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VASC Seminar: Katerina Fragkiadaki
Multi-Granularity Steering for Human Actions: Motion, Pose and Intention

Katerina Fragkiadaki
PhD Candidate, University of Pennsylvania

May 06, 2013, 3pm - 4pm, NSH 1507

Tracking people and their body pose in videos is a central problem in computer vision. Standard tracking representations typically reason about temporal coherence of detected bodies and parts. They have difficulty tracking people under partial occlusions or wild body deformations, where people and body pose detectors are often inaccurate, due to the small number of training examples in comparison to the exponential variability of such configurations.

In this talk, I will present novel tracking representations that allow to track people and their body pose by exploiting information at multiple granularities when available, whole body, parts or pixel-wise motion correspondences and their segmentations. A key challenge is resolving contradictions among different information granularities, such as detections and motion estimates in the case of false alarm detections or leaking motion affinities. I will introduce graph steering, a framework that specifically targets inference under potentially sparse unary detection potentials and dense pairwise motion affinities - a particular characteristic of the video signal - in contrast to standard MRFs.

We will present three instances of steering. First, we study people detection and tracking under persistent occlusions. I will demonstrate how to steer dense optical flow trajectory affinities with repulsions from sparse confident detections to reach a global consensus of detection and tracking in crowded scenes. Second, we study human motion and pose estimation. We segment hard to detect, fast moving body limbs from their surrounding clutter and match them against pose exemplars to detect body pose and improve body part motion estimates with kinematic constraints. Finally, I will show how we can learn certainty of detections under various pose and motion specific contexts, and use such certainty during steering for jointly inferring multi-frame body pose and video segmentation.

We show empirically that such multi-granularity tracking representation is worthwhile, obtaining significantly more accurate body and pose tracking in popular datasets.

Additional Information

Host: Martial Hebert

Appointments: Bernardo Pires (

Speaker Biography

Katerina Fragkiadaki is a Ph.D. student in Computer and Information Science in the University of Pennsylvania. She received her diplomat in Computer Engineering from the National Technical University of Athens. She works on tracking, segmentation and pose estimation of people under close interactions, for understanding their actions and intentions. She also works on segmenting and tracking cell populations for understanding and modeling cell behavior.