The locations of the fiducial facial landmark points around facial components
and facial contour capture the rigid and non-rigid facial deformations due to
head movements and facial expressions. They are hence important for various
facial analysis tasks. Many facial landmark detection algorithms have been
developed to automatically detect those key points over the years, and in this
paper, we perform an extensive review of them. We classify the facial landmark
detection algorithms into three major categories: holistic methods, Constrained
Local Model (CLM) methods, and the regression-based methods. They differ in the
ways to utilize the facial appearance and shape information. The holistic
methods explicitly build models to represent the global facial appearance and
shape information. The CLMs explicitly leverage the global shape model but
build the local appearance models. The regression-based methods implicitly
capture facial shape and appearance information. For algorithms within each
category, we discuss their underlying theories as well as their differences. We
also compare their performances on both controlled and in the wild benchmark
datasets, under varying facial expressions, head poses, and occlusion. Based on
the evaluations, we point out their respective strengths and weaknesses. There
is also a separate section to review the latest deep learning-based algorithms.
The survey also includes a listing of the benchmark databases and existing
software. Finally, we identify future research directions, including combining
methods in different categories to leverage their respective strengths to solve
landmark detection "in-the-wild".

综述了面部关键点检测算法的发展历程，将其分为三种不同的分类方法：全局方法、约束局部模型方法和回归方法，并对其进行了分类讨论，比较了它们在受不同因素影响下的性能表现。此外，还列举了面部关键点检测的基准数据库和现有软件，并指出了未来的研究方向。