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Version: 2.0

Action units

Action units (AUs) are individual facial musculature arrangements specified in the Facial Action Coding System (FACS), combinations of which can account for all possible facial expressions1. OpenDBM outputs framewise values for AU presence and intensity for the following AUs:

Action unit numberDescription
AU1Inner brow raiser
AU2Outer brow raiser
AU4Brow lowerer
AU5Upper lid raiser
AU6Cheek raiser
AU7Lid tightener
AU9Nose wrinkler
AU12Lip corner puller
AU15Lip corner depressor
AU16Lower lip depressor
AU20Lip stretcher
AU23Lip tightener
AU26Jaw drop

For each of the AUs in the table above, the following raw variables are calculated:

Raw Variables

VariableDescription
fac_auXXpresAction Unit presence, where XX refers to the action unit. This is a binary (1/0) variable, where 1 signifies presence of the action unit and 0 signifies its absence as determined by OpenFace.
fac_auXXintAction Unit intensity, where XX refers to the action unit. This is a continuous (0-5) variable, where 0 signifies no presence and 5 signifies maximum presence, as determined by OpenFace.

Derived Variables

VariableDescription
fac_auXXpres_pctAction Unit presence percentage. Using the binary vector fac_auXXpres, the percentage of video frames where an AU was present is calculated into this variable.
fac_auXXint_meanAction Unit intensity mean. Mean value of fac_auXXint over the course of the video.
fac_auXXint_stdAction Unit intensity standard deviation. Standard deviation of fac_auXXint over the course of the video.

  1. Ekman, R. (1997). What the face reveals: Basic and applied studies of spontaneous expression using the Facial Action Coding System (FACS). Oxford University Press, USA.