Alphashape and PolygonPatch: basic example doesn't work. Why?

Question

I am trying to use the package Alphashape from here.

Although I correctly copied and pasted the example in the initial pages, I get the following error:

  File "/anaconda3/lib/python3.8/site-packages/descartes/patch.py", line 87, in PolygonPatch
    return PathPatch(PolygonPath(polygon), **kwargs)
  File "/anaconda3/lib/python3.8/site-packages/descartes/patch.py", line 62, in PolygonPath
    vertices = concatenate([
  File "/anaconda3/lib/python3.8/site-packages/descartes/patch.py", line 63, in <listcomp>
    concatenate([asarray(t.exterior)[:, :2]] +
IndexError: too many indices for array: array is 0-dimensional, but 2 were indexed

The code I am trying:

import numpy as np
from descartes import PolygonPatch
import matplotlib.pyplot as plt
import alphashape


points_2d = [(0., 0.), (0., 1.), (1., 1.), (1., 0.),
(0.5, 0.25), (0.5, 0.75), (0.25, 0.5), (0.75, 0.5)]

fig, ax = plt.subplots()
ax.scatter(*zip(*points_2d))
alpha_shape = alphashape.alphashape(points_2d, 0.)
ax.add_patch(PolygonPatch(alpha_shape, alpha=0.2))
plt.savefig(f"./test.png")

Do you why this is not working?

Many thanks!

Answer 1

So from what I could tell, this issue comes from a broken implementation of shapely within descartes.

My speculation is that shapely changed how it handles Polygon exteriors and descartes simply hasn't been updated.

I don't know if it is the best idea, but I edited my installation of descartes directly to fix this issue:

Navigate to your descartes installation and open patch.py.

At line 62 you should see this piece of code:

 vertices = concatenate([
 concatenate([asarray(t.exterior)[:, :2]] + [asarray(r)[:, :2] for r in t.interiors])
 for t in polygon])

Simply change t.exterior to t.exterior.coords. This hopefully should fix your issue.

vertices = concatenate([
concatenate([asarray(t.exterior.coords)[:, :2]] + [asarray(r)[:, :2] for r in t.interiors])
for t in polygon])