## 3 Sensor Coverage/Deployment in WVSNs  
In WSNs, the sensors can cover an event if it is within sensing range. However, in WVSNs, besides the sensing range coverage, the event must also fall within the view angle of the camera on the sensor. Because the view angle of the camera equipped on the sensor node is limited, directional coverage instead of omni-directional coverage is applied in WVSNs. As indicated in Figure 2b, only two nodes (i.e., nodes \(C\) and _E_) are inside the coverage area of the camera sensor. Hence, there are two requirements (sensing range coverage and view angle coverage) for sensor coverage in WVSNs. The directional coverage literature in WSNs is surveyed in [15]. However, besides these two requirements, there is another factor (i.e., the occlusion problem) that needs to be addressed. In the WVSN, the camera on the sensor node could only capture the images of the object without any obstacles. Hence, line-of-sight is required between the camera on the sensor and the object. If there is an obstacle (e.g., a tree) between the sensor and the object, the sensor cannot capture the object image even if the object is within the view angle coverage of the sensor. This is known as occlusion. To be more precise, the FoV coverage in WVSNs should consider the view angle of the camera and the occlusion at the same time. Hence, as compared to the sensor coverage problem in WSNs that only needs to consider the sensing range coverage, in the sensor coverage problem in WVSNs, there are three criteria to be considered (i.e., sensing range coverage, view coverage, occlusion). In Figure 2, we show the differences between the sensor coverage problems in WSNs and WVSNs. In Figure 2a, the sensor could sense and capture the data from the nodes that are within sensing range. Hence, the sensor could capture the data from five nodes (i.e., nodes _A, B, C, D_ and _E_). In Figure 2b, besides the sensing range, the view angle from the camera on the sensor nodes pose another constraint \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Platform**} & \multirow{2}{*}{
\begin{tabular}{c} **Image Capturing** \\ **Device** \\ \end{tabular} } & \multirow{2}{*}{**Memory and Storage**} & \multirow{2}{*}{**Radio**} & \multicolumn{2}{c}{**Energy**} \\  & & & & **Consumption** \\ \hline \multirow{3}{*}{Cyclops [6]} & CMOS imaging & 64 KB SRAM, 512 KB & \multirow{3}{*}{Zigbee} & 0.8–110.1 \\  & device & FLASH & & \\ \cline{1-1} \cline{4-5} \multirow{2}{*}{Meerkats [3]} & CCD webcam & 32 MB FLASH and 64 MB & \multirow{2}{*}{IEEE802.11b} & \multirow{2}{*}{49.2–3,500} \\  & CCD webcam & & DRAM & \\ \cline{1-1} \cline{4-5} \multirow{2}{*}{Panopes [4]} & CCD webcam & 64 MB & IEEE802.11 & 58–5,300 \\  & CMOS imaging & 64 KB SRAM, 256 KB & \multirow{2}{*}{Zigbee} & 1.8–175.9 \\  & device & FLASH & \\ \cline{1-1} \cline{4-5} \multirow{2}{*}{MicrelEye [8]} & CMOS imaging & \multirow{2}{*}{32 KB SRAM, 1 MB FLASH} & \multirow{2}{*}{Bluetooth} & \multirow{2}{*}{\(\sim\)500} \\  & device & & & \\ \cline{1-1} \cline{4-5} \multirow{2}{*}{Vision Motes} & CMOS imaging & \multirow{2}{*}{64 MB SRAM, 128 MB} & \multirow{2}{*}{Zigbee} & 5.2–489.6 \\  & device & FLASH & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hardware components in WVSN platforms. on the coverage. The FoV of the sensor covers nodes \(C\) and \(E\). In Figure 2c, we could observe that the node \(C\) is behind the obstacle so that the FoV of the sensor could only cover node \(E\). Research on sensor coverage problems in WVSNs can be traced back to the art gallery problem. The art gallery problem is to identify how many guards are needed to guard an art gallery and how should they be placed [16]. There are polynomial algorithms to solve the 2-D art gallery problem. When the view angle of the camera on the sensor is 360 \({}^{\circ}\)and the sensing range is unlimited, then the sensor coverage problem in WVSN could be reduced to the art gallery problem.