Page 176 - Designing Sociable Robots

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10 Facial Animation and Expression

The human face is the most complex and versatile of all species (Darwin, 1872). For humans,
the face is a rich and versatile instrument serving many different functions. It serves as a
window to display one’s own motivational state. This makes one’s behavior more predictable
and understandable to others and improves communication (Ekman et al., 1982). The face
can be used to supplement verbal communication. A quick facial display can reveal the
speaker’s attitude about the information being conveyed. Alternatively, the face can be used
to complement verbal communication, such as lifting of the eyebrows to lend additional
emphasis to a stressed word (Cassell, 1999b). Facial gestures can communicate information
on their own, such as a facial shrug to express “I don’t know” to another’s query. The face can
serve a regulatory function to modulate the pace of verbal exchange by providing turn-taking
cues (Cassell & Thorisson, 1999). The face serves biological functions as well—closing
one’s eyes to protect them from a threatening stimulus and, on a longer time scale, to sleep
(Redican, 1982).

10.1 Design Issues for Facial Animation

Kismet doesn’t engage in adult-level discourse, but its face serves many of these functions
at a simpler, pre-linguistic level. Consequently, the robot’s facial behavior is fairly complex.
It must balance these many functions in a timely, coherent, and appropriate manner. Below,
I outline a set of design issues for the control of Kismet’s face.

Real-time response Kismet’s face must respond at interactive rates. It must respond in a
timely manner to the person who engages it as well to other events in the environment. This
promotes readability of the robot, so the person can reliably connect the facial reaction to
the event that elicited it. Real-time response is particularly important for sending expressive
cues to regulate social dynamics. Excessive latencies disrupt the flow of the interaction.

Coherence Kismet has fifteen facial actuators, many of which are required for any single
emotive expression, behavioral display, or communicative gesture. There must be coher-
ence in how these motor ensembles move together, and how they sequence between other
motor ensembles. Sometimes Kismet’s facial behaviors require moving multiple degrees
of freedom to a fixed posture, sometimes the facial behavior is an animated gesture, and
sometimes it is a combination of both. If the face loses coherence, the information it contains
is lost to the human observer.
Synchrony The face is one expressive modality that must work in concert with vocal
expression and body posture. Requests for these motor modalities can arise from multiple
sources in the synthetic nervous system. Hence, synchrony is an important issue. This is of
particular importance for lip synchronization where the phonemes spoken during a vocal
utterance must be matched by the corresponding lip postures.

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