Page 186 - Designing Sociable Robots

breazeal-79017 book March 18, 2002 14:11

Facial Animation and Expression 167

There are nine basis (or prototype) postures that collectively span this space of emotive
expressions. Although some of these postures adjust specific facial features more strongly
than the others, each prototype influences most if not all of the facial features to some degree.
Forinstance,thevalenceprototypeshavethestrongestinfluenceonlipcurvature,butcanalso
adjustthepositionsoftheears,eyelids,eyebrows,andjaw.Thebasissetoffacialpostureshas
been designed so that a specific location in affect space specifies the relative contributions of
the prototype postures in order to produce a net facial expression that faithfully corresponds
to the active emotion. With this scheme, Kismet displays expressions that intuitively map
to the human emotions of anger, disgust, fear, happiness, sorrow, and surprise. Different
levels of arousal can be expressed as well from interest, to calm, to weariness.
There are several advantages to generating the robot’s facial expression from this affect
space. First, this technique allows the robot’s facial expression to reflect the nuance of the
underlying assessment. Even through there is a discrete number of emotion processes,
the expressive behavior spans a continuous space. Second, it lends clarity to the facial
expression since the robot can only be in a single affective state at a time (by choice) and
hence can only express a single state at a time. Third, the robot’s internal dynamics are
designed to promote smooth trajectories through affect space. This gives the observer a lot
of information about how the robot’s affective state is changing, which makes the robot’s
facial behavior more interesting. Furthermore, by having the face mirror this trajectory, the
observer has immediate feedback as to how their behavior is influencing the robot’s internal
state. For instance, if the robot has a distressed expression upon its face, it may prompt
the observer to speak in a soothing manner to Kismet. The soothing speech is assimilated
into the emotion system where it causes a smooth decrease in the arousal dimension and a
push toward slightly positive valence. Thus, as the person speaks in a comforting manner,
it is possible to witness a smooth transition to a subdued expression. However, if the face
appeared to grow more aroused, then the person may stop trying to comfort the robot
verbally and perhaps try to please the robot by showing it a colorful toy.
The six primary prototype postures sit at the extremes of each dimension (see figure 10.5).
They correspond to high arousal, low arousal, negative valence, positive valence, open
(approaching) stance, and closed (withdrawing) stance. The high arousal prototype, P high ,
maps to the expression for surprise. The low arousal prototype, P low , corresponds to the
expression for fatigue (note that sleep is a behavioral response, so it is covered in the facial
display subsystem). The positive valence prototype, P positive , maps to a content expression.
The negative valence prototype, P negative , resembles an unhappy expression. The closed
stance prototype, P closed , resembles a stern expression, and the open stance prototype, P open ,
resembles an accepting expression.
The three affect dimensions also map to affective postures. There are six basis postures
defined which span the space. High arousal corresponds to an erect posture with a slight