Assessing naturalness and emotional intensity: a perceptual study of animated facial motion

Animated characters appear in applications for entertainment, education, and therapy. When these characters display appropriate emotions for their context, they can be particularly effective. Characters can display emotions by accurately mimicking the facial expressions and vocal cues that people display or by damping or exaggerating the emotionality of the expressions. In this work, we explored which of these strategies would be most effective for animated characters. We investigated the effects of altering the auditory and facial levels of expressiveness on emotion recognition accuracy and ratings of perceived emotional intensity and naturalness. We ran an experiment with emotion (angry, happy, sad), auditory emotion level (low, high), and facial motion magnitude (damped, unaltered, exaggerated) as within-subjects factors. Participants evaluated animations of a character whose facial motion matched that of an actress we tracked using an active appearance model. This method of tracking and animation can capture subtle facial motions in real-time, a necessity for many interactive animated characters. We manipulated auditory emotion level by asking the actress to speak sentences at varying levels, and we manipulated facial motion magnitude by exaggerating and damping the actress's spatial motion. We found that the magnitude of auditory expressiveness was positively related to emotion recognition accuracy and ratings of emotional intensity. The magnitude of facial motion was positively related to ratings of emotional intensity but negatively related to ratings of naturalness.