Key framing
Key framing requires the complete specification of the model at each extreme, or at least the storage of the differences between facial positions. Additionally, any unique subtle movement of the face must be constructed as a complete model, with the result that key framing is data-intensitive and lacks specific manipulation.
Parameterization
Parameterization avoids this problem of rigidity (cfr. key framing) by grouping
vertices together to perform specified tasks. However, generality is lost as
soon as the process is applied to new facial topology. Only by maintaining the
same topological mesh will the parameterization hold true.
Investigation by Parke on the conformation of faces deals with the problem of utilizing these constraints, but it is doubtful whether the generality of such a topology will hold true over a wider range of facial types.
Facial parameterization have dealt principally with the surface characteristics of the skin and have not been concerned with the motivators of the dynamics.
A Structural approach
Investigation by Badler into the structural bases
for the upper face dealt with the elastic nature of muscle and skin.
The process is iterative in nature and deals adequately with the motivators of the
actions.
However, the complexitites of the lower face jaw rotations render the processes
unperformable.
Viewer expectations
The closer the images get to reality, the more critical the viewer becomes.
- If the images are clearly perceived as artificial or synthetic, the viewer seems
willing to be somewhat forgiving and accept them as such.
- If the image is clearly a caricature, the viewer recognizes and accepts this.
- If the image is clearly supposed to be realistic, the viewer is very sensitive to any flaws.
Also it is important to animate certain features than others for expressing emotion. High priority features are the eyes,mouth,eyebrows and eyelids. Low on the list are the cheeks,nose,tongue,ears and hair.
By far, the most important is the eye and surprising degree of expression can be achieved by animating the pupil.
Also, animating features symmetrically will produce lifeless and unconvincing results. This is the principle of traditional animation not just restricted to the face where it is called "twins".
An extension of this is Character Animation where in addition to the face, synchronization of the lips and the other actions is also carried out.
