# A Survey on Monitoring Quality Assessment for Wireless Visual Sensor Networks
## 6 Discussion and Evaluations
### The Literature on Visual Quality Monitoring  
This review article surveyed the total of 62 works. This is a considerable amount of articles for two particular reasons. In first place, as it was mentioned before, we narrowed down the survey scope to articles that have proposed metrics for quality assessment on wireless visual sensor networks. That way, it was not considered articles addressing visual monitoring quality if these topics are not related to a visual sensors network or if they are not associated with at least one quantitative metric. Secondly, because we limited the search for the past 15 years (2007 to 2021, inclusive), in order to track the current contributions to this research field. The works are almost equally distributed between publications in conferences (32) and journals (30), which indicates that this study area is well discussed among the peers in conferences, in addition to receiving deeper and more detailed contributions in scientific journals. The distribution of the published works over the years is shown in Figure 8, which may indicate that this topic has been receiving adequate attention recently, especially in the last three years.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Work** & \begin{tabular}{c} **Dependability** \\ **Attribute** \\ \end{tabular} & **Coverage** & \begin{tabular}{c} **Hardware** \\ **Failures** \\ \end{tabular} & \
\begin{tabular}{c} **Communication** \\ **Failures** \\ \end{tabular} &
\begin{tabular}{c} **Energy** \\ **Efficiency** \\ \end{tabular} & **Redundancy** & **Occlusion** \\ \hline
[27] & Availability & Target & X & X & X & – \\ \hline
[51,52] & Availability & Area & X & X & X & – \\ \hline
[6] & Availability & Area & X & X & X & X & X \\ \hline
[92,94] & Availability & Target & – & – & – & X & – \\ \hline
[35] & Availability & Target & – & – & – & X & – \\ \hline
[93] & Availability & Target & – & – & – & X & X \\ \hline
[78] & Reliability & Not specified & – & – & X & X & – \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of the performed dependable quality reviewing.  
According to Figure 9, which shows the monitoring purpose of the applications described in each work, the majority of works addressed applications focused on target coverage (36), followed by area (21) and barrier (7) coverage, while two works did not specify their monitoring purpose. It is important to remark that some works addressed more than one monitoring objective (purpose), as discussed before. Furthermore, the purpose of monitoring not necessarily affects the same spatial coverage evaluation, i.e., the quality of a target coverage application not necessarily must be assessed through a spatial coverage quality metric (target coverage metric, in this case). This is illustrated in Figure 10, which shows the distribution of works addressing each group from the proposed classification: only 36 out of the 62 surveyed works assessed their monitoring performance using a spatial coverage quality metric.  
Figure 11 shows more details by the viewpoint of the classified groups and metrics, with a special remark to sharpness metrics regarding content quality, target and area coverage metrics regarding spatial coverage quality. These classes of metrics was addressed 21, 18 and 15 times, respectively, in the literature. Again, it is worth noting that more than one quality metric is addressed in some works.  
Figure 8: Published works per year.  
Figure 9: Percentage distribution of articles according to the application monitoring purpose.