
232 Image Analysis and Modeling in Ophthalmology
where I(v
i
) is the value of the level of the nearest neighbor of V
i
and
is the direction of
the external force for each vertex, which is dened in Equation 13.10:
F
C
i
i
i
ext ,
=
−
v
(13.10)
Therefore, the external force on each vertex can be written as in Equation 13.11:
FFF
i
ext ext ext
,
= (13.11)
The displacement contour is based on the composition of the internal and external forces
on the contour vertices. Each vertex is replaced by the iterative Equation 13.12:
ii
tt
+
=
++−
int,
xt,
(13.12)
where β is a dened weight that controls the rate of ow and the con