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

Action-Action Context

Context

What other people are

doing ?

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 Descriptorτ

action

τ

z

+action

Focal person Context

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

Action Retrieval

Rank according to the relevance to falls

Query : fall

Learning

• Input: document-rank pair (xi,yi)

• Optimization

Joachims, KDD 06

Ranking SVM

• Ranking function h(x)

h(x)

Rank r1Rank r2Rank r3

Action Retrieval - training

irrelevant

very relevant

relevant

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

• Nursing Home DatasetDataset

Dataset• Collective Activity Dataset

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

Retrieval Results

Nursing Home Dataset

Retrieval Results

Collective Activity Dataset

Retrieval Results

Collective Activity Dataset

Retrieval Results

Collective Activity Dataset

1 2

3 4

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Action Classification

[10] Choi et al. in VS. 09

Collective Activity Dataset

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

Thank you!

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