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A hierarchical model of the face provides a
natural and obvious set of control parameters for the face
. Conceptually, this approach decomposes
into six levels of abstraction involving representations that exploit
what we know about the psychology of human facial expressions, the
anatomy of facial muscle structures, the histology and biomechanics of
facial tissues, and the facial skeleton and kinematics.
- EXPRESSION. At the highest level of abstraction, the face model
executes expression commands. For instance, it can synthesize any of
the six primary expressions within a given time interval and with
specified degrees of emphasis.
- CONTROL. A muscle-control process translates expression
instructions into coordinated activation of muscle groups on the
- MUSCLES. As in real faces, muscles comprise the basic acquisition
mechanism of the model. Each muscle model consists of a bundle of
muscle fibers. When fibers contract, they displace their points of
attachment in the facial tissue or the jaw.
- PHYSICS. The face model incorporates a physical approximation to
human tissue, implemented as a lattice of point masses connected by
nonlinear elastic springs. Large-scale synthetic tissue deformations
are simulated numerically by continuously propagating through the
lattice the stresses induced by activated muscle fibers.
- GEOMETRY. The geometric representation of the facial model is a
non-uniform mesh of polygonal elements whose size depends on the
curvature of the neutral face. Muscle-induced synthetic tissue
deformations distort the neutral geometry into an expressive geometry.
- IMAGES. After each simulation time step, standard visualization
techniques implement by dedicated graphics hardware render the
deformed facial geometry in accordance with viewpoint, light source,
and skin reflectance information to produce a continuous stream of
facial images, the least abstract of the representations in the