What is FACS rigging?
Updated: May 21, 2021
Facial Action Coding System (FACS) is a way to anatomically measure and break down facial expressions. With this theory, emotions can be “built” by combining different movements in different sections of the face.
To put it simply: Surprise Emotion = Raised Eyebrows + Open Jaw
The FACS Index System
FACS can be thought of as an index for facial expressions. Each expression is given an index in Action Units (AU) and a basic description.
I've also read they can be further described by an Intensity Score of A-E. (A = a traced amount of movement, B = slight, C = pronounced, D = severe, E= extreme)
The image above displays examples of different expressions, their assigned Action Unit index (AU1), and their name (Inner Brow Raiser).
Additionally, the letters L and R can also be used to indicate whether it’s happening Left or Right. (not shown above)
U is used for an unilateral action. It occurs on only one side of the face, but can happen in either side (example: winking an eye).
A is an unilateral action that has a stronger side (example: raising brows but the left side is more pronounced).
FACS theory is used in rigging to create more natural and appealing facial expressions in characters. It is possible to make FACS rigs that are joint based, and/or blend shape based. It can also include normal maps that can blend to create the illusion of photo-realistic wrinkles.
Here's a pretty cool example of something done with FACS:
Aaaand here's a rudimentary prototype of a FACS based rig I'm currently working on:
I'm planning on doing a breakdown of the whole process once it's finished, but this is your basic introduction to what I know about FACS!