How to split polygon with a line?

[ganisback]

Hi All, is there any way to split a polygon? very appreciated if have an example.

[piyushvaish]

Any updates on the same?

[macdrevx]

I haven't used it personally, but a quick web search seems to suggest that the geos polygonize functionality might be able to solve this for you. Seems like the workflow for your specific case (polygon + line) might be something like:

1. get boundary of polygon
2. union the boundary and your line
3. polygonize the result of the union to get split polygons

GEOSwift 5 has support for 1 (boundary) and 2 (union) but not 3 (polygonize). Let's mark this as a feature request and get it added to a future version of the library.

In the meantime, you might be able to accomplish what you want by using geos directly instead of GEOSwift. If you do, please share anything you learn to help us add support for polygonize to GEOSwift.

[macdrevx]

@ganisback @piyushvaish if you still need this feature, I've just pushed a branch that exposes polygonize and enables this use case. Here's an example of how you can do it:

let polygon = try! GEOSwift.Polygon(exterior: GEOSwift.Polygon.LinearRing(points: [
    Point(x: 0, y: 0),
    Point(x: 1, y: 0),
    Point(x: 1, y: 1),
    Point(x: 0, y: 1),
    Point(x: 0, y: 0)]))
let line = try! LineString(points: [
    Point(x: -1, y: 2),
    Point(x: 2, y: -1)])

print(try! polygon.boundary().union(with: line).polygonize())

which results in

GeometryCollection(geometries: [
  Polygon(exterior: Polygon.LinearRing(
    points: [
      Point(x: 1.0, y: 0.0),
      Point(x: 0.0, y: 0.0),
      Point(x: 0.0, y: 1.0),
      Point(x: 1.0, y: 0.0)]),
    holes: [])),
  Polygon(exterior: Polygon.LinearRing(
    points: [
      Point(x: 0.0, y: 1.0),
      Point(x: 1.0, y: 1.0),
      Point(x: 1.0, y: 0.0),
      Point(x: 0.0, y: 1.0)]),
    holes: []))
])

(cleaned up for readability).

If you have a chance, give it a try and let me know how it works for your use case.

[macdrevx]

This is now available in v6.2.0. Enjoy!