Face Detection and Modeling for Recognition - Biometrics Research ...

(a) (b) (c) (d)
Figure 5.6. Shadow maps: (a) and (c) are luma components of face images in Figs.
5.4(a) and 5.4(c), overlaid with rectangles within which the average values of skin
intensity is calculated; (b) and (d) are shadow maps where bright pixels indicate the
regions that are darker than average skin intensity.
representations. The explicit contour representation has the advantage of maintaining
the geometric topology. The implicit contour representation requires topological
constraints on the implicit function.
5.3.1 Interacting Snakes and Energy Functional
Active contours have been successfully used to impose high-level geometrical constraints
on low-level features that are extracted from images. Active contours are
iteratively deformed based on the initial configuration of the contours and the energy
functional that is to be minimized. The initial configuration of interacting snakes is
obtained from the coarsely-aligned semantic face graph, and is shown in Fig. 5.8(c).
Currently, there are eight snakes interacting with each other. These snakes describe
the hair outline, face outline, eyebrows, eyes, nose, and mouth of a face; they are
denoted as V (s) = ⋃ N
j=1 {v i(s)}, where N (= 8) is the number of snakes, and v i (s) is
the i th snake with the parameter s ∈ [0, 1].
The energies used for minimization include the internal energy of a contour (i.e.,
smoothness and stiffness energies), and the external energy (i.e., the inverse of edge
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