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Retrieving Actions in Group Contexts
Tian Lan, Yang Wang, Greg Mori, Stephen Robinovitch Simon Fraser University
Sept. 11, 2010
Outline
• Action Retrieval as Ranking
• Results and Future Work
• Contextual Representation of Actions
Nursing Home
• Fall analysis in nursing home surveillance videos– a system automatically rank the videos according
to the relevance to fall action is expected
Actions in Group Context
• Motivation– human actions are rarely performed in isolation,
the actions of individuals in a group can serve as context for each other.
• Goal– explore the benefit of contextual information in
action retrieval in challenging real-world applications
Action Context Descriptor
Feature Descriptor
Multi-class SVM
action class
scor
e
action class
scor
e
…action class
scor
e
max
action classsc
ore
e.g. HOG by Dalal & Triggs
Outline
• Action Retrieval as Ranking
• Results and Future Work
• Contextual Representation of Actions
Classification or Retrieval
• Previous Work–Most work in human action understanding
focuses on action classification.
Classification or Retrieval • Most surveillance tasks are typical retrieval
tasks– retrieve a small video segment contains a
particular action from thousands of hours of videos.
• The “action of interest” is rare event– Extremely imbalanced classes
Outline
• Action Retrieval as Ranking
• Results and Future Work
• Contextual Representation of Actions
Dataset
• Nursing Home Dataset • 5 action categories: walking, standing, sitting, bending
and falling. (per person)• 18 video clips.• Query: fall
• Collective Activity Dataset (Choi et al. VS. 09)
• 5 action categories: crossing, waiting, queuing, walking, talking. (per person)
• 44 video clips.• Query: each of the five actions
System Overview
Person
DetectorPerson
DescriptorVideo
u
v
RankSVM
• Pedestrian Detection by Felzenszwalb et al.• Background Subtraction
• HOG by Dalal & Triggs • LST by Loy et al. at cvpr 09
Baselines
• Context vs No Context– Action Context Descriptor– Original feature descriptors, e.g. HOG (Dalal & Triggs at CVPR
05), LST (Loy et al. at CVPR 09)
• RankSVM vs SVM
• Methods– Context + RankSVM (our method)– Context + SVM– No Context + RankSVM– No Context + SVM
Conclusion
• A new contextual feature descriptor to represent actions– action context (AC) descriptor
• Formulate our problem as a retrieval task.
Future Work
• Contextual Feature Descriptors– How to only encode useful context?
• Rank-SVM loss, optimize the NDCG score