Physically Based

A hierarchical model of the face provides a natural and obvious set of control parameters for the face [133]. 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.

1. 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.

2. CONTROL. A muscle-control process translates expression instructions into coordinated activation of muscle groups on the facial model.

3. 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.

4. 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.

5. 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.

6. 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 hierarchy.