Re: [Matplotlib-users] matplotlib.nxutils.pnpoly returns True for some vertexes, and False for others

[Maria L. <li...@us...>]

Ben,

Many thanks! Will try to use shapely package then.

Masha
--------------------
li...@us...




On Jan 30, 2013, at 6:59 AM, Benjamin Root wrote:

> 
> 
> On Tue, Jan 29, 2013 at 5:55 PM, Maria Liukis <li...@us...> wrote:
> Hello,
> 
> I tested the following code on my Mac laptop and our production Linux server both running matplotlib V1.0.1. Both machines observe the same output from the code, so I was wondering if somebody is aware of the problem or if it's some undocumented feature of "pnpoly()" function from matplotlib.nxutils?
> 
> I use matplotlib.nxutils.pnpoly() function from matplotlib to determine if point belongs to the polygon. 
> The following code:
> 
> >>> import numpy as np
> >>> import matplotlib.nxutils as nx
> >>> coords = np.array([[4.0, 1.0], [4.0, 4.0], [5.0, 5.0], [6.0, 4.0], [5.0, 0.0]])
> 
> >>> nx.pnpoly(4.0, 1.0, coords)
> 1
> >>> nx.pnpoly(4.0, 4.0, coords)
> 1
> >>> nx.pnpoly(5.0, 5.0, coords)
> 0
> >>> nx.pnpoly(6.0, 4.0, coords)
> 0
> >>> nx.pnpoly(5.0, 0.0, coords)
> 0
> 
> The question is why first two vertexes are considered to be inside of defined polygon, and last 3 vertexes are not? My guess, it's treating the polygon as a semi-open set, and I wonder if it can be changed to make all vertexes inclusive?
> 
> Any help would be greatly appreciated.
> 
> Many thanks,
> Masha
> 
> The documentation for pnpoly() for that version states:
> 
> """
> A point on the boundary may be treated as inside or outside.
>         See `pnpoly <http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html>`_
> """
> 
> Note that in version 1.2.0, the nxutils module was deprecated.  pnpoly() and points_inside_poly() are now merely wrappers around the polygon's implementations of point-testing, which differs from the nxutils' implementation, so you may get slightly different results.
> 
> Do note that the point-testing algorithm in matplotlib was more geared for visualization purposes rather than for strict geometric needs.  If you need a more well-behaved point-tester (and faster if the polygon is "prepared"), use the shapely package instead.
> 
> I hope that clears things up!
> 
> Cheers!
> Ben Root
>