Finding if two polygons intersect in Python

Question

I'm looking for an algorithm, a high-level solution, or even a library which can help me determine if two polygons intersect, in Python.

I have the vertices of the two polygons (These are single-part polygons without any holes) in two different arrays. The polygons are 2D (i.e. just X and Y coordinates)

I'll like to make a function which will return a boolean indicating whether these two polygons intersect.

Please note that I cannot use arcpy, or any arcgis components in this.

Can you suggest an algorithm or library for doing this?

Answer 1

You could try shapely.

They describe spatial relationships and it works on Windows

The spatial data model is accompanied by a group of natural language relationships between geometric objects – contains, intersects, overlaps, touches, etc. – and a theoretical framework for understanding them using the 3x3 matrix of the mutual intersections of their component point sets

The following code shows how you can test for intersection:

from shapely.geometry import Polygon
p1 = Polygon([(0,0), (1,1), (1,0)])
p2 = Polygon([(0,1), (1,0), (1,1)])
print(p1.intersects(p2))

Answer 2

You can use the GDAL/OGR Python bindings for that.

from osgeo import ogr
wkt1 = "POLYGON ((1208064.271243039 624154.6783778917, 1208064.271243039 601260.9785661874, 1231345.9998651114 601260.9785661874, 1231345.9998651114 624154.6783778917, 1208064.271243039 624154.6783778917))"
wkt2 = "POLYGON ((1199915.6662253144 633079.3410163528, 1199915.6662253144 614453.958118695, 1219317.1067437078 614453.958118695, 1219317.1067437078 633079.3410163528, 1199915.6662253144 633079.3410163528)))"
poly1 = ogr.CreateGeometryFromWkt(wkt1)
poly2 = ogr.CreateGeometryFromWkt(wkt2)
intersection = poly1.Intersection(poly2)
print intersection.ExportToWkt()

It returns None if they don't intersect. If they intersect it returns the geometry where both intersect.

Also, you can find further info in the GDAL/OGR Cookbook.

• Devdatta Tengshe: I would love to use this, but I'm on windows, and on both the systems i've tried, I can't get the python bindings to work. I run into the problem described in this post: gis.stackexchange.com/questions/44958/…
• Evil Genius: Just in case someone else stumbles upon this, it is possible to use GDAL/OGR with Python in Windows (and within ArcGIS, no less): gis.stackexchange.com/questions/74524/…
• Max: you also can write intersection = poly1.Intersect(poly2) --- the value of intersection will be TRUE or FALSE depending on if the polygons intersect

Answer 3

I know this is an old question, but I've written a python library for handling collisions between concave and convex polygons, as well as circles.

It's pretty simple to use, here you go!

Example:

from collision import *
from collision import Vector as v
p0 = Concave_Poly(v(0,0), [v(-80,0), v(-20,20), v(0,80), v(20,20), v(80,0), v(20,-20), v(0,-80), v(-20,-20)])
p1 = Concave_Poly(v(20,20), [v(-80,0), v(-20,20), v(0,80), v(20,20), v(80,0), v(20,-20), v(0,-80), v(-20,-20)])
print(collide(p0,p1))

You can also have it generate a reponse, which includes:

overlap (how much they overlap)
overlap vector (when subtracted from second shapes position, the shapes will no longer be colliding)
overlap vector normalized (vector direction of collision)
a in b (whether the first shape is fully inside the second)
b in a (whether the second shape is fully inside the first)

https://github.com/QwekoDev/collision

Answer 4

You can also use PyQGIS.

from qgis.core import QgsGeometry
p1 = QgsGeometry.fromWkt('POLYGON ((0 0, 1 1, 1 0, 0 0))')
p2 = QgsGeometry.fromWkt('POLYGON ((0 1, 1 0, 1 1, 0 1))')
intersection = p1.intersection(p2)
print(intersection)
# OUT
# <QgsGeometry: Polygon ((1 1, 1 0, 0.5 0.5, 1 1))>

Answer 5

If you want to know the level you can use this. As an argument, you can give a list of polygons. And as a return value, you get a list of levels. In the list of levels, there are the polygons.

from shapely.geometry import Point
from shapely.geometry.polygon import Polygon
def isPolygonInPolygon(poly1,poly2):
 poly2 = Polygon(poly2)
 for poi in poly1:
 poi = Point(poi)
 if(poly2.contains(poi)):
 return True
def polygonTransformHierarchy(polygon_list):
 polygon_list_hierarchy = []
 for polygon1 in polygon_list:
 level = 0
 for polygon2 in polygon_list:
 if(isPolygonInPolygon(polygon1, polygon2)):
 level += 1
 if(level > len(polygon_list_hierarchy)-1):
 dif = (level+1)- len(polygon_list_hierarchy)
 for _ in range(dif):
 polygon_list_hierarchy.append([]) 
 polygon_list_hierarchy[level].append(polygon1)
 return polygon_list_hierarchy

Answer 6

If you know or are interested in learning R it has some useful spatial packages. http://cran.r-project.org/web/views/Spatial.html There is Python module to interact with R (RPy*)