Unlike the eye region, the mouth does not have a blob-like, circular outline.
When it is closed or slightly open, the mouth is a thin, elongated structure
or a limb. Thus, it may not be detected by the symmetry transform's interest
map. We propose the use of a limb extraction stage as outlined by Kelly
[20] to detect the mouth. The limb extraction process begins with
the computation of the points of symmetry as displayed in
Equation . This is an intermediate computation that was
used to compute the interest map. Furthermore, only annuli of radius *r*=1 to
*r*=6 are used, since the mouth's vertical thickness is slightly smaller than
the vertical thickness of the eye region (from the top of the eyebrows to the
bottom of the eye orbit). We only compute dark symmetry since the lips and the
interior of the mouth are darker than the surrounding skin.

The result of the symmetry computation is an image at each value of *r* (*r*=1
to *r*=6) with each point containing 8 magnitudes for each symmetry
orientation. Thus, a symmetry magnitude is computed for each point *p* for
each *r* from *r*=1 to *r*=6 and for each
from =1 to =8 (8
symmetry orientations as in Figure ). The resulting
symmetry points are output as in Figure .