Coons bicubic surface

1. 1
BIPARAMETRIC SURFACES
1. Bilinear surface
2. Synthetic Surfaces
AML710 CAD LECTURE 18
PARAMETRIC REPRESENTAION
In parametric surfaces a vector valued function
P(u,v) of two variables is used as follows:
A surface may be one patch or constructed using
several patches. All complex surfaces are
represented using many patches
maxminmaxmin
;
)],(),(),([][),(
vvvuuu
vuzvuyvuxzyxvuP
££££
==
P(u,v)
x
y
v
u
u
v
Parametric space
Cartesian space

2. 2
q
f
q=0
f=p/2
f=p/4
q=p/2
]cossinsinsincos[),(
ffqfqfq=Q
SPHERICAL PATCH
]cossinsinsincos[),( ffqfqfq=Q
Parametric Derivatives
]0sincossinsin[),( fqfqfq
q
-=Q
]sincossincos[cos),( ffqfqfq
f
-=Q
The four patch boundaries are defined by 4 position vectors
and 8 tangent vectors 2 at each corner. The tangent vectors
can be obtained by finding the derivatives w.r.t qand f.
Also the shape of the interior near the 4 corners is
influenced by 4 twist vectors defined as the cross
derivatives of the parameters qand f.
]0coscoscossin[),(
22
,
fqfq
qffq
fq
fq
-=
¶¶
¶
=
¶¶
¶
=
QQ
Q

3. 3
Surface Normal
]cossinsinsinsinsincos[
sincossincoscos
0sincossinsin),(),(
222
ffqfqfq
ffqfq
fqfqfqfq
fq
--=
-
-=´
kji
QQ
The surface normal at any point is calculated as the cross
product of the parametric derivatives.
0]sincossincos].[cos0sincossinsin[),().,( =--=ffqfqfqfqfqfq
fqQQ
On a surface patch the isoparametric lines are orthogonal.
Therefore the
dot product of parametric derivativesis zero.
Surface Mapping
Two parameter surfaces from the parametric (rectangular) space
to three dimensional xyz object space.
C
1
C
2
C
3
C
4
u
w
214
213
432
431
CuCCw
CuCCw
CwCCu
CwCCu
££=
££=
££=
££=
),(
),(
),(
wuzz
wuyy
wuxx
=
=
=

4. 4
3D Surface Mapping
Example of 3D surface mapping -R&A Example 6.8.
0
0.2
0.4
0.6
0.8
1
-1
-0.5
0
0.5
1
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
0
0.5
1
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
0
0.5
1
-1
-0.5
0
0.5
1
0
0.2
0.4
0.6
0.8
1
0
0.5
1
0
0.2
0.4
0.6
0.8
1
z-component
x-component
y-component
Total surface
uwz
wuy
wux
=
-=
-=
2
2
)(
Bilinear Surface
A bilinear patch or surface is constructed by the four corners of a
unit square P(0,0),P(0,1),P(1,1) and P(1,0). Any point in the
interior of the surface is obtained by interpolation between thetwo
boundary curves.
uwPwuPwuPwuPwuQ )1,1()1()0,1()1)(1,0()1)(1)(0,0(),(
+-+-+--=
[ ]





-






-=
w
w
PP
PP
uuwuQ
1
)1,1()0,1(
)1,0()0,0(
1),(
0
0.2
0.4
0.6
0.8
1
0
0.5
1
7
7.2
7.4
7.6
7.8
8
8.2
[ ]5.05.05.0)5.0,5.0( =Q
Example 6.9 R&A

5. 5
Ruled and Developable Surface
A ruled surface is produced when a straight line(curve) is movedalong a
path with one degree of freedom.
Test for Ruled Surface: At any point on the surface rotate a plane
containing the surface normal. If at least at one orientation all the points on
the edge of the plane contact the surface, then the surface is ruled in that
direction.
wuPwuPwuQ )1,()1)(0,(),(
+-=
[ ][ ]






-==
)1,(
)0,(
1),(),(),(),(
uP
uP
wwwuzwuywuxwuQ
u
w
Parametric space
P(0,w)
P1,w)
uwPuwPwuQ ),1()1)(,0(),( +-=Alternately
Surface Curvature: Gaussian and Mean Curvatures
Q(u,w)
n

6. 6
Developable Surfaces
Developmentof a surface is the process of unfolding it onto a planar face
without stretching or tearing.
Not all ruled surfaces are developable. However, all developable surfaces
are ruled.
Application: Very useful in plate / sheet metal and fabric industries.
3
22
maxmin
2
||2||
2
wu
uwuw
QQ
QCQBQQA
H
´
+-
=
+
=
kk
wuPwuPwuQ )1,()1)(0,(),(
+-=
4
2
maxmin
.
wu
QQ
BAC
K
´
-
==
kk
The
average and Gaussiancurvature for a surface are defined respectively as:
[ ][ ][ ]
wwuwuuwu
QQQQQCBA .´=
Where
For a developable surface K =0everywhere
Surface Characteristics
A developablesurface has the property that it can be made out of sheet
metal, since such a surface must be obtainable by transformationfrom a
plane (which has Gaussian curvature0) and every point on such a surface
lies on at least one straight line.
A surface of constant Gaussian curvature:It is a surface where K=const.
Minimal Surface
Minimal surfaces are defined as surfaces with zero mean curvatureH=0. A
minimal surfaceparametrizedas
therefore satisfies Lagrange's equation,
Complete Surface
A complete surface is surface which has no edges

A surface which is simultaneously complete and minimal. There have been
a large number of fundamental breakthroughs in the study of such
surfaces in recent years, and they remain the focus of intensivecurrent

7. 7
Surface Characteristics
Complete Minimal Surface
A surface which is simultaneously complete and minimal. There have been
a large number of fundamental breakthroughs in the study of such
surfaces in recent years, and they remain the focus of intensivecurrent
research
Beltrami’sTheorem
Let be a surface mapping. If either M or N has constant
curvature, then both surfaces have constant curvature
Smooth Surface
A surface parameterizedin variables u and v is called smooth if the
tangent vectorsin the u and v directions satisfy
0¹´
vu
TT
Helicoid: A minimal Surface
The (circular)helicoidis the minimal surfacehaving a (circular) helixas its
boundary. It is the only ruledminimal surfaceother than the plane
The parametric equation of helicoid is:
q
q
q
cz
uy
ux
=
=
=
sin
cos

8. 8
Shape and Curvature of Developable Surfaces
Cylindrical /
Conical
0One or
both zero
Hyperbolic< 0Opp. sign
Elliptic> 0Same sign
ShapeKK
min,K
max
Linear Coons Surface
If four boundary curves, P(u,0), P(u,1), P(0,w) and P(1,w) are known, and
bilinear blending function is used for the interior of the surface, a linear
Coons surface can be generated.
It appears that we can simply add two singly ruled interpolations to obtain
this. This poses some problems at the corners of the patch, e.g.P(0,0) will
be added twice. To do the corrections some terms at the end are
subtracted
uwPuwPwuPwuPwuQ ),1()1)(,0()1,()1)(0,(),(
+-++-=
uwPwuP
wuPwuP
uwPuwPwuPwuPwuQ
)1,1()1()0,1(
)1)(1,0()1)(1)(0,0(
),1()1)(,0()1,()1)(0,(),(
---
-----
+-++-=
Wrong
Right
u
w
Parametric space
P(0,w)
P(1,w)
P(u,1)
P(u,0)

9. 9
Coons BicubicSurface
If we use higher order polynomials for boundary curves and interior
blending functions then we get a more flexible surfaces. Patchesof
surfaces are joined together to generate the required surface. Such
surfaces are calledsculpturedsurfaces.
Coons bicubic surface patch uses normalised cubic splines for all four
boundary curves and also for blending functions to define the interior.
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P
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P(u,1)
P(u,0)
P(1,w)
P(1,w)
n
Geometry for Coons patch
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Coons BicubicSurface
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)1,1()0,1()1,1()0,1(
)1,0()0,0()1,0()0,0(
)1,1()0,1()1,1()0,1(
)1,0()0,0()1,0()0,0(
)()()()(),(
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wF
wF
wF
wF
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PPPP
uFuFuFuFwuQ
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ww
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-
-
=
vectors
twist
vectors
tangnet
vectors
tangnet
vertices
Conrner
][
u
w
P

10. 10
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[][][][][]
[][][][][]
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T
ww
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uu
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uw
T
w
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u
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Derivatives of Coons BicubicSurface
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wW
uU
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Surface normal is given by
wu
QQn ´=
Coons Patch and Shape Control
We have 4 corner vertices, 8 tangent vectors and 4 twist vectorsas input
data. The tangent vectors and twist vector magnitude and directions can
be changed to influence the shape.
Effect of changing the magnitude of tangent vectors: Consider the position
vectors on y=0 plane. If the y magnitude of the tangent vectors are zero
then the surface is flat. If it is not zero then it is bowed. Aswe increase the
magnitude the surface bulges up and results in self intersectionpatch
when the order of magnitude is increased further.
P(u,1)
P(u,0)
P(1,w)
P(1,w)
n
Geometry for Coons patch

11. 11
Coons Patch and Shape Control
Effect of changing the direction of tangent vectors
When we change the direction (sign) of the tangent vectors at any vertex it
changes the local convex portion of the patch to concave shape.
Effect of changing the TWIST vectors: We can control the interior shape of
the surface by changing twist vectors keeping the tangent vectors at the
corners the same. By modifying the twist vector (both magnitude and
direction) we can influence the shape of the surface. Modifying one twist
vector magnitude affects about a quarter portion near to that corner.
Increasing the magnitude lifts up the surface further. However in contrast
to what happens at higher orders of magnitude in the case of tangent
vectors, the self intersectingsurface does not result in this case.
The surface with zero twist vectors is called Ferguson or F patch.
P(u,1)
P(u,0)
P(1,w)
P(1,w)
n
Geometry for Coons patch