A Stochastic Attribute Grammar for Robust Cross-View Human Tracking

In computer vision, tracking humans across camera views remain challenging, especially for complex scenarios with frequent occlusions, significant lighting changes, and other difficulties. Under such conditions, most existing appearance and geometric cues are not reliable enough to distinguish humans across camera views. To address these challenges, this paper presents a stochastic attribute grammar model for leveraging complementary and discriminative human attributes for enhancing cross-view tracking. The key idea of our method is to introduce a hierarchical representation, parse graph, to describe a subject and its movement trajectory in both space and time domains. These results in a hierarchical compositional representation, comprising trajectory entities of varying level, including human boxes, 3D human boxes, tracklets, and trajectories. We use a set of grammar rules to decompose a graph node (e.g., tracklet) into a set of children nodes (e.g., 3D human boxes), and augment each node with a set of attributes, including geometry (e.g., moving speed and direction), accessories (e.g., bags), and/or activities (e.g., walking and running). These attributes serve as valuable cues, in addition to appearance features (e.g., colors), in determining the associations of human detection boxes across cameras. In particular, the attributes of a parent node are inherited by its children nodes, resulting in consistency constraints over the feasible parse graph. Thus, we cast cross-view human tracking as finding the most discriminative parse graph for each subject in videos. We develop a learning method to train this attribute grammar model from weakly supervised training data. To infer the optimal parse graph and its attributes, we develop an alternative parsing method that employs both top-down and bottom-up computations to search the optimal solution. We also explicitly reason the occlusion status of each entity in order to deal with significant changes of camera viewpoints. We evaluate the proposed method over public video benchmarks, and demonstrate with extensive experiments that our method clearly outperforms the state-of-the-art tracking methods.