I. Introduction
Facial expressions are an important means for humans to communicate information. In 1872, Darwin proposed that individuals of different nationalities and races exhibit similar facial expressions when experiencing the same emotional state [1]. Based on this, in 1967, psychologist Mehrabian and others suggested that 7% of human emotional expression is conveyed through language, 38% through tone of voice, and 55% through facial expressions [2]. Therefore, research on facial expression recognition is crucial for enabling machines to “understand” human emotions, comprehend human emotional needs, achieve more advanced artificial intelligence, and facilitate more intelligent human-machine interaction. As a result, facial expression recognition technology has become an important research field in artificial intelligence.