algorithmcomputational-geometrypoint-in-polygonconvex-polygon

How to find the covering polygon if I know a point all the lines around it


I have a collection of lines in my diagram and I also have a point. What I want is a collection of lines which will together form a polygon through a ordered traversal. I don't need implementation or anything all I want is someone to direct me towards the algorithm I can use. Similar problem like this have been asked but won't work for me because

One of the common asked problems is that given a polygon I need to find whether the point lies inside it or not but this won't work for me because I don't have any polygons I only have a collection of lines.

the final polygon can be convex too so simply drawing rays on every side from that point and finding intersections won't work I need something more advanced.

Sorry for all the confusion : See this for clarity https://ibb.co/nzbxGF


Solution

  • You need to store your collection of segments inside a suitable data structure. Namely, the chosen data structure should support the concept of faces, as you're looking for a way to find the face in which a given point resides. One such data structure is the Doubly Connected Edge List.

    The Doubly Connected Edge List is a data structure that holds a subdivision of the plane. In particular, it contains a record for each face, edge, and vertex of the subdivision. It also supports walking around a face counterclockwise, which allows you to know which segments bound a particular face (such as the face containing the point you're searching for).

    You can use a Sweep Line Algorithm to construct the Doubly Connected Edge List in O(nlog(n)+klog(n)) where n is the number of segments and k is the complexity of the resulting subdivision (the total number of vertices, edges, and faces). You don't have to code it from scratch as this data structure and its construction algorithm have already been implemented many times (you can use CGAL's DCEL implementation for example).

    With the Doubly Connected Edge List data structure you can solve your problem by applying the approach you've suggested in your post: given an input point, solve the Point in Polygon problem for each face in the Doubly Connected Edge List and return the set of segments bounding the face you've found. However, this approach, while might be good enough for somewhat simple subdivisions, is not efficient for complex ones.

    A better approach is to transform the Doubly Connected Edge List into a data structure that is specialized in point location queries: The Trapezoidal Map. This data structure can be constructed in O(nlog(n)) expected time and for any query point the expected search time is O(log(n)). As with the Doubly Connected Edge List, you don't have to implement it yourself (again, you can use CGAL's Trapezoidal Map implementation).