Over the last two decades, face alignment or localizing fiducial facial
points has received increasing attention owing to its comprehensive
applications in automatic face analysis. However, such a task has proven
extremely challenging in unconstrained environments due to many confounding
factors, such as pose, occlusions, expression and illumination. While numerous
techniques have been developed to address these challenges, this problem is
still far away from being solved. In this survey, we present an up-to-date
critical review of the existing literatures on face alignment, focusing on
those methods addressing overall difficulties and challenges of this topic
under uncontrolled conditions. Specifically, we categorize existing face
alignment techniques, present detailed descriptions of the prominent algorithms
within each category, and discuss their advantages and disadvantages.
Furthermore, we organize special discussions on the practical aspects of face
alignment in-the-wild, towards the development of a robust face alignment
system. In addition, we show performance statistics of the state of the art,
and conclude this paper with several promising directions for future research.

本研究综述了人脸对齐技术在非受控环境下所遇到的挑战、方法和性能表现，并提出了未来研究的方向。