Parameterized Models for Facial Animation

There are several approaches to applying computer graphics techniques to facial animation:

Key frame animation. The desired image is specified for a certain point in time and then again for another point in time a certain number of frames later. A computer algorithm generates the necessary images for the frames in between these key frames.
This technique has been successfully adapted to 2D and 3D animation.

Parameterized models. Two basic idease are involved in the creation of parametrized graphical models:

determine the appropriate set of parameters, and
develop image synthesis models based on these parameters.

Both approaches have some drawbacks, here are some suggested by K. Waters.

Developing parameter sets

How is a facial parameter set developed?
Several approaches are possible:

Observe the surface properties of faces and develop ad hoc sets that allow these observed characteristics to be specified parametrically.
Studying the underlying structure, or facial anatomy, and developing a set of parameters based on it -- the approach used by Platt and Badler.
Parameters are based on structural understanding, wherever possible, and are supplemented as necessary by parameters based on observation -- the approach used by Parke.

There are two broad categories of parameters; those controlling the

conformation, or structure, of an individual face.
expression, or emotional content.

What should the parameters for an expression model be? A possible foundation -- and it appears to be the best basis -- is the Facial Action Coding System, or FACS, developed by Paul Ekman.

Expression parameters

Which parameters are best included in a simple expression model remains unresolved. Of primary importance in expression are the eyes and mouth, so most parameters relate to these areas.

For the eyes:

pupil dilation
eyelid opening
eyebrow position and shape
the direction in which the eyes are looking

For the mouth:

jaw rotation (controls mouth opening)
width of the mouth
mouth expression (smiling, frowning, ...)
position of the upper lip
positions of the corner of the mouth

others are:

size of the nostrils
the orientation of the face

Conformation parameters

Changes in the confirmation of faces (those aspects that vary from individual to individual and make each perso unique) require a different set of parameters. Again, the ideal set is unknown.

Conformation parameters:

control the color of the eyebrows, eyelashes, iris, lips, ...
use relative size (scale), shape and positioning information to control:
- neck length and shape
- chin, forehead, cheek, and cheekbone shape
- eyelid, eyeball, and iris size and the position and separation of the eyes
- jam width
- nose length and the width of the bridge and end of the nose
- chin and forehead scale and the scale vis-a-vis the rest of the face. The development of truly complete confirmation parameters appears very difficult.

One such parametrized model was given by K Waters

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