shapely parallel_offset sometimes does not generate closed ring

Question

I am using the parallel_offset function of the shapely package to get offset structures to some polygons that are closed rings. I have several polygons at once, many with similar shapes. Around 10-25% of them, however, do not generate a closed ring from the parallel_offset. Here is a MWE of a shape that does not work:

import matplotlib.pyplot as plt
from shapely.geometry.polygon import LinearRing

def plot_line(ax, ob, color):
    x, y = ob.xy
    ax.plot(x, y, color=color, alpha=0.7, linewidth=3, 
            solid_capstyle='round', zorder=2)

polygon = [[-29.675, -30.675],
           [-28.4094, -29.4094],
           [-28.325, -29.325],
           [-28.325, -29.764],
           [-28.325, -29.7933],
           [-28.4587, -29.8274],
           [-28.4676, -29.8297],
           [-28.5956, -29.8814],
           [-28.6041, -29.8848],
           [-28.724, -29.953],
           [-28.732, -29.9576],
           [-28.8417, -30.0413],
           [-28.849, -30.0469],
           [-28.9466, -30.1445],
           [-28.9531, -30.151],
           [-29.0368, -30.2607],
           [-29.0424, -30.268],
           [-29.1106, -30.3879],
           [-29.1152, -30.3959],
           [-29.1669, -30.5239],
           [-29.1703, -30.5324],
           [-29.2044, -30.6661],
           [-29.2067, -30.675],
           [-29.6457, -30.675],
           [-29.675, -30.675]]

poly_line = LinearRing(polygon)
poly_line_offset = poly_line.parallel_offset(0.05, side="left", resolution=16, 
                                             join_style=2, mitre_limit=1)
fig = plt.figure()
ax = fig.add_subplot(111)
plot_line(ax, poly_line, "blue")
plot_line(ax, poly_line_offset, "green")
plt.show()

As you can see, the green offset polygon does not close at the point that is first/last in the list of vertices. Other very similar shapes, however, do work as intended. They have the same data structure and also have the same start/end point, as does my example above. The join_style attribute does not change the outcome to what I want. Changing the resolution or distance does not help either. The documentation also does not address this issue.

Do you have any guidance? I am using shapely 1.6.3.

Answer 1

not completely sure why this happens, nevertheless you might use a workaround based on the buffer method:

poly_line = LinearRing(polygon)
poly_line_offset = poly_line.buffer(0.05, 
                       resolution=16, join_style=2, mitre_limit=1).exterior

With your data, this produces the (probably) desired result: