The muscle that dominates our mental model of a smile is zygomaticus major. This muscle pulls our lip corners upward, outward, and backward. In FACS, or the Facial Action Coding System (a descriptive tool I use to discuss facial movements), the action driven by zygomaticus major is referred to as “lip corner puller,” or AU12.
NOTE: In this post, “smile” refers specifically to lip corner puller (AU12).
Closed vs. Open Smiles in Facial Rigs
In most facial rigs, the base smile shape is sculpted with the lips closed. Open-lip versions come from combinations: artists layer in other shapes, commonly upper lip raiser and lower lip depressor, to part the lips. This seems intuitive. Based on the names alone, upper lip raiser looks like the obvious choice for separating the top lip from the bottom. However, relying on upper lip raiser as your sole source of upper lip separation misses the other actions and interactions that part the lips in a natural smile.
Closed vs. Open Smiles on Real Faces
On a real face, the lips can part during a smile for one or more of several reasons:
(1) When zygomaticus major’s pull is strong enough, it can separate the lips. Zygomaticus major is a long muscle that originates on the zygomatic bone. As it draws the lip corners upward, outward, and backward, the tension it exerts on the lips causes them to shorten in vertical height, which often creates a slight separation.
(2) One or more of the upper lip elevators becomes active, lifting the top lip away from the bottom lip. Upper lip elevators include: levator labii superioris alaeque nasi (nose wrinkler, AU9), zygomaticus minor (nasolabial furrow deepener, AU11), and levator labii superioris (upper lip raiser, AU10).
(3) Depressor labii inferioris (lower lip depressor, AU16) kicks in to lower the bottom lip.
(4) The jaw actively or passively opens, forcing the lips to separate.
Why You Should Change What You’re Doing
When the zygomaticus major action is strong, it stretches the fleshy parts of the lips laterally while pulling the lip corners diagonally. In some cases, this is what causes the lips to part. In other cases, the separation is a consequence of a complex system of interconnected muscles and their interactions.
To create an appealing, toothy smile: do not simply crank up upper lip raiser. Instead:
The remaining content is for Premium Members only. If you ARE a member, sign in and go back to this page :)!
Great info and thanks for posting. I would add that much of a smile’s meaning is also determined by lower lid compression of the rising cheek. Under stress(malice) the eyelids are often very wide or when a “false” smile is performed.
Thank you 😀 really appreciate that!
Thank You! These posts are Golden!