Re: Inpolygon equivalent

[Stefan van der Walt]

Octave-forge includes non-free bindings to the General Polygon
Clipping library, which makes this fairly easy to implement.

The following code comes from the Polygon-module for Python, that
wraps GPC.  It is licensed under LGPL and can be found at

http://www.dezentral.de/soft/Polygon

The algorithm, explained at

http://graphics.cs.ucdavis.edu/~okreylos/TAship/Spring2000/PointInPolygon.html

is elegant and simple, and can easily be implemented in octave.


int poly_c_point_inside(gpc_vertex_list *vl, double x, double y){
  int i, j, c=0;
  gpc_vertex *vi, *vj;
  for (i=0, j=vl->num_vertices-1; i < vl->num_vertices; j = i++) {
      vi = vl->vertex+i;
      vj = vl->vertex+j;
      if ((((vi->y <= y) && (y < vj->y)) || ((vj->y <= y) && (y < vi->y))))
          if ((x < (vj->x - vi->x) * (y - vi->y) / (vj->y - vi->y) + vi->x))
              c = !c;
  }
  return c;
}


Regards
Stéfan

On Wed, Nov 30, 2005 at 06:40:08PM -0800, Henry F. Mollet wrote:
> From http://octave.sourceforge.net/index/index.html:
> 
> inpolygon= use David Doolin's inpoly.m, but with [analysis]
> "http://www.che.wisc.edu/octave/mailing-lists/octave-sources/1999/13";>caveat
> s
> 
> Henry



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