building smiles – the right way

The smile is an essential expression for emotion and communication. Smiles form when our lip corners are pulled diagonally by a muscle called “zygomaticus major.” In the Facial Action Coding System (a descriptive tool I use to discuss facial movements), the diagonal action for the smile is referred to as “lip corner puller,” or AU12.

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NOTE: For the purposes of this post, “smile” will specifically refer to the action lip corner puller/AU12/zygomaticus major pulling the lip corners.

Open & Closed-Mouth Smiles

  • closed-mouth smile:
    • lips remain closed/together
  • open-mouth smile:
    • lips are parted
    • occurs when zygomaticus major’s pull is strong enough to separate the lips
    • can also occur when other facial actions induce lip separation (such as upper lip raiser)
  • open-mouth, open jaw smile:
    • both lips and jaw are parted
    • occurs when the jaw opens
    • parted lips may be due to –
      • opening of the jaw
      • strength of zygomaticus major
      • induction of additional facial actions

When artists develop a set of expression shapes, they often need to create different states for open mouth shapes, like the smile – i.e. One cannot create a single smile shape and have it exist in both open and closed form without the addition of other shapes.

In the process of putting together shapes to achieve an open-mouth smile, it is commonplace for artists to add actions such as upper lip raiser and lower lip depressor to a closed-lip smile. This method seems intuitive; based on the names of each action, one would assume upper lip raiser is the obvious choice for getting the top lip to separate from the bottom lip. However: Using upper lip raiser to separate the top lip from the bottom lip is bad form.

Why You Should Change What You’re Doing

When zygomaticus major is strong, it separates the lips on its own in a unique way. It stretches the fleshy parts of the lips laterally, while pulling the lip corners diagonally; this is what causes the lips to part. 

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Upper lip raiser is its own thing; it is not a component of the smile. Upper lip raiser is induced by a completely different muscle: levator labii superioris. Upper lip raiser is most notable for its association with emotions such as:

  • disgust
  • anger
  • contempt

Lip corner puller, on the other hand, is most commonly used to paint joy-based expressions.

If you want to create an authentic-looking, joy-based smile: DO NOT ADD UPPER LIP RAISER.

I understand there are tool limitations and an urgency to reduce the number of shapes in a given expression library; however, smiles are too critical in our communication to be reduced to hacks. Smiles should be given priority over other shapes. Rather than using upper lip raiser or other shapes to open the lips for smile, it is important to consider creating a new shape** for an open-mouth smile.

**UPDATE: After writing this post, I had a discussion with Terry Bannon – Facial Animation Supervisor at DNEG – about how to solve the open-mouth smile issue. We concluded a corrective shape for pulling up the top lip (in the manner it would be pulled by a natural smile) would likely be the best solution.

If you are limited in space and cannot create an additional smile shape – smiles are so important, I would suggest compromising upper lip raiser to mimic the movement of the upper lip in smile.

When smiles are combined with upper lip raiser, there is often a subtle touch of malice. Even if the end-shape for an open-mouth smile with upper lip raiser looks fine, the smile will be compromised during the animation when the top lip is pulled up at a steeper, straighter angle rather than the oblique way it is intended to be pulled.  Animation for smiling is especially important. One of the key ways we evaluate a person’s authenticity and trustworthiness is in the trajectory and dynamics of their smile. There are many studies that have revealed how critical dynamics are in expression perception. Appropriate movement is a huge piece of the puzzle.

While the difference between a naturally-formed open-mouth smile and an induced open-mouth smile is subtle, the nuances of the face are what make our expressions so rich and complex. Every detail has meaning. Every detail matters. These concepts transcend stylization and apply whether your art is simple or complex.

 (Exception: If you’ve created a character you wish to be perceived as sinister, using an upper lip raiser-based smile would be fine and appropriate.)

Observing the Differences

Please observe the following side-by-side photos and watch the included videos (both realtime and slow motion) to identify the differences in natural open-mouth smiles and induced open-mouth smiles (with upper lip raiser). 

NOTE: Many of the examples with upper lip raiser also include lower lip depressor. The same concept for upper lip raiser and smile applies to lower lip depressor as well.

When observing the differences, you will notice the following: 

  • Upper lip raiser smiles have . . .
    • narrower mouth widths
    • higher pull in the center of the top lip 
      • exposing more gum and tops of teeth
    • straighter upward pull of the top lip
      • more evident in animation
    • depending on the strength of the smile vs. upper lip raiser – the area under the top lip may be rounded (if upper lip raiser is especially strong)
      • This happens in Fig. 1 but not Fig. 2
    • downward-sloping lip corners
      • because the pull is concentrated in the center of the lip, there will be a disportionate raise in the center of the lips, leaving the areas of the upper lip near the lip corners behind)
  • Natural, zygomaticus major-based open-mouth smiles have . . .
    • wider mouth widths
      • Both Fig. 1 and Fig. 2 show teeth further back in the mouth.
    • more stretching in the center of the fleshy areas of the lips
    • higher angled lip corners 
      • because the sloping effect from upper lip raiser is absent
    • straighter or downward curve in the center of the area under the upper lip 
      • See Fig. 1
    • More exposure of inside of mouth around the lip corners
      • There is a certain unlocking mechanism you can notice in the lip corners when the lips in a smile part naturally (from zygomaticus major). The unlocking mechanism appears to curl out the lip corners to expose more of the inside flesh of the mouth. The unlocked lip corners are evident in both Fig.1 and Fig. 2.
        • NOTE: I have not seen this detail discussed anywhere. I first noticed it when working on expression shapes with character artist, Giovanni Nakpil. We have coined this characteristic the “lip corner unlocking mechanism.” If you are aware of a text that has already defined this feature, please contact me with your source, and I will edit this post to reflect the original terminology.
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Fig. 1 (L: natural smile, R: upper lip raiser smile)
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Fig. 2
Video above: Smile starting in realtime, freezes on open-mouth smile, then resumes. Shows open-mouth smile when the lips part naturally from the pull of zygomaticus major, as opposed to parting from “upper lip raiser” and/or “lower lip depressor.”
Video above: Slow motion capture of open-mouth smile caused by addition of “upper lip raiser” and “lower lip depressor.” Watch with sound on. “Upper lip raiser” appears at the first “bboooiiing” sound effect, and “lower lip depressor” appears at the second “bboooiiing” sound effect.