A Muscle Model for
Animating Three-Dimensional Facial Expressions

Keith Waters
Computer Graphics, July 1987

Abstract

The development of a parameterizerd facial muscle process, that incorporates the use of a model to create realistic facial animation is described. Existing methods of facial parameterization have the inherent problem of hard-wiring performable actions. The development of a muscle process that is controllable by a limited number of parameters and is non-specific to facial topology allows a richer vocabulary and a more general approach to the modelling of the primary facial expressions.

Summary

Few living forms can be determined by simple parameters such as width, height, diameter, etc. which are possible for geometric primitives. The fifty independent facial actions can give rise to several thousands of muscle combinations. These are encoded in the Facial Action Coding System (FACS) as action units (AU). Six categories are described: Anger, Fear, Surprise, Disgust, Happiness, and Sadness, each using multiple combinations of action units in various degrees.

The human face contains at least the following types of muscles: Uppers and downers, which move the face upwards towards the brow and conversely towards the chin; those that contract horizontally towards the ears and conversely towards the center line of the face; oblique muscles that contract angularly from the lips to the cheeks; orbitals that circle around the mouth and eyes; and sheet muscles that cover the neck. Among these, the muscles of the mouth carry out the most complex interactions. The primary muscle is a sphincter muscle with no bony attachment.

Neutral face with muscles relaxed

Happiness: the corners of the lips are drawn back and raised obliquely by the zygomatic major muscle

Fear: the inner brows are raised by the inner frontalis muscle, the eyes are wide with pupils dilated. The jaw is rotated and the lips drawn back.

Anger: the brows are lowered and the inner part drawn together. The jaw is not rotated and the lips are tight.

Disgust: the alaeque nasi muscle raises the upper lip pulling the skin around the nose and causing the nostrils to dilate.

Surprise: the brows are curved and high, the eyelids wide and the pupils dilated.