926 X.P. Zhang et al. / Computer Aided Geometric Design 26 (2009) 923–940
where M =
ABC
BDE
CEF
and
A
= H
y
H
zx
− H
z
H
yx
,
B
= (H
z
H
xx
− H
x
H
zx
+ H
y
H
zy
− H
z
H
yy
)/2,
C
= (H
y
H
zz
− H
z
H
yz
+ H
x
H
yx
−
H
y
H
xx
)/2,
D
= H
z
H
xy
− H
x
H
zy
,
E
= (H
x
H
yy
− H
y
H
xy
+ H
z
H
xz
− H
x
H
zz
)/2,
F
= H
x
H
yz
− H
y
H
xz
.
To solve Eqs. (1), let
H
z
= 0 without losing generality, and substituting
dz
from II(d
x
) = 0to
I
(d
x
) = 0, we have
III(dx, dy) = Udx
2
+2Vdxdy+ Wdy
2
= 0(2)
where
U
= AH
2
z
− 2
CH
x
H
z
+ FH
2
x
,
V
= BH
2
z
− CH
y
H
z
− EH
x
H
z
+ FH
x
H
y
, and
W
= DH
2
z
− 2
EH
y
H
z
+ FH
2
y
, respectively.
From Eqs. (1) and (2), we can derive the principal directions, respectively:
T
1
=
⎛
⎝
(−
V +
√
V
2
− UW)H
z
UH
z
(V −
√
V
2
− UW)H
x
− UH
y
⎞
⎠
,
T
2
=
⎛
⎝
(−
V −
√
V
2
− UW)H
z
UH
z
(V +
√
V
2
− UW)H
x
− UH
y
⎞
⎠
(3)
We write T
i
= ( X , Y , Z)
T
,
i
= 1, 2. Thus the differential equation of a line of curvature on
H
(x) = 0 can be rewritten as
dx
X
=
dy
Y
=
dz
Z
, whose a special case is
dx
ds
=
T
i
T
i
(4)
where
s
is the parameter of arc length.
2.2. Umbilical points and Monge form
We now list some useful results about umbilical points and the patterns of LOC in proximity to them using Monge form.
2.2.1. Umbilical points
It is an essential step to find the umbilical points for accurate computation of
P .
Theorem 2.1. (See Che et al., 2007.) p is an umbilical point if and only if U
= V = W = 0.
Theorem 2.1 provides the foundation to trace umbilical points in our work.
2.2.2. Monge form
To obtain
P accurately, we also need to compute limit principal directions of LOC tending to an umbilical point p, which
can be determined by the Monge form of the surface S. Suppose T
p
S is the tangent plane of S at p, then:
Definition 2.2. (See Cazals and Pouget, 2005; Porteous, 2001, p. 200.) A limit principal direction of p is a direction in T
p
S
which is tangent to a line of curvature ending at p.
For simplicity, we denote such a limit principal direction of p by D
p
.
Consider a surface of the form as S
=[u, v, w(u, v)]
T
. If the origin of p is umbilical and T
p
S coincides with the u − v
plane, then we can Taylor expand the w-component of S to the Monge form as:
w(u, v) =
κ
n
(0, 0)
2
u
2
+ v
2
+
1
6
w
uuu
(0, 0)u
3
+3w
uu v
(0, 0)u
2
v + 3w
uvv
(0, 0)uv
2
+ w
vvv
(0, 0)v
3
+o
u
3
+ v
3
(5)
where κ
n
(0, 0) is the identical normal curvature at the origin. Eq. (5) contains a cubic form
w(u, v) = w
uuu
(0, 0)u
3
+3w
uu v
(0, 0)u
2
v + 3w
uvv
(0, 0)uv
2
+ w
vvv
(0, 0)v
3
(6)
An umbilical point of U is a singularity of F
1
and F
2
. Generic umbilical points can be categorized into three types: a star,
a monstar and a lemon as in Fig. 1(a)–(c) respectively, also referred to as ordinary umbilical points in Porteous (2001). Their
classifying form is just Eq. (6). Other umbilical points exist for degenerative but non-vanishing Eq. (6) (Gutiérrez et al., 2004;
Porteous, 2001). If Eq. (6) vanishes, higher order umbilical points need to be considered, but we find that it is discussed
quite rarely in literature and are not computationally robust. A special pattern is one through which an infinite number of
LOC pass in all directions (Fig. 1(d)), called a perfect star in this paper. Although it is non-generic, a perfect star is often
present in geometric surfaces of modeling, especially symmetrical ones.
To compute all
D
p
s, we can further express the Jacobian cubic form of Eq. (6) as
w
uu v
(0, 0)u
3
−
w
uuu
(0, 0) − 2w
uvv
(0, 0)
u
2
v +
w
vvv
(0, 0) − 2w
uu v
(0, 0)
uv
2
− w
uvv
(0, 0)v
3
(7)
whose root lines are precisely D
p
s (Maekawa et al., 1996; Hallinan et al., 1999).