How can I determine whether a 2D Point is within a Polygon?

Question

I\'m trying to create a fast 2D point inside polygon algorithm, for use in hit-testing (e.g. Polygon.contains(p:Point)). Suggestions for effective tech

Answer 1

This is a presumably slightly less optimized version of the C code from here which was sourced from this page.

My C++ version uses a std::vector> and two doubles as an x and y. The logic should be exactly the same as the original C code, but I find mine easier to read. I can't speak for the performance.

bool point_in_poly(std::vector>& verts, double point_x, double point_y)
{
    bool in_poly = false;
    auto num_verts = verts.size();
    for (int i = 0, j = num_verts - 1; i < num_verts; j = i++) {
        double x1 = verts[i].first;
        double y1 = verts[i].second;
        double x2 = verts[j].first;
        double y2 = verts[j].second;

        if (((y1 > point_y) != (y2 > point_y)) &&
            (point_x < (x2 - x1) * (point_y - y1) / (y2 - y1) + x1))
            in_poly = !in_poly;
    }
    return in_poly;
}

The original C code is

int pnpoly(int nvert, float *vertx, float *verty, float testx, float testy)
{
  int i, j, c = 0;
  for (i = 0, j = nvert-1; i < nvert; j = i++) {
    if ( ((verty[i]>testy) != (verty[j]>testy)) &&
     (testx < (vertx[j]-vertx[i]) * (testy-verty[i]) / (verty[j]-verty[i]) + vertx[i]) )
       c = !c;
  }
  return c;
}