Facial animation isn't always involving complex anatomical models. Often, animators will approximate the movements of the face to give an imitation of reality without going into too much detail. Many of the famous Pixar films were done using simple facial animation. It is the animation of the figure as a whole, rather than just the facial movements, that creates a successful image with believable expressions. The key facets to making animation work are: good body and head motion; good eye motion and good mouth motion. As a companion to simple facial animation comes simple facial shapes. These can be as incredibly stark, a sphere for example, but somehow they can still give a believable effect. The animators at Pixar have created their own language, Menv, to ease the process of animation. A bonus of simple facial animation is that the expressions can be exaggerated to give a more interesting and "alive" effect.
The animation for the short film, "Tin Toy", was on the more complex side of facial animation. For the toy, Tinny, they used the simple techniques described earlier, but for the baby, Billy, they wanted a more realistic image. To animate Billy, they had to create software which could handle complex facial animation. They did not want to have to sculpt each facial expression and then digitise it, so they created a muscle model to work on a set of 3-D points which represented Billy's skin. The first step was to create a clay model of Billy with a neutral expression and then digitise it. (Only one half as they took advantage of the symmetry of the face.) The Pixar people have a rule that they only use polygons when representing flat things, so the usual technique of breaking the face up into triangles was not an option. Instead, they used a series of Catrom patches and a formulation by DeRose/Barsky for smoothing the resulting surface.
To animate Billy's body, an articulated, hierarchical skeleton was made out of simple cylinders. Billy's face was animated using a muscle-based model, similar to that of Keith Waters. Three types of muscles: linear (43 of them), sphincter (4) and rotational (for areas where the muscles were in contact with bone) were used in Billy's face. The placement of these muscles was based on the illustrations in Ekman and Friesen's FACS manual. A system of macros was developed to give a higher level of control to the animation system. The result was a very successful short film.
Synopsis by Valarie Hall