Graphics Reference
In-Depth Information
10.3.3
Muscle models
Parametric models encode geometric displacement of the skin in terms of an arbitrary parametric value.
Muscle-based models (e.g.,
Figure 10.13
, Color Plate 11) are more sophisticated, although there is wide
variation in the reliance on a physical basis for the models. There are typically three types of muscles
that need to be modeled for the face: linear, sheet, and sphincter. The
linear muscle
is a muscle that
contracts and pulls one point (the
point of insertion
) toward another (the
point of attachment
). The
sheet
muscle
acts as a parallel array of muscles and has a line of attachment at each of its two ends rather than
a single point of attachment as in the linear model. The
sphincter muscle
contracts a loop of muscle. It
can be thought of as contracting radially toward an imaginary center. The user, either directly or indi-
rectly, specifies muscle activity to which the facial model reacts. Three aspects differentiate one
muscle-based model from another: the geometry of the muscle-skin arrangement, the skin model used,
and the muscle model used.
The main distinguishing feature in the geometric arrangement of the muscles is whether they are
modeled on the surface of the face or whether they are attached to a structural layer beneath the skin
(e.g., bone or tissue
[27]
,
[33]
). The former case is simpler in that only the surface model of the face is
needed for the animation system (
Figure 10.14
)
. The latter case is more anatomically correct and thus
promises more accurate results, but it requires much more geometric structure in the model and is there-
fore much more difficult to construct (
Figure 10.15
)
.
The model used for the skin will dictate how the area around the point of insertion of a (linear)
muscle reacts when that muscle is activated; the point of insertion will move an amount determined
(Image courtesy of Arun Somasundaram.)
A
B
FIGURE 10.14
Part of the surface geometry of the face showing the point of attachment (A) and the point of insertion (B) of a
linear muscle; point B is pulled toward point A.
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