  The goal of human action recognition is to temporally or spatially localize
the human action of interest in video sequences. Temporal localization (i.e.
indicating the start and end frames of the action in a video) is referred to as
frame-level detection. Spatial localization, which is more challenging, means
to identify the pixels within each action frame that correspond to the action.
This setting is usually referred to as pixel-level detection. In this chapter,
we are using action, activity, event interchangeably.
