The biological structure of the face reveals various notions of genericity at different levels. While building the face, features always ``order'' in the same way: the nose is in between the two eyes, the lips are under the nose, and so on. In other words, the morphology varies while the base structure is kept unchanged. The skeleton defines the overall shape of the face, while the muscles smooth and deform that shape. Skin layers influence in a noticeable way the final shape: fat accumulation in the dermis creates cheeks, double chins, and other phenomena.
Also, while deforming the face, each muscle is a generic object. However, its shape and contractions are sightly different from one individual to another. This generates slightly different behaviors as muscles act. Some people smile with the extremities of the lips higher than the middle point of the superior lips, while others don't, but we detect a smile in both cases.
Wrinkles also carry a notion of genericity: the skin bulges under muscle compression, and wrinkles depend on the nature of the skin and on muscular contractions.
The process of aging is a complex one. It depends not only on the structure of the initial face but also on many aspects of one's life, including climatic, psychological and other parameters. Moreover, some aspects of aging are linked to the face's structure: fat accumulation, hollow places or wrinkle directions are well known [17]. Hence some generic aspects of aging can be drawn.
The idea of genericity is to use these properties of ``similarity'' between faces to generate tools that allow an easy creation of new synthetic faces from existing ones. Several systems provide various tools acting on an initial mask to create new faces [143][138][107][80]. In addition, the new structure may inherit some parts of the deformation tools used to create facial actions. As soon as action units of the system are respected, higher level controls (expression libraries or sequences of animation) become available to the new model.
The design of orthogonal functionalities for facial animation which can be inherited and interactively modified frees a facial animation process from excessive complexity. It also makes facial animation software easily amenable to the inclusion of new developments.