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A deformable model is active in the sense that it can adopt itself
to fit the given data. It is a useful shape model because of its flexibility,
and its ability to both impose geometrical constraints on the shape
and to integrate local image evidences.
There has been a substantial amount of research on deformable models in recent
years. These activities can be partitioned into two classes:
- free-form models, and
- parametric models.
By free-form deformable models, we mean that there is no global structure
of the template except for some general regularization
constraints; the template is constrained only by continuity and/or
smoothness of the boundary. Such a free-form model can
be deformed to match salient image features like lines and edges using potential fields (energy functions) produced by those features. Since there is no
global structure for the template, it can represent an arbitrary shape as long
as the regularization requirements are satisfied. On the
other hand, parametric deformable models control the deformations using
a set of parameters which are capable of encoding a
specific characteristic shape and its variations.
This type of model is used when more specific shape information is available,
which can be described by a set of parameters.
There are two ways to parameterize the shape variation: (i) handcraft
a parametric formula for the curves (surfaces) in the shape template such that
different
shape instances can be obtained using different parameter values;
(ii) design a prototype for a shape class, and then apply a
parametric transformation on the prototype to obtain different
deformed templates.
Bob Fisher
Wed May 5 18:16:24 BST 1999