## 4 Experiment
### Threshold Estimation and Scenario-Based Experimental Results
#### 4.2.1 Threshold Estimation  
An experiment was conducted to estimate the similarity comparison threshold. If a threshold is set to a low value, two different videos are considered to be equal. If the threshold is set too high, a manipulated video is considered to be unrelated. Because this experiment focused on the detection of all crime videos, a threshold was set, even if different videos were recognized as similar. Although the videos used during the experiment were not based on actual crime videos, they reflected the features of illegally filmed videos. As shown in Table 3, the videos were classified according to the criteria of brightness, object perspective, and shooting angle. Table 4 shows the eight environments created through the intersection of the features. All videos were classified into eight experimental environments. Each environment had 311 frames of similar videos. Therefore, the experiment used frames with a total of 2488 frames, and the threshold for each environment was set. \begin{table}
\begin{tabular}{c c c} \hline
**Brightness** & **Object Perspective** & **Shooting Angle** \\ \hline Bright (B) & Near (N) & Middle angle (M) \\ Dark (D) & Far (F) & High angle (H) \\ \hline \end{tabular}
\end{table} 
Table 3: Video classification criteria. Figure 5 illustrates the criteria for perspective and shooting angle. The comparison in terms of perspective was based on how far or near an object is. The target object must be determined by an investigator. Because the perspective is a qualitative area, an investigator should determine how far or near an object is. During this experiment, a nearby object was shot within a 1 m distance from the camera; in addition, a distant object was shot at a distance of at least 1-m from the camera. The shooting angle is classified into a middle angle (the horizontal angle of a camera for an object) and high angle (from-top-to-bottom angle of a camera for an object). The shooting angle was also a qualitative area. Therefore, an investigator should select the shooting angle type based on judgment. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Environment** & **Quantitative** & **Qualitative** \\ \hline BNM & Bright & Near + middle angle \\ BNH & Bright & Near + high angle \\ BFM & Bright & Far + middle angle \\ BFH & Bright & Far + high angle \\ DNM & Dark & Near + middle angle \\ DNH & Dark & Near + high angle \\ DFM & Dark & Far + middle angle \\ DFH & Dark & Far + high angle \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Criteria for classification of experimental environment. Figure 4: Criteria of brightness. Figure 5: Based on perspective and shooting angle. Figure 6a illustrates a graph of correlation similarities for similar videos in an BNM environment. In the correlation algorithm-based histogram comparison, the comparison is evaluated into a number with a scale of \(1\) to \(-1\), with \(1\) being identical, \(-1\) being a perfectly reversed image frame, and zero being uncorrelated. To find appropriate thresholds, this study selected frames with noticeable changes of objects and used them as experimental data, rather than the frames that look similar with the naked eye. As for the experimental data used in this study, exceptional situations arising in illegally filmed videos, such as shifting an object horizontally, making an object disappear temporarily, or adding a different object to its surroundings while the shooting angle remains unchanged in a single space, were taken into consideration. To find a minimum threshold, this study moved an object as much as possible in the comparative experiment. Even if two frames look totally different with the naked eye, they are sometimes considered similar in similarity comparisons. In such a case, a comparison similarity becomes an outlier. With the similarities of the frames, variance of a sample was calculated. Among 311 frames, outliers were removed, and their mean was calculated. In this way, a threshold was estimated. The threshold of the correlation algorithm was set as 0.47 or more.