Facial Animation - Computer Animation: Algorithms and Techniques - page 316

CHAPTER

10

Facial Animation

Realistic facial animation is one of the most difficult tasks that a computer animator can be asked to do.
Human faces are familiar to us all. Facial motion adheres to an underlying, well-defined structure but is
idiosyncratic. A face has a single mechanical articulator but has a flexible covering capable of subtle
expression and rapid, complex lip movements during speech.
The face is an important component in modeling a figure, because it is the main instrument for
communication and for defining a figure's mood and personality. Animation of speech is especially
demanding because of the requirement for audio synchronization (and, therefore, is often referred
to as lip-sync animation ). In addition, a good facial model should be capable of geometrically repre-
senting a specific person (called conformation by Parke [ 24 ], static by others, e.g., [ 28 ] ).
Facial models can be used for cartooning, for realistic character animation, for telecommunications
to reduce bandwidth, and for human-computer interaction (HCI). In cartooning, facial animation has to
be able to convey expression and personality that is often exaggerated. In realistic character animation,
the geometry and movement of the face must adhere to the constraints of realistic human anatomy.
Telecommunications and HCI have the added requirement that the facial model and motion must
be computationally efficient. In some applications, the model must correspond closely to a specific
target individual.
In addition to the issues addressed by other animation tasks, facial animation often has the con-
straint of precise timing with respect to an audio track during lip-synching and some expressions.
Despite its name, lip-synching involves more than just the lips; the rigid articulation of the jaw and
the muscle deformation of the tongue must also be considered.

10.1 The human face
10.1.1 Anatomic structure
The human face has an underlying skeletal structure and one main skeletal articulatory component—
the jaw. In addition to the jaw, the other rigid articulatory components are the eyes.
The surface of the skull is covered with muscles, most of which (at least indirectly) connect areas of
the skin to positions on the skull. These muscles tug on the skin to create movement, often recognizable
as expressions. Features of the skin include eyelids, mouth, and eyebrows. The skin has various expres-
sive wrinkles such as on the forehead and around the mouth.
As with the rest of the body, muscles provide the driving force for the face ( Figure 10.1 , Color
Plate 7). However, unlike the rest of the body, the muscles mainly move the skin into interesting