Facial expression of emotions

The particular subset of basic emotions that we have adopted is the one proposed by Ekman [6]--anger, disgust, fear, happiness, sadness, and surprise--with the exception of disgust, since this emotion does not make sense for the kind of interactions that humans can have with Feelix. The reason for choosing this classification is that its main criterion to define emotions as basic is their having distinctive (universal) facial expressions [6].

As for facial primitives to express emotions, we have largely adopted the features concerning eyebrows and lips (the only elements that Feelix can use to express emotions) described in [9]. This coding system is inspired by Ekman's Facial Action Coding System [5].

Concerning an observer's perception of emotional expressions, we have adopted the hypothesis proposed by De Bonis [4] that the upper and lower parts of the face function as the building blocks at the basis of emotion perception, rather than the building blocks of finer granularity postulated by other authors. We have thus taken the most telling feature of each part of the face--eyebrows and lips--to express emotions in Feelix, making the hypothesis that these two features should allow humans to recognize its emotional expressions. Feelix' face is thus closer to a caricature than to a realistic model of a human face. Moreover, also following [4], we have assumed one of the parts of the face to be dominant for some emotions (e.g., the upper part in fear and sadness, the lower part in happiness and disgust); in those cases, the perception of the dominant part expressing a given emotion should be enough for a human observer to recognize Feelix' expression. In other cases, the perception of the expression in both parts of the face is needed for its recognition.