I. Introduction
We humans share emotions through verbal or non-verbal communication. Communication plays an important role in expressing human behavior. Facial expression is an essential part of non-verbal communication. It constitutes an important part of social interaction and human behavior. Automatic facial expression recognition is a challenging task due to complexity and diversity of facial expressions, inter-person facial differences and the variation of performing facial expression. Therefore, recognizing facial emotion expressions has attracted a great deal of interest by researchers in the last two decades. It has applications in the different fields such as Human Computer Interaction, virtual reality, video games, computer graphics, biometrics, psychology, detecting driver fatigue, analyzing customer satisfaction, educational software, lie detection and pain assessment. For the human being, it is easy to recognize facial expressions on the fly, however for computers it is difficult to recognize.