Facial Representation for Automatic Facial Action Unit Analysis System

Facial Action Coding System (FACS) is the most influential sign judgment method for facial behavior, and it is a comprehensive and anatomical system which could encode various facial movements by the combination of basic AUs (Action Units). However, automatic facial AUs recognition remains challenging due to several open questions such as rigid and non-rigid facial motions, multiple AUs detection, intensity estimation and naturalistic context application. This paper introduces recent advances in automatic facial action unit recognition, focusing on the fundamental components of face detection and registration, facial action representation, feature selection and classification. We discussed the role of each component, main techniques with their characteristics, challenges and potential directions of facial action unit analysis.