  We present a novel framework for finding complex activities matching
user-described queries in cluttered surveillance videos. The wide diversity of
queries coupled with unavailability of annotated activity data limits our
ability to train activity models. To bridge the semantic gap we propose to let
users describe an activity as a semantic graph with object attributes and
inter-object relationships associated with nodes and edges, respectively. We
learn node/edge-level visual predictors during training and, at test-time,
propose to retrieve activity by identifying likely locations that match the
semantic graph. We formulate a novel CRF based probabilistic activity
localization objective that accounts for mis-detections, mis-classifications
and track-losses, and outputs a likelihood score for a candidate grounded
location of the query in the video. We seek groundings that maximize overall
precision and recall. To handle the combinatorial search over all
high-probability groundings, we propose a highest precision subgraph matching
algorithm. Our method outperforms existing retrieval methods on benchmarked
datasets.
