  The performance of face recognition systems can be negatively impacted in the
presence of masks and other types of facial coverings that have become
prevalent due to the COVID-19 pandemic. In such cases, the periocular region of
the human face becomes an important biometric cue. In this article, we present
a detailed review of periocular biometrics. We first examine the various face
and periocular techniques specially designed to recognize humans wearing a face
mask. Then, we review different aspects of periocular biometrics: (a) the
anatomical cues present in the periocular region useful for recognition, (b)
the various feature extraction and matching techniques developed, (c)
recognition across different spectra, (d) fusion with other biometric
modalities (face or iris), (e) recognition on mobile devices, (f) its
usefulness in other applications, (g) periocular datasets, and (h) competitions
organized for evaluating the efficacy of this biometric modality. Finally, we
discuss various challenges and future directions in the field of periocular
biometrics.
