How to access Polygon.vertices() from python?

Question

I'd like to access the vertex list of a mshr Polygon but I can't work out how to do so. My first attempt:

from dolfin import *
from mshr import *

poly = Polygon([Point(0,0), Point(1,0), Point(1,1), Point(0,0)])
[v for v in poly.vertices()]

results in

TypeError: 'SwigPyObject' object is not iterable

Attempting "poly.vertices()[0]" also fails:

TypeError: 'SwigPyObject' object has no attribute '__getitem__'

The only reasonable thing I can find on the object returned by vertices() is a next() function, but it just returns "None".

How can I get the vertices out of a polygon?

Answer 1

Hi steve, the polygon object is not a mesh, so it does not have what you need. For this, you might first create a mesh with

resolution = 10
mesh = generate_mesh(poly, resolution)
vertices = mesh.coordinates()

Best regards!

Comments

Is there no way to get the vertices which define the Polygon from mshr prior to mesh generation?

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None that I know of, and as steve mentioned the methods that seems reasonable give only nulls. Maybe if he gives some details on the problem we could find another way.

Best regs

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Thanks for confirming my fears :-) In the end, however, I solved my problem by avoiding it!

For my application, I have boundaries generated by another system that I am converting into fenics to solve the Poisson equation (an electrostatics problem). The reason I want the vertices is to set up the boundary conditions. In the end, I just went back to the original input polylines for the boundary vertices.

Incidentally, my first attempt was to do as you suggest -- generate a mesh and obtain the boundaries from it. For inner "hole" boundaries, this works fine since the part that is meshed is outside the problem domain: I was able to essentially do a set intersection to find the (common) boundary edges. But this doesn't work well for the outer domain boundary -- the "rough mesh for boundary creation" also contains vertices interior to the boundary and gets in the way of my naive approach.