# Research on Automatic Generation of Park Road Network Based on Skeleton Algorithm
## 3 Principles of the Method  
### Basic Principle of Skeleton  
In 1967, [PERSON] [22; 23] provided the initial definition of the skeleton using the Grassfire Model. He postulated that the boundary points of the graph were ignited simultaneously, and a fire propagated uniformly in all directions towards the interior until extinguished. The set of extinguished points formed the skeleton of the graph. [PERSON] also presented an equivalent definition of the skeleton as a collection of centers of maximal disks, where a maximal disk is entirely contained within and not enclosed by any other disk within the graph. The skeleton serves as a graphical structure that concurrently encapsulates both shape and topological characteristics, situated internally and depicted as a branching network composed of polylines (Figure 1a). Furthermore, it can effectively handle scenarios involving \"holes\" within its interior (Figure 1b). Additionally, internal line segments are treated akin to degenerate polygons with processing flow similar to that for \"holes\" and line segments alike (Figure 1c).  
According to the definition of a skeleton, it is composed of individual segments of skeletal branches, which exist in two forms.  
The first form is the primary skeleton that is not directly connected to the polygon. It consists of the centers of the maximum inscribed circles within the graph, with each segment preserving local symmetrical information from its corresponding curve. This representation can convey both shape and topological relationships within the graph as a whole. In this study, the primary skeletal branches are abstracted as main pathways within a park.  
The second form is secondary skeletons directly linked to polygon vertices. These serve as offshoots from the main skeleton and provide insight into vertex distribution across polygons. In this study, secondary skeletons are conceptualized as secondary pathways in a park, connecting main pathways with environmental boundaries.  
Currently, the skeletal algorithms for road generation can approximately be categorized into the following types. The first type is the approach based on topology and geometric analysis, which acquires the skeleton by constructing the Voronoi diagram of the model [24]. The second type is the topology thinning method (topology thinning) [25]. The third type of approach is the one based on the distance field [26], and the fourth type is the generalized potential field method [27].  
In 1997, [PERSON] et al. [28] postulated that the internal structure of simple polygons, namely the straight skeleton, has found extensive applications in numerous fields, such as computing indoor navigation path networks [29], generating roofs [30], and so forth. This approach takes into account both the computing speed and readability, and the processing based on vector graphics is more consistent with the procedure of this study. Hence, in this study, the straight skeleton algorithm is employed to generate the road network scheme.  
Figure 1: Schematic diagram of the skeleton: (**a**) Graphical skeleton with outer boundaries only; (**b**) graphical skeleton with ’holes’; (**c**) graphic skeleton with ’line segments’.