1 Introduction
With rapid advancements in technology, advanced and user-friendly human-computer interaction (HCI) systems should become capable of considering human affective states in interactions to promote mutual sympathy between humans and machines. In human communication, the expression and understanding of emotions can help to achieve mutual sympathy. To develop an emotion-based HCI, we need to equip machines with the abilities to understand and identify human emotions without any input devices to translate users’ intentions. However, emotion is a psychological and physiological expression and is associated with mood, temperament, personality, disposition, and motivation, which are produced by cognitive processes, subjective feelings, motivational tendencies, behavioral reactions, and physiological arousal. Therefore, developing an effective emotion recognition system for identifying various emotions is an interesting and challenging topic.