An Unsupervised Approach to Anonymous Crowd Monitoring

Abstract

With over 4.2 million CCTV cameras in the UK alone, it would be useful have to an automated system to monitor over wide, open spaces. In such areas, we can observe emergent behaviour in crowd movements, often changing dynamicall over time as crowds form and disperse. Trained operators can often notice trouble the moment, if not before, it happens. Unfortunately, the high number of cameras watching over the public has generated a feeling of unease within the populus as people feel increasingly that their privacy is being invaded. We propose a system that, using an offline, unsupervised learning process, will anonymously detect patterns of motion within a scene and describe it as usual or unusual. The system is trained on footage of the scene, recorded using a single camera. Flow is detected using the KLT tracker [3] to accumulate, at chosen granularity, 'track-lets' [1] of elemental motion. These tracks are quantised and their distribution in spatial and temporal windows around each pixel clustered to generate acceptable patterns. In scenes with changing behaviour, there may be several candidates at each position. During testing, similarly generated patterns are tested for plausibility by proximity to acceptable clusters. Early results show promise and may be tuned via various parameters of the system.

Full text

An Unsupervised Approach to Anonymous
Crowd Monitoring
Ian Hales, Roger Boyle, Kia Ng
School of Computing,
University of Leeds, LS2 1HE
{i.j.hales06, r.d.boyle, k.c.ng}@leeds.ac.uk
March 29, 2010
1 Abstract
With over 4.2 million CCTV cameras in the UK alone [2], it would be useful have
to an automated system to monitor over wide, open spaces. In such areas, we can
observe emergent behaviour in crowd movements, often changing dynamicall over time
as crowds form and disperse. Trained operators can often notice trouble the moment,
if not before, it happens. Unfortunately, the high number of cameras watching over the
public has generated a feeling of unease within the populus as people feel increasingly
that their privacy is being invaded.
We propose a system that, using an offline, unsupervised learning process, will
anonymously detect patterns of motion within a scene and describe it as usual or un-
usual. The system is trained on footage of the scene, recorded using a single camera.
Flow is detected using the KLT tracker [3] to accumulate, at chosen granularity, ‘track-
lets’ [1] of elemental motion.
These tracks are quantised and their distribution in spatial and temporal windows
around each pixel clustered to generate acceptable patterns. In scenes with changing
behaviour, there may be several candidates at each postion. During testing, similarly
generated patterns are tested for plausibility by proximity to acceptable clusters.
Early results show promise and may be tuned via various parameters of the system.
References
[1] Hannah M. Dee, David C. Hogg, and Anthony G. Cohn. Scene modelling and
classification using learned spatial relations. In Kathleen S. Hornsby, Christophe
Claramunt, Michel Denis, G´
erard Ligozat, Kathleen S. Hornsby, Christophe Clara-
munt, Michel Denis, and G´
erard Ligozat, editors, COSIT, volume 5756 of Lecture
Notes in Computer Science, pages 295–311. Springer, 2009.
[2] Michael McCahill and Clive Norris. Cctv in london. Report to the European
Commission Fifth Framework RTD as part of UrbanEye: on the threshold of the
urban panopticon, 2002.
[3] J. Shi and C. Tomasi. Good features to track. Proceedings of the Conference on
Computer Vision and Pattern Recognition, pages 593–600, June 1994.
1