ARKit to FACS Cheat Sheet
- Melinda Ozel
- Published: May 1, 2022
- Updated: Feb. 16, 2025
Welcome to the ARKit to FACS Cheat Sheet! Here you will find breakdowns of how to translate ARKit face shapes into their Facial Action Coding System (FACS) equivalents. Due to the difficulties in distinguishing similar FACS shapes as well as the lack of clear explanations in the Apple’s devkit, there are many mistranslations of ARKit-to-FACS out there. Beware. This guide is meant to clear up confusion and undo common misconceptions!
Scroll down to jump right into the Cheat Sheet.
Difficulties With AR Development Toolkits
If you or your team are using open-source face tracking kits to:
- animate faces
- overlay virtual content
- create expression-based events
. . . figuring out what’s what can be challenging – especially if you or your team do not have a strong background in:
- facial expressions
- face tracking
- the Facial Action Coding System (FACS)
Vaguely-defined Terms
Face tracking software development kits (SDKs) can be confusing for a number of reasons. A large contributor to this confusion is the lack of detailed documentation available for users.
Human facial expressions are complex and rich with nuance. Understanding how to identify and break down expressions is not always simple or intuitive. Despite this reality, face tracking kits often only provide minimalist definitions for the expression shapes in their libraries.
Minimally-defined expression shapes:
- create room for user misinterpretation.
- increase user’s likelihood to confuse similar-looking shapes.
- limit the user’s potential to effectively use the product.
Getting Around the Ambiguity
Expression shapes in most face tracking products (despite their names) are primarily FACS-based. For those who may be skeptical: FACS is derived from anatomy; so unless a face kit has completely annihilated the foundations of human facial anatomy, all shapes will have FACS equivalents.
If you wish to foster a better understanding of the face tracking products you are using, you should familiarize yourself with FACS.
FACS naming is standardized. FACS is consistent. Each FACS shape has a detailed, well-defined, and heavily researched description. If you are well-versed in FACS, you can equip yourself with the tools you need to compensate for the ambiguity of most expression libraries.
Whether or not you are FACS-savvy, if you want a clearer breakdown of ARKit facial expression shapes, this is the document for you 🙂
The Guide
QUICK TABLE
ARKit label
corresponding FACS name(s)
corresponding muscle(s)
reference
browInnerUp
AU 1 – inner brow raiser
frontalis, medial portion
browOuter (left & right)
AU2 – outer brow raiser
frontalis, lateral portion
browDown (left & right)
AU4 – brow lowerer
corrrugator supercilii, depressor supericilii, procerus
eyeWide (left & right)
AU5 – upper lid raiser
levator palpebrae superioris
cheekSquint (left & right)
AU6 – cheek raiser
orbicularis oculi, orbital portion
eyeSquint (left & right)
AU7 – lid tightener
orbicularis oculi, palpebral portion
eyeBlink (left & right)
AU45 – blink
relaxation of levator palpebrae superioris & contraction of the palpebral portion of orbicularis oculi
eyeLookUp (left & right)
M63 – eyes up
superior rectus & inferior oblique**
eyeLookDown (left & right)
M64 – eyes down
inferior rectus & superior oblique**
eyeLookIn (left & right)
AU66 – crosseye (when applied to both eyes at once); otherwise, eyeLookInLeft must be paired with eyeLookOutRight for rightward gaze (FACS M62), while eyeLookInRight must be paired with eyeLookOutLeft for leftward gaze (FACS M61)
medial rectus**
crosseye
rightward gaze
eyeLookOut (left & right)
AU65 – walleye (when applied to both eyes at once); see above for leftward vs. rightward gaze
lateral rectus**
walleye
NOTE: I couldn’t do walleye for real, so I Photoshopped my screen-right eye. Don’t use this one as any sort of legit reference lol. (You can see the inner eye orbit unnaturally shift.)
leftward gaze
noseSneer (left & right)
AU9 – nose wrinkler
levator labii superioris alaeque nasi
mouthUpper (left & right)
AU10 – upper lip raiser
levator labii superioris
mouthSmile (left & right)
AU12 – lip corner puller
zygomaticus major
mouthDimple (left & right)
AU14 – dimpler
buccinator
mouthFrown (left & right)
AU15 – lip corner depressor
depressor aguli oris
mouthLowerDown (left & right)
AU16 – lower lip depressor
depressor labii inferioris
mouthShrugUpper
AU17 – chin raiser – upper split
secondary effect from the mentalis
CRITICAL NOTE: mouthShrugUpper is **CONTINGENT ON mouthShrugLower and is only applicable when the lips are already touching or WILL BE touching due to the intensity of mouthShrugLower.
I have confirmed this distinction with former members of FaceShift (the company who created the original face shapes that ARKit is derived from before Apple bought them out).
If you are using the ICT-FaceKit, they have misappropriated mouthShrugUpper as “upper lip raiser” and have mistranslated the actual “upper lip raiser” shapes, mouthUpperUp_L & mouthUpperUp_R as “nasolabial furrow deepener.”
mouthShrugLower
AU17 – chin raiser – lower split
mentalis
SEE NOTE ABOVE in mouthShrugUpper section.
cheekPuff
AD34 – puff
buccinator
mouthPucker
AU18 – lip pucker
incisivus labii superioris, incsivus labii inferioris
mouthStretch (left & right)
AU20 – lip stretcher
risorius
mouthFunnel
AU22 – lip funneler
orbicularis oris
mouthPress (left & right)
AU24 – lip presser
orbicularis oris
mouthClose
AU8, but see notes in Comprehensive Guide.
orbicularis oris, but see notes in Comprehensive Guide.
mouthRoll (upper & lower)
AU28 – lips suck
orbicularis oris
jawOpen
AU26 – jaw drop or AU27 – mouth stretch
(AU26) relaxation of masseter, temporalis, and internal pterygoid muscles or (AU27) lateral pterygoid and the suprahyoid (anterior digastric, geniohyoid, and mylohyoid) muscles
jaw (left & right)
AD30 – jaw sideways
pterygoids and temporalis
jawForward
AD29 – jaw thrust
pterygoids and masseter
** = Indicates an oversimplification of muscular correlates. For example, the muscles related to gaze are much more complex than “superior rectus = eyes look up.” Rather, they work together in a system of relativity. For the purposes of this guide, however, movements are matched with the most relevant and idealistic muscles.
OTHER NOTES
ARKit is missing some important actions for everyday expression and emotion. Missing FACS-official shapes include:
- AU11 – nasolabial furrow deepener – zygomaticus minor
- AU13 – sharp lip puller – levator anguli oris
- AU23 – lip tightener – orbicularis oris
- AU38 – nostril dilator – dilator naris
- AU39 – nostril compressor – depressor septi & transverse nasalis
Other missing shapes:
- vertical lip tightener – orbicularis oris – *See FACS Cheat Sheet for more.
ADDITIONAL RESOURCES
- For more references and information on FACS (Facial Action Coding System), check out the FACS Cheat Sheet.
- Check out my collaboration post with product designer, Pooya Deperson: The Ultimate Guide to Creating ARKit’s 52 Facial Blendshapes! This guide (major thanks to Pooya) is filled with visuals that clearly illustrate the function of ARKit’s facial blendshapes.
