  Most state-of-the-art methods for action recognition rely only on 2D spatial
features encoding appearance, motion or pose. However, 2D data lacks the depth
information, which is crucial for recognizing fine-grained actions. In this
paper, we propose a depth-aware volumetric descriptor that encodes pose and
motion information in a unified representation for action classification
in-the-wild. Our framework is robust to many challenges inherent to action
recognition, e.g. variation in viewpoint, scene, clothing and body shape. The
key component of our method is the Depth-Aware Pose Motion representation
(DA-PoTion), a new video descriptor that encodes the 3D movement of semantic
keypoints of the human body. Given a video, we produce human joint heatmaps for
each frame using a state-of-the-art 3D human pose regressor and we give each of
them a unique color code according to the relative time in the clip. Then, we
aggregate such 3D time-encoded heatmaps for all human joints to obtain a
fixed-size descriptor (DA-PoTion), which is suitable for classifying actions
using a shallow 3D convolutional neural network (CNN). The DA-PoTion alone
defines a new state-of-the-art on the Penn Action Dataset. Moreover, we
leverage the intrinsic complementarity of our pose motion descriptor with
appearance based approaches by combining it with Inflated 3D ConvNet (I3D) to
define a new state-of-the-art on the JHMDB Dataset.
