# A Framework for Coverage Path Planning Optimization Based on Point Cloud for Structural Inspection
## 2 Proposed Framework  
CPP is the task of determining a path that passes through all points of a determined area. [PERSON] et al. [30] proposed the classification of coverage algorithms in two types:online and offline. The Offline algorithms depend only on stationary information, and the environment is known. On the other hand, online algorithms do not assume complete prior knowledge of the environment to be covered. In addition, they use real-time sensor measurements to scan the target space. In large structures, such as slopes and hydroelectric dams, UAV missions require extensive pilot experience to cover the large area. In addition, the missions' repeatability is affected, preventing reconstructions at different times from being compared. Another aspect is the waste of the UAV energy since, in a manual flight, the flight time and the images' overlay depend only on the operator's skill. It is possible to generate a mission with optimized aspects concerning the aircraft and the obtained images using information from a previously performed 3D reconstruction. Thus, this research lists the challenges of working with these structures and the missions' generation. The methods can be divided into the structure's analysis to be delivered, data filtering, metaheuristic optimization, and dynamic objectives identification. Figure 1 presents the framework diagram. Initially, the structures' data can be imported in the formats of a point cloud, mesh, or structure representing the surface. The data must then be filtered to identify the surface shape, remove outlier, and reduce the optimization algorithm's points. The optimization algorithm will create a waypoint mission that meets the mission time and photometry criteria. This mission will be sent to the UAV to start the flight. During the route, each waypoint identifies the Point of Interest (POI). In case this point is identified, the framework realizes a local mini-mission, increasing the image data of that region, and then, the UAV goes to the next waypoint. In a negative case, the aircraft will move to the next waypoint. Differently, if the waypoint is the last of the horizontal movement, the UAV checks if it is the global mission's end. If not, the mission is optimized again to lessen the impacts of local missions and any flight delays. If it is the endpoint, the mission will end, making the UAV returning to the takeoff location. At each waypoint within the horizontal transfer, the framework performs a POI identification. According to [31], 3D reconstructions can be broadly categorized into three categories: (i) voxel-based representations; (ii) point-based representations; and (iii) mesh representations. Voxel representations are a straightforward generalization of pixels to the 3D case. However, the cost of memory in this representation grows cubically with resolution. In this way, alternative representations are point clouds and meshes. They use appropriate functions to decrease losses without dramatically increasing the cost of memory. However, the point clouds do not have the mesh connectivity structure, and, therefore, this representation needs additional processing steps to extract the geometry from the 3D model [32]. Figure 1: Framework diagram. The framework proposed in this article requires the surface's shape, which can be in any of these categories. The reconstructions must conform to the UAV's coordinate system, with emphasis on the ECEF and GPS. ECEF, which is an acronym for earth-centered, earth-fixed, is a geographic and Cartesian coordinate system. It represents positions as X, Y, and Z coordinates. The point (0, 0, 0) is defined as the center of mass of Earth [33]. The GPS is a satellite-based radio navigation system representing the terrestrial globe's position by latitude, longitude, and height relative to an ellipsoidal Earth model [34]. Both position models' representation can be converted to each other using nonlinear optimization [35].