Detect if a point (X, Y) is within a given polygon (Set of X, Ys) .

import pip

// Polygons are self closing (i.e. you don't need to add first point at end)
rectangle := pip.Polygon{
  Points : []pip.Point {
    pip.Point{X : 1.0, Y : 1.0},
    pip.Point{X : 1.0, Y : 2.0},
    pip.Point{X : 2.0, Y : 2.0},
    pip.Point{X : 2.0, Y : 1.0},
  },
}

pt1 := pip.Point{X : 1.1,  Y : 1.1}

pip.PointInPolygon(pt1, rectangle) // Test - Should return true

• Currently no support for holes although this should be easy to account for if the hole is tested as a separate polygon.
• Currently no support for points that reside on edges (returns outside, i.e. false).
• Beware of roundoff errors
• The direction that you list the vertices (clockwise or counterclockwise) does not matter.

Based on the example code given in this article by W. Randolph Franklin. The method is named PNPOLY.

MIT License Original C code : Copyright (c) 1970-2003, Wm. Randolph Franklin Modified Go code: Copyright 2016 James Milner