Facial Action Coding System is the most influential sign judgment method for facial behavior, and it is a comprehensive and anatomical system which could encode various facial movements by the combination of basic AUs (Action Units). AUs define certain facial configurations caused by contraction of one or more facial muscles, and they are independent of interpretation of emotions. However, automatic facial action unit recognition remains challenging due to several open questions such as rigid and non-rigid facial motions, multiple AUs detection, intensity estimation and naturalistic context application. This paper introduces recent advances in automatic facial action unit recognition, focusing on the fundamental components of face detection and registration, facial action representation, feature selection and classification. The comprehensive analysis of facial representations is presented according to the facial data properties (image and video, 2D and 3D) and characteristics of facial features (predesign and learning, appearance and geometry, hybrid and fusion). Facial action unit recognition involves AUs occurrence detection, AUs temporal segment detection and AUs intensity estimation. We discussed the role of each component, main techniques with their characteristics, challenges and potential directions of facial action unit analysis.

中文翻译：

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自动面部动作单元分析综合综述

面部动作编码系统是最有影响力的面部行为符号判断方法，是一个综合的、解剖学的系统，可以通过基本AU（动作单元）的组合对各种面部动作进行编码。AU 定义了由一个或多个面部肌肉收缩引起的某些面部构造，并且它们独立于情绪的解释。然而，由于一些开放性问题，例如刚性和非刚性面部运动、多个 AU 检测、强度估计和自然上下文应用，自动面部动作单元识别仍然具有挑战性。本文介绍了自动面部动作单元识别的最新进展，重点介绍了面部检测和配准、面部动作表示、特征选择和分类的基本组成部分。根据面部数据属性（图像和视频、2D 和 3D）和面部特征的特征（预设计和学习、外观和几何、混合和融合）呈现面部表征的综合分析。面部动作单元识别涉及 AU 发生检测、AU 时间段检测和 AU 强度估计。我们讨论了每个组件的作用、主要技术及其特征、面部动作单元分析的挑战和潜在方向。AU 时间段检测和 AU 强度估计。我们讨论了每个组件的作用、主要技术及其特征、面部动作单元分析的挑战和潜在方向。AU 时间段检测和 AU 强度估计。我们讨论了每个组件的作用、主要技术及其特征、面部动作单元分析的挑战和潜在方向。