3.7. SMOOTH MANIFOLD 21
Figure 3.7 represents the transition functions between two charts defined on the same man-
ifold.
Figure 3.7: Given two neighborhoods U
i
and U
j
, each one with an associated coordinate map '
i
and
'
j
respectively, the transition function '
i;j
is defined on the intersection of U
i
and U
j
; in particular
'
i;j
must be compliant with both the '
i
and '
j
[217].
3.7 SMOOTH MANIFOLD
Smooth manifolds are manifolds whose transition functions have derivatives of all orders. Smooth
surfaces, which are particularly important for shape analysis, belong to this class. Formally:
Smooth manifold A k-dimensional topological manifold with (resp. without) boundary is called
a smooth manifold or C
1
with (resp. without) boundary, if all transition functions '
i;j
are smooth.
If all transition functions '
i;j
are n times continuously differentiable, then the manifold is called
a C
n
manifold.
Smooth surface A smooth 2-dimensional manifold with (resp. without) boundary is called a
smooth surface, or simply surface, with (resp. without) boundary.
3.8 ORIENTABILITY
Orientability can be introduced in many ways. e intuition behind it is to be able to orient the
normals at every point of our object in a consistent manner, for instance by using the well-known
right-hand rule. Beside the standard Euclidean space, the concept of orientability can be extended
to more general spaces, such as manifolds, where the role of normals is taken by the Jacobian.²
A manifold M is called orientable if there exists an atlas f.U
i
; '
i
/g on it such that the
Jacobian of all transition functions '
i;j
from a chart to another is positive for all intersecting pairs
of regions. Manifolds that do not satisfy this property are called non-orientable.
²e Jacobian matrix extends the notion of gradient to multi-variate functions and represents the matrix of all the first-partial
derivatives; the Jacobian is the determinant of the Jacobian matrix. For the notions of differentiability we refer the reader to
standard books of real analysis [118, 123].

Get Mathematical Tools for Shape Analysis and Description now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.