A unified tree-based framework for joint action localization, recognition and segmentation

A unified tree-based framework for joint action localization, recognition and segmentation is proposed. An action is represented as a sequence of joint hog-flow descriptors extracted independently from each frame. During training, a set of action prototypes is first learned based on k-means clustering, and then a binary tree model is constructed from the set of action prototypes based on hierarchical k-means clustering. Each tree node is characterized by a hog-flow descriptor and a rejection threshold, and an initial action segmentation mask is defined for leaf nodes (corresponding to a prototype). During testing, an action is localized by mapping each test frame to its nearest neighbor prototype using a fast tree search method, followed by local search based tracking and global filtering based location refinement. An action is recognized by maximizing the sum of the joint probabilities of the action category and action prototype given an input sequence. An action pose from a test frame can be segmented by GrabCut algorithm using the initial segmentation mask from the matched leaf node as the user labeling. Our approach does not rely on background subtraction, and enables action localization and recognition in realistic and challenging conditions (such as crowded backgrounds). Experimental results show that our approach achieves start-of-art performances on the Weizmann dataset, CMU action dataset and UCF sports action dataset.

► We adopt a HOG-based shape feature to encode shape without background subtraction.
► We introduce a tree-based approach for multiclass action localization and recognition.
► We propose a probabilistic framework to determine action labels and action prototypes.
► We propose a silhouette-based mask as a prior for GrabCut-based action segmentation.