  The automatic recognition of facial expressions has been an active research
topic since the early nineties. There have been several advances in the past
few years in terms of face detection and tracking, feature extraction
mechanisms and the techniques used for expression classification. This paper
surveys some of the published work since 2001 till date. The paper presents a
time-line view of the advances made in this field, the applications of
automatic face expression recognizers, the characteristics of an ideal system,
the databases that have been used and the advances made in terms of their
standardization and a detailed summary of the state of the art. The paper also
discusses facial parameterization using FACS Action Units (AUs) and MPEG-4
Facial Animation Parameters (FAPs) and the recent advances in face detection,
tracking and feature extraction methods. Notes have also been presented on
emotions, expressions and facial features, discussion on the six prototypic
expressions and the recent studies on expression classifiers. The paper ends
with a note on the challenges and the future work. This paper has been written
in a tutorial style with the intention of helping students and researchers who
are new to this field.
