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The first "baseline" system was designed to reproduce
the performance equivalent to that achieved during the ISCAPS project.
The second step added the concept of ownership while the third
augmented the ownership concept with social relations.
The baseline system was implemented using the
functionality previously explored in ISCAPS, where the threat
(i.e. abandonment) was defined by two simple rules:
- • Bag unattended if no person within 2 meters
- • Bag abandoned if unattended for 30 seconds
The performance of the baseline subsystem was
evaluated and the results are shown in the second column of the
table below. The baseline definition fails to raise the correct
alarm in all cases because there is always some individual in
the surveyed scene that is sufficiently close to the baggage such
that the alarm condition is never reached.
The second step involved implementation of an
enhanced threat assessment which included the notion of ownership.
In this case the rules were extended to:
- • Bag owner is nearest person on appearance
- • Bag unattended if owner is not within 2 meters
- • Bag abandoned if unattended for 30 seconds
Thus for the threat assessment to be correct,
the system was required to raise an alarm following a potential
threat in addition to correctly identify both the ID of the
abandoned bag and the ID of the owner. Again the results of the
system evaluation are shown in table below. The Ownership definition
still makes several errors, but performs better than the simple
ISCAPS functionality.
The final step of the development was to augment
the notion of ownership by incorporating inferred information on
social relations into the threat assessment. This was achieved by
running an "inverse-SFM" procedure at each time-step. This enables
the system to recognise situations in which someone known to the
owner (e.g. a friend or relative) is looking after a bag whilst
the owner moves away. Without the knowledge of the social
relationship, this situation would otherwise raise a false alarm.
The threats are now defined as follows:
- • Bag owner is nearest person on appearance
- • Bag unattended if nobody from owner's group is within 2 meters
- • Bag abandoned if unattended for 30 seconds
Once more the system was evaluated and the results
are shown in the final column of the table. The results using the
rules based on Ownership plus Social Groups again show an improvement
in performance. Where an error did occur it was through failing to
correctly assign two individuals to the same group; which in turn
was due to insufficient evidence of their relationship prior to
abandonment.
The simple reasoning engine used to implement the
three versions of the logic outlined above was also used to correct
three types of error in the output from image analysis and to improve
the assignment of ownership.
- • Occasionally changes in the ID of a person or bag due to
temporary occlusion occurred, which was corrected by the
implementation of a simple rule to detect such situations and
substitute the original ID.
- • Bags only appear in the scene when deposited by the owner.
A bag appearing without an assigned owner was assumed to be
spurious and removed from the world model.
- • For many scenes, it can be assumed that individuals only
appear close to the boundaries of the video frame. The system
therefore assumed any person hypothesis that appears away from
this boundary was spurious and removed it from the world model.
In the logic implemented for the tests reported
above, the owner of a bag is the nearest person to the bag on
appearance. Unfortunately, there were cases where the appearance
of a bag was delayed due to the nature of the image analysis
algorithm, such that a second individual was closer to the bag
on appearance. A solution was implemented requiring that the person
depositing a bag also paused at the location of its appearance.
This additional constraint reduced the chance of an incorrect
assignment.
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