“…The research area of facial behaviour analysis includes the problems of: i) the recognition of the so-called six universal expressions (i.e., Anger, Disgust, Fear, Happy, Sad, Surprise), plus Neutral, influenced by the seminal work of Ekman [12], ii) the recognition of spontaneous expressions including mental states (pain intensity [17] and compound expressions [10]), iii) the detection of the facial Action Units (AU) and estimation of their intensity, according to the Facial Action Coding System [11] which provides a standardised taxonomy of facial muscles' movements, iv) the detection of micro-expressions, and v) the estimation of facial affect in a continuous dimensional space (e.g., valence and arousal). Related research can assist in flagging complex behavioral patterns such as deception, depression, autism, spectrum disorders and schizophrenia [1], [20], [27], [44], [56], [57].…”