## 2 Related Work  
Recently, maximizing the coverage of an area, improving the resolution and increasing the field of view in WMSNs have all been studied in great depth. Equipping sensor nodes used in regular WMSNs with some extra resources or using image processing methods are instances of methods used in previous works which tried to satisfy these goals. Cameras which have the ability of pan, tilt, and zoom as well as moveable platforms are samples of resources that sensor nodes are equipped with [7, 8, 9]. Creating accurate or high-resolution images by processing a sequence of rough images, utilizing image fusion, and using new compression methods over row data are instances of image processing ways [10, 11]. Some of related literature is described in the following paragraphs. In [7], the authors have maximized multimedia coverage in WMSNs by considering video sensor orientation. This method finds the best direction of camera sensors dynamically and depends on the circumstances not on setting camera sensors orientation once they have been deployed. The distributed algorithm which is proposed in order to perform this goal has two main steps: (i) minimizing the effects of occlusion in the environment and (ii) improving the cumulative quality of the information sensed from the region of interest. This algorithm improves robustness of WMSNs, because the direction of camera sensors could be updated after nodes fail due to the battery outage or external effects. In [8], the authors have improved their previous work in [7] such that each sensor node determines the most beneficial orientation for its multimedia sensor so that the entire image of a field can be constructed using low-resolution snapshots from multiple sensors. In overall, this approach has some benefits: (i) the proposed algorithm is fully distributed using local information, so communication overhead is incurred only among neighboring nodes, (ii) with the flexibility to adjust orientations of the multimedia sensors, multimedia sensor nodes update the orientation of multimedia sensors on the fly to increase the multimedia coverage significantly, (iii) overlapped and occluded regions in the sensing field can be decreased by collecting the current pose of neighboring nodes and (iv) coverage is increased even for sparse networks by using self-orientation instead of random orientations when arbitrary obstacles exist in the sensor field. Although the mentioned approaches have solved some problems in WMSNs, they have some weak points. First and foremost, these approaches are based on single-tier topology, whereas according to [12] the topology of WMSNs must be multi-tier in order to achieve a balance between cost, coverage, functionality, and reliability. Also, these approaches are costly because they use strong nodes which are also equipped with extra resources like a rotatable platform for cameras. On the other hand, they are not suitable for real-time application because changing the orientation of camera sensors is performed by mechanical devices. Moreover, since there is no guarantee of covering the area related to a node by its neighbors, these approaches tried to reduce the number of sensor nodes, so the reliability of the WMSN is decreased. Therefore, if a node fails, some part of the field will not be covered. In [9], authors discussed a system architecture that uses controlled motion to provide virtual high-resolution in a network of camera sensors. The motion abilities have been added to camera sensors include pan, tilt, and zoom, help avoiding obstacles and camera overlap. After detecting an interesting phenomenon, camera sensor tries to provide a new image of it which has acceptable level of details for recognition. This method is not suitable for real-time application, because if the tolerable delay in sensing is small, then only a limited range of motion may be feasible and capturing an image with adequate details for recognition is impossible.