## 2 Literature Review
### Surface Model Creation Techniques for PCAM  
Once the point-cloud is available and processed based on the aspects described above, a surface model creation is considered. Since the point-clouds are obtained from RE, a great emphasis is given on the creation of freeform surfaces. In this regard, the curve-based modeling and the polygon-based modeling are the most commonly used methods [36]. These issues are described as follows. In the curve-based modeling, it includes three steps, namely, segmentation, curve fitting, and skinning. A Non-Uniform Rational B-Spline (NURBS) curves using the least square method is used in the curve-based modeling technique. For example, [PERSON] et al. [43] have developed a construction tree model for surface model creation from the point-cloud. [PERSON] et al. [44] have generated a 3D model by building a left feature through some curves generated by slicing the point-cloud wherein the point-cloud data are obtained from 3D sketching. [PERSON] et al. [45] have implemented a virtual approach for slicing the point-cloud for AM process. [PERSON] et al. [46] have developed the algorithms for generating blocked patterns automatically from the point-cloud data that can be used in garment manufacturing. [PERSON] and [PERSON] [47] have developed an algorithm for the surface reconstruction of a piecewise planar object or CAD model from the point-cloud. [PERSON] [48] have developed a tangent plane method for generating 3D geometry from the point-cloud. [PERSON] et al. [49; 50] have implemented a point-cloud based approach to directly slicing of NURBS based model for 3 DP rather than STL-based slicing. They used original NURBS model and converted the model into a point-cloud, and the applied direct slicing. The polygon-based modeling (on the other hand) consists of four steps, namely, triangulation, decimation, subdivision, and triangle to NURBS fitting. The triangulation can be performed by using the methods called Delaunay triangulation, alpha shaping, crusting, and volumetric triangulation. Recently, many authors have improved the efficiency of the abovementioned methods. For example, [PERSON] et al. [32] have developed a method for a reconstruction of polygonal faces from the point-cloud and mapped each point-cloud onto a 2D image for detecting the bounded planar faces. [PERSON] et al. [51] have developed an umbrella facet matching algorithm to construct the watertight manifold triangle meshes from the point-cloud. [PERSON] et al. [52] have developed an inverse distance square method for direct slicing of the spatial point-cloud data obtained from the RE for rapid prototyping [26] applications. They also implemented the extended extrapolation method to obtain a vertical slicing contour, wherein the contour points in each layer are searched, sorted, and reconstructed by interpolation of NURBS. [PERSON] et al. [53] have developed an algorithm based on the moving least-squared process that can intersect with lines, planes, polygonal mesh, and NURBS surface for the direct manufacturing of object given by a massive point-cloud without building the CAD model. [PERSON] and [PERSON] [54] have developed a genetic algorithm for direct slicing of the point-cloud for RP. It can be used to overcome the limitations of polygonization of the point-cloud. [PERSON] et al. [55] have developed a method for surface reconstruction from the point-clouds for creating the textures on the surface which can be used in a medical scaffold. It is worth mentioning that, compared to the curve-based modeling processes, the polygon-based modeling systems take less time regardless of the geometric complexity of the scanned parts. However,it is still challenging to reconstruct the fine details, such as the sharp corners and edges using the methods mentioned above. Apart from the point-clouds obtained by using data acquisition system, e.g., a 3D scanner, laser probe, and alike, as explained in above, a point-cloud can also be created by using mathematical process. Some authors have also worked in the other direction.