What is FACS?
Facial Action Coding System
What does it have to do with Face Rigging?
you create a face rig that mimics the action of the muscle groupings identified by FACS, usually using blendshapes
Example list of Facial Shapes (not truly FACS-based)
Jaw Open
Jaw Side
Jaw Fwd
Smile
Frown
Narrow
Wide
Upper Lip Up/Dn
Lower Lip Up/Dn
Mouth L/R
Sneer
Squint
Outer Brows Up/Dn
Mid Brow Up/Dn
Blink
Andrew Silke's Face Blendshape Tutorials
didn't go through em all, but it looks like a nice basic introduction... AND shows you how to get them out of Mudbox.
How do you keep blend shapes from intersecting each other and causing ugly shapes?
(ie., keeping the 'smile' shape looking good as the jaw opens, or using 'smile' and 'mouth narrow' at the same time..)
How do you neatly split a blend shape vertically (up and down, instead of L/R)?