# A Survey on Monitoring Quality Assessment for Wireless Visual Sensor Networks
## 2 Proposed Classification and Comparisons  
Considering the literature in the area of wireless visual sensor networks, different subjects have been discussed and analyzed, especially when considering networking, processing and energy issues. However, new approaches related to visual monitoring assessment, as well as its recurrent challenges, have been not properly discussed in a comparative way. As a result, it may become hard to compare and classify existing solutions aimed at the evaluation of visual monitoring quality. In this sense, the performed review of the literature in this article is centered on the proposal of a new classification methodology targeted at quality evaluations of visual sensor networks, grouping different subjects in this group of potential solutions. Therefore, concerning the intended classification, it was performed a substantial literature review focused on articles that have proposed metrics for quality assessment on wireless visual sensor networks. Our goal is to understand how the distribution, configuration and connection of visual nodes working together in an application can be quantitatively compared. Thereby, we did not review articles addressing merely quality of image or video, occlusion, optimization of coverage, energy consumption or lifetime, if these topics are not related to a visual network or not associated with a quantitative metric. Doing so, the surveyed results could better contribute to the construction of a comprehensive classification focused on the quality of visual monitoring tasks. The conceptual organization of the proposed classification is depicted in Figure 1, highlighting each of the three major classification groups. We propose that the quality assessment of visual monitoring tasks performed by wireless visual sensor networks can be organized in three major groups: (a) Spatial coverage quality, (b) Content quality and (c) Dependable quality. These groups were identified as a result of the reviewing process, allowing the grouping of quality assessment following visual coverage parameters. The purpose of categorization into these specific groups is to analyze visual quality beyond merely the image quality. The performed review revealed that the visual monitoring task can be qualified regarding other aspects related to the required visual information for an application. A short description of these groups is presented as follows:  
* **Spatial coverage quality**: it is related to the common types of coverage that visual sensors application executes: target coverage, area coverage and barrier coverage [18; 19]. Whatever the case, many applications aim to cover a space from which visual data will be retrieved and analyzed to quantify how much space is covered. Then, for all kinds of coverage, quality is generally assessed through the amount of coverage collaboratively performed by all sensors in relation to this quantified space, i.e., the quantity of targets, amount of area or the length of a barrier;
* **Content quality**: the amount of coverage might express the quality of a network, but it does not express the quality of the received visual data (content), and so,for some applications it may not be an appropriate quality measurement [20]. For these cases, a content quality is defined in terms of how well information can be extracted from the visual data. Thereby, the assessment of content quality should take into account properties of the gathered images, like resolution, definition and sharpness. On the other hand, content quality can be also assessed considering aspects that indirectly affect those properties, such as distance from camera to the aimed coverage objective or camera's facing angle, which is also referred as \"perspective of view\".