A Muscle Model

In this example an abstract muscle model controls facial expressions on a facial geometry. The muscle model provides a direct linkage to muscle-based coding schemes, such as the Facial Action Coding Scheme, and is independent of a specific facial geometry [144].

The underlying process is as follows:

1. Muscles are described as belonging to one of three primary groups: Linear, sphincter, and sheet. Each group describes a specific geometric deformation function emulating muscle action on skin tissue.

2. The muscles control parameters are (1) location on bone and skin, (2) zones of influence, and (3) type of contraction profile (linear, non-linear). Each muscle functions independently.

3. Muscles are then labeled to correspond to Action Units (AU's) in FACS. Consequently, facial expressions can be created by orchestrating a sequence of muscle AU contractions.

4. Muscle control parameters are then scripted into a keyframe animation system. At each frame in the sequence, muscles are contracted that in turn deform the facial geometry. The resulting geometry is rendered in accordance with viewpoint, light source, and skin reflectance information.