Geometry-I
Geometry-I
What’s done• The two-tier representation• Topology-the combinatorial
structure• Geometry –The actual
parameters representing the geometric face.
In this Talk (and henceforth!)
GEOMETRY
The Problem Before Us…
The geometric representation of edges/co-edges/faces.
• Edges/Coedges-part of a curve
• Faces-part of a surface
2 Questions
• How does one represent a curve/surface• How does one represent a part of eitherImplicit-surface as an equation f(x,y,z)=0 curve as two such equationsParametric-edge is (x(t),y(t),z(t)) surface is (x(t,u),y(t,u),z(t,u)) Thus surface /curve :parameters on which the
coordinates depend.
An Example
Implicit Parametric
Curve
(circle)
x^2 +y^2-1=0
z=0
x=(2t)/(1+t^2)
y=(1-t^2)/(1+t^2)
z=0
Surface
(cylinder)
x^2 +y^2 –1=0 x=(2t)/(1+t^2)
y=(1-t^2)/(1+t^2)
z=u
In general
Implicit Parametric
Curve F(x,y,z)=0
G(x,y,z)=0
X=x(t)
Y=y(t)
Z=z(t)
Surface F(x,y,z)=0 X=x(t,u)
Y=y(t,u)
Z=z(t,u)
Another Example-Torus
Torus-occurs as a blend
Parametric
x=(R+r sin u)cos t
y=(R+r sin u)sin t
z=r cos u
Implicit-tedious
x^2 +y^2=
(R +/- sqrt(r^2 –z^2))^2
The Twisted Torus
This occurs in a slanted blend
• Parametric is
difficult• Implicit is (practically)
Impossible
Implicit-cost benefits
Easy• Testing if point on
curve/surface
• Deciding which side point of surface
Hard
• Generating points on curve/surface
Parametric-cost benefits
Hard
• Testing if point on curve/surface
• Deciding which side point of surface
Easy
• Generating points on curve/surface
Exactly the Opposite!
Our Decision-Parametric !
Reasons• Generating points on
surfaces/curves is very important
• Interpolation/Approximation theory-creation of surfaces/curves from points is easy
Our Decision-Parametric !
Reasons• Generating points on
surfaces/curves is very important
• Interpolation/Approximation theory-creation of surfaces/curves from points is easy
So Then -Parametric
• Curves: One parameter
X=x(t) Y=y(t) Z=z(t)
Domain of definition: an interval
• Surfaces: Two parameters
X=x(u,v) Y=y(u,v) Z=z(u,v)
Domain of definition: an Area
Parametric Representation-Edges
Edge• End vertices v1 , v2
• Interval [a,b]
• C: the curve function from
parameter space [a,b] to model space R3
Edge – image of [a,b]
Example
e1
e2
e1: part of a line
X=1+t; Y=t, Z=1.2+t
t in [0,2.3]
e2: part of a circle
X=1.2 +0.8 cos t
Y=0.8+0.8 sin t
Z=1.2
T in [-2.3,2.3]
Parametric Representation-Face
Face• Domain D subset of R2
• S: surface function from
parameter space R2 to model space R3
Face – image of D
Example
f1
f2
f1: part of cylinder
X=1.2 +0.8 sin v
Y=u
Z=2.1 +0.8 cos v
f2: part of a plane
X=u
Y=v
Domains Domains• P-curves in parameter
space• pi:[ai,bi] to parameter space R2
• Domain Loops
(p1,-p2,p3,p4)
• Normal Data
Example Domain
f1
f2
Parameter Space
u
v
Part removed by the boss
Part of Cylinder
P-Curves
Parameter Space
u
vC1
C2
C3
C4
C5
C6A total of 6 p-curves
•All but c5 easy (lines)
• c5 inverse image of a cylinder-cylinder intersection.
•Only Approximately Computed!
Co-edges-The image of this p-curve is only an approximation to the correct intersection
-This results in 3 separate paramets of the same intersection curve
-all of these are required!
Recap
Entity Toplogical data Geometric Data
Face Loops,
^co-edges
^domain
Surface function S
Edge ^vertices
[a,b]
Curve function C
Co-edge [a,b] : P-curve domains
P-curve functions C