Unexpected behavior of buffer with LinearRing object

Expected behavior and actual behavior.

Maybe I’m using the LinearRing in an unsupported fashion, but when I use LinearRing.buffer I get more vertices than expected around the first (identical to the last since it’s a ring) input vertex.

Steps to reproduce the problem.

# define an equilateral triangle as LinearRing (just to make sure than all three corners are comparable
lr = shapely.geometry.LinearRing([[0, 0],
                                  [50, (100**2-50**2)**0.5],
                                  [100, 0]])
print(lr.is_closed)  # is closed as expected
print(list(lr.coords))  # the three given coordinates plus the closing [0, 0]
lr_buffered = lr.buffer(2, 2)
print(list(lr_buffered.exterior.coords))  # UNEXPECTED: more vertices than expected around [0, 0]

The output for the last print is a mess, but let me describe it here:

1. 4 vertices around the first input vertex.
2. 4 vertices around the second input vertex.
3. 4 vertices around the third input vertex.
4. Another 8 vertices around the fourth/first input vertex.

Operating system

Linux manjaro 5.3.7-2-MANJARO SMP PREEMPT Tue Oct 22 18:56:05 UTC 2019 x86_64 GNU/Linux

Shapely version and provenance

1.6.4.post2 installed from PyPI using pip

Thanks for your work on shapely and for looking into this issue.

[sgillies]

I’m going to close this one. GEOS 3.10.x is making its way into distributions of shapely.

[jorisvandenbossche]

With latest Shapely and GEOS (3.10.1), I get the expected result (4 vertices in each corner):