To simulate muscle action, we define regions on the face mesh which correspond to the anatomical description of the facial region on which a muscle action is desired. The face mesh can then be deformed by either adjusting weights on control points, moving the control points, or both. The region inside control points then deform like a flexible volume corresponding to the displacement or weight of each control point. Physical properties of the skin surface such as mass, stiffness, and elasticity can also be incorporated as a parameter in the deformation.
A minimum perceptible action is a basic facial motion parameter. The range of this motion is normalized between 0 and 1 or -1 and 1. MPAs include facial feature movement (eyebrows, jaw, mouth) as well as non-facial actions such as nods or turns of the head or eye movement. The following is a list of available MPAs:
| MPA Name | Intensity |
|---|---|
| Raise_eyebrow | -1 to 1 |
| Squeeze_eyebrow | 0 to 1 |
| Move_horizontal_eye | -1 to 1 |
| Move_vertical_eye | -1 to 1 |
| Close_upper_eyelids | -1 to 1 |
| Close_lower_eyelids | -1 to 1 |
| Stretch_nose | -1 to 1 |
| Raise_nose | 0 to 1 |
| Raise_upper_lip | 0 to 1 |
| Pull_upper_lip | 0 to 1 |
| Lower_lower_lip | 0 to 1 |
| Pull_lower_lip | 0 to 1 |
| Raise_corner_lip | 0 to 1 |
| Lower_corner_lip | 0 to 1 |
| Stretch_corner_lip | 0 to 1 |
| Mouth_beak | 0 to 1 |
| Zygomatic_cheek | 0 to 1 |
| Puff_cheek | -1 to 1 |
| Nod_head | -1 to 1 |
| Turn_head | -1 to 1 |
| Roll_head | -1 to 1 |
An expression is then made up of a set of MPAs, providing a higher level of abstraction. The MPAs can combine to show natural expressions such as anger, fear, or surprise, or unnatural and ideosyncratic expressions. Expressions also have associated intensities to reflect stronger or more feeble expressions.