Review of face emotion recognition systems using images: techniques, databases, and challenges

Artificial Intelligence (AI) technologies have decreased the gap between humans and machines. Particularly, machine learning algorithms and deep learning algorithms are applied in various applications. One of these applications is facial emotion identification. Facial expressions are considered an important tool between people as a non-verbally communication and showing the people's emotional states. The technique of facial emotion recognition can be presented as an assistant tool and used in several domains such as the healthcare sector, education, and E-commerce. However, the recognition of human emotions by using image samples is still a heavily calculative, challenging, and complex task. Therefore, this paper presents an overview, and techniques used in systems of human facial emotion recognition based on image samples. Furthermore, this paper highlights the drawbacks of these systems, presents different image databases, and discusses the current challenges and key issues in systems of face emotion recognition.