Facial Action Coding System

Facial Action Coding System (FACS) is a system originally developed by Paul Ekman and Wallace Friesen in 1976, to taxonomize every conceivable human facial expression. It is the most popular standard currently used to systematically categorize the physical expression of emotions, and it has proven useful both to psychologists and to animators.

It defines expressions as one of 46 "Action Units", which is a contraction or relaxation of one or more muscles. For example, it can be used to distinguish the two types of smiles as follows:


 * insincere and voluntary Pan American smile: contraction of zygomatic major alone
 * sincere and involuntary Duchenne smile: contraction of zygomatic major and inferior part of orbicularis oculi, and pars orbitalis

Although the labeling of expressions currently requires trained experts, researchers have had some success in using computers to automatically identify FACS codes, and thus quickly identify emotions. 

Computer graphical face models, such as CANDIDE or Artnatomy, allow expressions to be artificially posed by setting the desired action units.

Codes for common action units
(Also see the list of facial muscles.)

Action units involving facial muscles

 * 1      Inner Brow Raiser -- Frontalis (pars medialis)
 * 2      Outer Brow Raiser -- Frontalis (pars lateralis)
 * 4      Brow Lowerer -- Corrugator supercilii, Depressor supercilii
 * 5      Upper Lid Raiser -- Levator palpebrae superioris
 * 6      Cheek Raiser -- Orbicularis oculi (pars orbitalis)
 * 7      Lid Tightener -- Orbicularis oculi (pars palpebralis)
 * 9      Nose Wrinkler -- Levator labii superioris alaquae nasi
 * 10      Upper Lip Raiser -- Levator labii superioris
 * 11      Nasolabial Deepener -- Zygomaticus minor
 * 12      Lip Corner Puller -- Zygomaticus major
 * 13      Cheek Puffer -- Levator anguli oris (also known as Caninus)
 * 14      Dimpler -- Buccinator
 * 15      Lip Corner Depressor -- Depressor anguli oris (also known as Triangularis)
 * 16      Lower Lip Depressor -- Depressor labii inferioris
 * 17      Chin Raiser -- Mentalis
 * 18      Lip Puckerer -- Incisivii labii superioris and Incisivii labii inferioris
 * 19      Tongue Show
 * 20      Lip stretcher -- Risorius w/ platysma
 * 21      Neck Tightener
 * 22      Lip Funneler -- Orbicularis oris
 * 23      Lip Tightener -- Orbicularis oris
 * 24      Lip Pressor -- Orbicularis oris
 * 25      Lips part -- Depressor labii inferioris or relaxation of Mentalis, or Orbicularis oris
 * 26      Jaw Drop -- Masseter, relaxed Temporalis and internal pterygoid
 * 27      Mouth Stretch -- Pterygoids, Digastric
 * 28      Lip Suck -- Orbicularis oris
 * 29      Jaw Thrust
 * 30      Jaw Sideways
 * 31      Jaw Clencher
 * 32      Bite
 * 33      Blow
 * 34      Puff
 * 35      Suck
 * 36      Bulge
 * 37      Lip Wipe
 * 38      Nostril Dilator
 * 39      Nostril Compressor
 * 41      Lid droop -- Relaxation of Levator palpebrae superioris
 * 42      Slit -- Orbicularis oculi
 * 43      Eyes Closed -- Relaxation of Levator palpebrae superioris; Orbicularis oculi (pars palpebralis)
 * 44      Squint -- Orbicularis oculi (pars palpebralis)
 * 45      Blink -- Relaxation of Levator palpebrae superioris; Orbicularis oculi (pars palpebralis)
 * 46      Wink -- Relaxation of Levator palpebrae superioris; Orbicularis oculi (pars palpebralis)

Other action units

 * 51      Head turn left
 * 52      Head turn right
 * 53      Head up
 * 54      Head down
 * 55      Head tilt left
 * 56      Head tilt right
 * 57      Head forward
 * 58      Head back
 * 61      Eyes turn left
 * 62      Eyes turn right
 * 63      Eyes up
 * 64      Eyes down
 * 65      Walleye
 * 66      Cross-eye