  Most state-of-the-art approaches for Facial Action Unit (AU) detection rely
upon evaluating facial expressions from static frames, encoding a snapshot of
heightened facial activity. In real-world interactions, however, facial
expressions are usually more subtle and evolve in a temporal manner requiring
AU detection models to learn spatial as well as temporal information. In this
paper, we focus on both spatial and spatio-temporal features encoding the
temporal evolution of facial AU activation. For this purpose, we propose the
Action Unit Lifecycle-Aware Capsule Network (AULA-Caps) that performs AU
detection using both frame and sequence-level features. While at the
frame-level the capsule layers of AULA-Caps learn spatial feature primitives to
determine AU activations, at the sequence-level, it learns temporal
dependencies between contiguous frames by focusing on relevant spatio-temporal
segments in the sequence. The learnt feature capsules are routed together such
that the model learns to selectively focus more on spatial or spatio-temporal
information depending upon the AU lifecycle. The proposed model is evaluated on
the commonly used BP4D and GFT benchmark datasets obtaining state-of-the-art
results on both the datasets.
