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Page 20 Xu et al. J Surveill Secur Saf 2020;1:16-33 I http://dx.doi.org/10.20517/jsss.2020.04

Figure 2. Schematic diagram of block processing of video frames

The neighborhood size for solving the acceleration in the experiment was 5 × 5; (3) regarding anomaly
classification, most algorithms are based on empirical thresholds. The threshold ε in this paper is mainly the
continuous actual detection and analysis of different datasets; and (4) an optimal behavior determination
based on an optimal threshold is applied to a given video sequence. If it is greater than the threshold, it is
considered that there is an abnormal situation; otherwise, it is considered a normal video frame. Represent
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threshold as Ai. The detection process can be described as :
FA  normal , A < ε (1)
i
( ) = 
i
 anomaly , A ≥ ε
i
In real video images, the crowd leisurely walks in the direction they want to go. However, when there are
unexpected events, people will run around because of psychological panic. At this point, people usually get
away from danger quickly. This results in a very significant change in the speed of the human body, thus
the range of motion of the moving object is also large.

It should be noted that both normal behavior and abnormal behavior are continuous; they cannot occur
at a certain moment, and then disappear immediately. Therefore, occasional short-lived anomalous frames
that appear in normal video frames are still considered normal video frames. Conversely, several frames
of normal frames that occur by chance in an abnormal video frame are still determined as abnormal video
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frames .

3.2 Escape center
The anomaly detection object in this paper is a random group in ordinary public places. That is to say, the
movement direction of the crowd is irregular when there is no abnormality. Usually, sudden abnormal
situations for ordinary people will bring fear to people and then move away from the point where the
anomaly occurs. Therefore, according to this principle, the idea of escaping from the center is introduced,
and the place where the abnormality may occur is determined by determining the escape center. As shown
in Figure 3, one or more anomalies may occur in a scene. The squares in the figure represent moving
targets, the arrows on the squares indicate the direction of motion of the moving targets, and the blue dots
represent the locations where anomalies may occur .
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