Matching is an essential task for many computer vision applications.
A clear definition can be given for rigid or articulated
bodies: it is to recover rigid displacements of rigid parts.
The task is much more complicated for deformable objects,
with plastic or elastic deformations. In this case, there is not
a single definition of an ideal optimal match, but as many
definitions as practical applications. Each time, one has to
define precisely the set of deformations T which are explored
(rigid, affine, spline, free-form etc), and the type of features
which are used (points, curves, surfaces, intensities etc).