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

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shows examples of mouth energies. For the nose component, its GVF is usually weak,<br />

<strong>and</strong> it is difficult to construct an energy map <strong>for</strong> nose. Hence, <strong>for</strong> the nose, we utilize<br />

Tsai <strong>and</strong> Shah’s shape-from-shading (SFS) algorithm [179] to generate a boundary<br />

energy <strong>for</strong> augmenting the GVF <strong>for</strong> the nose component. The illumination direction<br />

used in the SFS algorithm is estimated from the average gradient fields of a face<br />

image [180] within a facial region. Figures 5.11(d) <strong>and</strong> 5.11(e) show examples of nose<br />

boundary energies in a 2D grayscale image <strong>and</strong> a 3D mesh plot, respectively.<br />

5.3.2 Parametric Active Contours<br />

Once we obtain the attraction <strong>for</strong>ce, we can make use of the implicit finite differential<br />

method [77], [127] <strong>and</strong> the iteratively updated repulsion <strong>for</strong>ce to de<strong>for</strong>m the snakes.<br />

The stopping criteria is based on the iterative movement of each snake. Figure 5.12(a)<br />

shows the initial interacting snakes, Fig. 5.12(b) shows snake de<strong>for</strong>mation without the<br />

eyebrow snakes, <strong>and</strong> Fig. 5.12(c) shows finely aligned snakes. Component matching<br />

scores in Eq. (5.4) are then updated based on the line <strong>and</strong> region integrals of boundary<br />

<strong>and</strong> component energies, respectively. We discuss another approach <strong>for</strong> de<strong>for</strong>ming the<br />

interacting snakes based on geodesic active contours <strong>and</strong> level-set functions in Section<br />

5.3.3.<br />

5.3.3 Geodesic Active Contours<br />

As implicit contours, geodesic snakes [181], which employ level-sets functions [182]<br />

are designed <strong>for</strong> extracting complex geometry. We initialize a level-set function using<br />

127

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