:
A curve p(u) over a unit domain u ∈ [0, 1], with A = p(0), B = p(1),
If its parameter
The curve is chord-length parameterized.
Interpreting chord-length parametrization
chord-length parametrizations are preserving the modulus of the ratios.
Define:
Defining bipolar coordinatesThe bipolar coordinates (u, ):ϕ
of a point ϕ p is the angle between the segments pB and Ap. ∈ (−π,π]ϕ
Isoparametric curves with constant , uϕ
A fixed value u
Isoparametric curves with constant ϕ are again circular arcs, the locus of points : see a segment AB with constant angle (π − ϕ).
A circle, has a centre lying on the line AB
Interpreting chord-length parametrization
To construct chord-length parametrized curves p(u), simply choose an arbitrary function (u).ϕ
Complex inversion
The inverse 1/p(u) of a chord-length parametrized curve p(u) : chord-length parametrization, too.
Circles and chord-length parametrization
Setting a constant ϕ, p(u): a chord-length parametrization , u ∈ [0, 1]
Standard Bézier arcs
Planar rational curves with chord-length parametrization Planar curves p(u) admit rational chord-length par
ametrization on u ∈ [0, 1].
Control the quartic using the following shape handlesEndpoints A,B, and angles α,β between the endpoint tangents and the segment AB.
Angle σ between chords AS and SB at S = p(1/2).Choose the position of S on the bisectorof ABsetting the height:
Equilateral hyperbola and Lemniscate of Bernoulli
Limaçon of Pascal
Space curves with chord-length parametrizationq(u) = {x(u), y(u), z(u)} is chord-length only if its associated planar curve p(u) is chord-length, too.
3D rational curve q(u) with chord-length parametrization
c(u): a rational unit circle
Conclusions: Connection between bipolar coordinates and curves
with chord-length parametrization.
Identify the curves with chord-length parameterization in two or higher-dimension.
Identify the curves with Rational chord-length para
meterization
Thank you
Questions ?