parallel_offset returns MultiString with missing parts

Hi,

I’m noticing strange behavior in the parallel_offset method.

Expected behavior and actual behavior.

parallel_offset should return a single LineString in the following example but returns a MultiLineString with a part missing. Works correctly for some other distances or side=‘left’ or when removing one of the points.

incorrect behavior for distance=10 correct behavior for distance=8

Steps to reproduce the problem.

import shapely.wkt
from matplotlib import pyplot as plt
from shapely.geometry import LineString

road = shapely.wkt.loads('LINESTRING ('
                         '424367.2267078362 4513099.023495375,'
                         ' 424370.9561004792 4513098.974304576,'
                         ' 424380.476033291 4513098.832676926,'
                         ' 424382.1039646821 4513098.816053259'
                         ')')
plt.plot(*road.coords.xy, marker='o')

line = road.parallel_offset(distance=10, side='right')

if isinstance(line, LineString):
    line = [line]

for l in line:
    plt.plot(*l.coords.xy, marker='o')

plt.gca().set_aspect('equal')
plt.show()

Operating system

Mac OS X 10.14.5

Shapely version and provenance

1.6.4.post1 installed from Conda

[Jeremiah-England]

A link to the ticket in the geos bug tracker: https://trac.osgeo.org/geos/ticket/806

[jorisvandenbossche]

Using the latest released shapely and GEOS versions, this is now working correctly (the offset curve implementation has been refactored):

# using example from above
road = ...

line = road.parallel_offset(distance=10, side='right')

from shapely.plotting import plot_line
plot_line(road)
plot_line(line, color="C1")