Facial Action Coding System

Facial Action Coding System (FACS) is a comprehensive, anatomically based system for describing all observable facial movement. It was developed by psychologists Paul Ekman and Wallace V. Friesen in the 1970s. The system is used to identify and classify physical expressions of human emotion, and it has applications in a wide range of fields, including psychology, cognitive science, and animation.

Overview

The Facial Action Coding System breaks down facial expressions into individual components of muscle movement, called "Action Units" or "AUs". Each AU represents a change in appearance in a particular part of the face. When combined, these AUs can represent a wide range of emotions. FACS is a standardized system, which means that it provides a method for coding facial expressions that is consistent across different observers.

Action Units

The system includes a total of 46 action units (AUs), although not all AUs are used to code for all expressions. Some of the most commonly referenced AUs include:

• AU1: Inner Brow Raiser
• AU2: Outer Brow Raiser
• AU4: Brow Lowerer
• AU5: Upper Lid Raiser
• AU6: Cheek Raiser and Lid Compressor
• AU12: Lip Corner Puller
• AU15: Lip Corner Depressor
• AU17: Chin Raiser
• AU20: Lip Stretcher
• AU25: Lips Part

Each of these AUs can be combined in various ways to form expressions. For example, happiness might be coded as AU6 + AU12, indicating a cheek raise and lip corner puller.

Applications

FACS has been applied in various fields, including psychology, for understanding the link between facial expression and emotion; in law enforcement, for the interpretation of suspect behaviors; in animation and CGI, for creating realistic facial expressions in characters; and in human-computer interaction, to develop systems that can recognize and interpret human emotions.

Training and Certification

Learning to use the Facial Action Coding System requires extensive training and practice. There are official FACS training programs available that teach individuals how to accurately code facial expressions. Certification is also available for those who demonstrate proficiency in using the system.

Limitations

While FACS is a powerful tool for analyzing facial expressions, it is not without its limitations. The system requires extensive training to use effectively, and even experienced coders may have difficulty interpreting subtle expressions. Additionally, FACS is primarily focused on the physical expression of emotion and does not account for the contextual factors that can influence emotional expression.

• 
  Side views of the muscles of facial expressions
• 
  Facial Action Coding System