Action Units
Facial Action Coding System (FACS) is a system to taxonomize human facial movements by their appearance on the face. Movements of individual facial muscles are encoded by FACS from slight different instant changes in facial appearance. Using FACS it is possible to code nearly any anatomically possible facial expression, deconstructing it into the specific Action Units (AU) that produced the expression. It is a common standard to objectively describe facial expressions.
OpenFace is able to recognize a subset of AUs, specifically: 1, 2, 4, 5, 6, 7, 9, 10, 12, 14, 15, 17, 20, 23, 25, 26, 28, and 45.
You can find more details about FACS and AUs here and here
AUs can be described in two ways
- Presence - if AU is visible in the face (for example AU01_c
- Intensity - how intense is the AU (minimal to maximal) on a 5 point scale
OpenFace provides both of these scores. For presence of AU 1 the column AU01_c in the output file would encode 0 as not present and 1 as present. For intensity of AU 1 the column AU01_r in the output file would range from 0 (not present), 1 (present at minimum intensity), 5 (present at maximum intensity), with continuous values in between.
OpenFace is able to extract Action Units both from images, image sequences and videos
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Static vs dynamic
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How they are trained and where the instructions for training are
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Datasets used for training