Image-Based 3D Face Modeling System 2075
Ellipse
Probe
Exterior probe neighborhood
Interior probe neighborhood
(a)
(b) (c) (d)
Figure 3: Face region detection with deformable model. (a) Deformable model. (b), (c), and (d) Convergence of the deformable model.
Here, R
n,x,y
denotes an (n × 7)-sized rectangle centered at
(x, y), while P
n,r
is an (n × n/3)-sized e llipse centered at r.
The control parameters are the scaling coefficient, α, and the
expected size of the e ye features, n.
The meaning of the variation image V
n,α
(x, y)canbede-
scribed as a dilatation of the high-frequency patterns in the
red channel facial image. The variation image is calculated
for several (n, α) pairs in order to cope with the high vari-
ance of the eye appearance, as shown in Figure 4. This results
in a stable and correct behavior for the images with differ-
ent lighting and quality. The connected components of the
pixels with high variation values are then tested to satisfy the
shape, size, and symmetry restriction in order to obtain the
best-matching eye position for each variation image inde-
pendently. Finally, different (n, α) configurations are sorted
so that the later ones generate a stronger response. Their re-
sults are combined in this order so that later results can either
provide an output if no response is generated previously, or
refine the previous results otherwise.
2.3. Eye contours detection
The eye contour model consists of an upper lid curve in a cu-
bic polynomial, a lower lid curve in a quadratic polynomial,
and an iris circle. The iris center and radius are estimated by
the algorithm developed by Ahlberg [23]. This is based on
the assumptions that the iris is approximately circular and
dark against the background, that is, the eye white. Conven-
tional approaches of eyelid contour detection use deformable
contour models attr acted as a result of the high values of the
luminance edge gradient [24]. Deformable models require
the careful formulation of the energy term and good initial-
ization, otherwise an unexpected contour extraction result
may be acquired. Moreover, it is undesirable to use lumi-
nance edges for contour detection, because eye area may have
many outlier edges.
This paper proposes a novel technique that achieves both
stability and accuracy. Taking the luminance values along a
single horizontal row of an eye image as a scalar function
L
y
(x), it can be seen that the significant local minima cor-
respond to the eye boundary points, as shown in Figure 5.
This observation is valid for many images taken under very
different lighting conditions and qualities. The detected can-
didate pixels of the eye boundary are filtered to remove the
outliers before fitting a curve to the upper eyelid points. On
the other hand, the lower lid is detected by fitting the eye
corners and the lower point of the iris circle with a quadratic
curve.
The eyebrows can be detected simply by fitting parabolas
to the dark pixels after binarizing the luminance image in the
areas above the eye bounding boxes.
2.4. Lip contour detection
In most cases, the lip color differs sig nificantly from that
of the skin. Iteratively refined skin and lip color models are
used to discriminate the lip pixels from the surrounding skin.
The pixels classified as skin at the face detection stage and
located inside the face ellipse are used to build a person-
specific skin-color histogram. The pixels with low values of a