## 3 Results
### Studies Implementing Vision Systems (VSS)
#### 3.2.1 Overall Statistics  
This section presents and discusses general statistical information regarding the studies in Table 1. The first analysis concerns the type of infrastructure addressed in the studies and the type of SHM implemented (Figures 3a and 3b, respectively). The analysis revealed that VSS have been implemented on different types of infrastructure, particularly on more general targets (such as bridges and buildings) or other more specific examples (such as containment vessels or retention ponds). The item \"structures (generic)\" refers to solutions suitable for multiple infrastructures. Regarding the type of SHM, the analysis revealed four categories, even though they are pairwise related. For example, the \"vibration estimation\" category includes those studies whose primary objective is to estimate vibration frequencies (mainly of bridges) starting from a displacement mea \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Author** & **Target** & **SHM** & **Study Objectives** & **Methods** & **Camera Features** & **Test Scenarios** & **Main Results** \\ \hline \multirow{9}{*}{[PERSON] et al., 2018 [83]} & \multirow{9}{*}{Bridges} & \multirow{9}{*}{Displacement estimation} & A framework to quantify the absolute displacement of a structure from videos taken by a UAV, using the targetless tracking algorithm based on the recognition of fixed points on the & \multirow{9}{*}{CV} & \multirow{9}{*}{In-field: mean (4.096 \(\times\) 2.160 px, 24 fps)} & \multirow{9}{*}{The estimated absolute displacement agrees with that manually measured because of the motion generated by the simulator. The mounted on UAV (video recording at 4.6 m from bridge)} & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & 1.2 pixels of resolution. \\  & & & & & & \\  & & & & & & \\  & & & & & & & \\  & & & & & & \\ \hline \multirow{9}{*}{[PERSON] et al., 2018 [84]} & \multirow{9}{*}{Retention ponds,
\begin{tabular}{} \end{tabular} } & \multirow{9}{*}{Damage detection dams} & \multirow{9}{*}{CV} & Proof of concept for using a USW with an onboard camera to visually assess the health status of a retention pond and a team, automatically detecting damaged areas. & Single-camera: not specified, mounted on a vehicle (USV). & In-field: retention pond and a seawall (outdoor) & Results related to the various steps of the proposed methodology were provided. \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & & & & \\ \cline{1-1} \cline{3-6}  & & & Pixel-wise deblurring imaging (PDI) system for compensation for blurring caused by one-dimensional motion at high speed (100 km/h) between a camera and special targets (black and white stripes) to improve image quality for crack detection in tunnels. & Single-camera: High-speed SPARK-12000M & In-field: highway tunnel (outdoor) & In-field: vibrating due to high bypass speed was compensated for by a PDI system; without it, cracks and stripes could not be distinguished at 100 Km/h due to the strong motion blurring. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont_. surement determined by the vision system. On the contrary, the \"displacement estimation\" category includes those studies that have as their primary objective the measurement of displacements (mainly of buildings) and only secondarily attempt to estimate vibration frequencies. The \"damage detection\" category includes studies that focus on general structural damage (including deformations, deflections, and corrosion). In contrast, the \"cracks on surface \" category includes studies addressing this type of damage exclusively. Another relevant item involving vision systems concerns the type of cameras and configurations used. Regarding configuration, 85% of studies implemented a single-camera solution, while the others adopted a multi-camera approach.