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Facial Animation and Expression 173

10.4 Analysis of Facial Expressions

Ekman and Friesen (1982) developed a commonly used facial measurement system called
FACS. The system measures the face itself as opposed to trying to infer the underlying
emotion given a particular facial configuration. This is a comprehensive system that distin-
guishes all possible visually distinguishable facial movements. Every such facial movement
is the result of muscle action (see figure 10.7 and table 10.3). The earliest work in this area
dates back to Duchenne (1806–1875), one of the first anatomists to explore how facial mus-
cles change the appearance of the face (Duchenne, 1990). Based on a deep understanding of
how muscle contraction changes visible appearance, it is possible to decompose any facial
movement into anatomically minimal action units. FACS has defined 33 distinct action
units for the human face, many of which use a single muscle. It is possible for up to two or

Corrugator
Epicranius major
Corrugator
supercilli
Levator labii
Obicularis oculi major superioris
alaeque nasi
Levator labii superioris
major
Zygomatic minor
Zygomatic major
Zygomatic minor
Levator labii superioris
Zygomatic major Levator anguli oris
Masseter
Risorius
Buccinator
Depressor anguli oris
Mentalis
Depressor labii inferioris
Obicularis oris
Platysma
Temporalis
Frontalis
Corrugator Epicranius
Depressor superclii
Orbicularis oculi
Nasalism.
Levator labii superioris
alaeque nasi
Levator labii
Zygomatic minor Semispinalis
Obicularis oris major Stemocleidomastoid
Zygomatic major
Risoris
Depressor labii inferioris m. Spenius capitis
Mentalis
Trapezius
Depressor anguli oris
Platysma
Figure 10.7
A schematic of the muscles of the face. Front and side views from Parke and Waters (1996).