Facial animation by computer is a complex subject involving advanced techniques in human-computer interaction, physical and geometrical simulation, computer graphics and also image recognition. A living face (human or animal) in motion is a complex animated structure in which fine details, rigid and deformable structures run together producing a variety of expressions. The choice of a specific facial model is dictated by the particular effects one expects to achieve during animation. See . As discussed above, all human faces possess the same physiological organization of bones, muscles and skin. People smile in the same way-contracting the big zygomatic and the lips corner elevator. Nevertheless, all faces are different.
The complexity of facial animation is due mainly to the fact that there is no unique model integrating all primary and secondary effects perceived on a face: each particular aspect requires specific modeling (facial animation, wrinkles, speech, etc.) and all aspects intersect each other in a complicated manner. This is why the design of highly structured facial animation systems in which all aspects of facial animation are clearly integrated is an important issue. High level tools allow the design of facial animation from a generic point of view; that is, independent of a given geometry. Since it is now possible to obtain several geometric data bases representing faces, the design of facial animation would be greatly simplified if one could utilize expressions libraries that could be applied to any face.