UnaryUnion gives error: found two shells in EdgeRing list

[theroggy]

For a combination of 3 specific valid polygons, applying unaryUnion on them gives an error using GEOS 3.14.1.

C:\Tools\miniforge3\envs\pysnippets\Library\bin\geosop.exe -a "GEOMETRYCOLLECTION (POLYGON ((-8.060879443418989 -2.6638274935170196, -8.194659017192153 -33.71089267415941, -11.242630721688515 -33.69775918301853, -11.10885114791535 -2.6506940023761434, -8.060879443418989 -2.6638274935170196)), POLYGON ((-84.63155622050482 -2.3338899473058037, -84.61842272936394 0.7140817571905578, -8.047745952278113 0.3841442109793418, -8.06087944341899 -2.6638274935170196, -84.63155622050482 -2.3338899473058037)), POLYGON ((-84.63155622050482 -2.3338899473058037, -84.61842272936394 0.7140817571905576, -11.095717656774472 0.3972777021202178, -11.108851147915349 -2.6506940023761434, -84.63155622050482 -2.3338899473058037)))" unaryUnion

Run-time exception: AssertionFailedException: found two shells in EdgeRing list

If all input polygons are oriented ccw first, the error does not occur, but e.g. nomalizing does not help.

Originally posted here: shapely/shapely#1639 (comment)

Visualization of input:

Full script to reproduce some different scenarios:

import os
import matplotlib.pyplot as plt
import shapely
from shapely import normalize, orient_polygons, plotting, unary_union, wkt

buffer_0 = wkt.loads('POLYGON ((-8.060879443418989 -2.6638274935170196, -8.194659017192153 -33.71089267415941, -11.242630721688515 -33.69775918301853, -11.10885114791535 -2.6506940023761434, -8.060879443418989 -2.6638274935170196))')
buffer_1 = wkt.loads('POLYGON ((-84.63155622050482 -2.3338899473058037, -84.61842272936394 0.7140817571905578, -8.047745952278113 0.3841442109793418, -8.06087944341899 -2.6638274935170196, -84.63155622050482 -2.3338899473058037))')
buffer_2 = wkt.loads('POLYGON ((-84.63155622050482 -2.3338899473058037, -84.61842272936394 0.7140817571905576, -11.095717656774472 0.3972777021202178, -11.108851147915349 -2.6506940023761434, -84.63155622050482 -2.3338899473058037))')
buffers = [buffer_0, buffer_1, buffer_2]

print(f"{all([b.is_valid for b in buffers])=}") # True

try:
    print(f"{unary_union(buffers)=}")
except Exception as e:
    print(f"Exception: {e}")

try:
    print(f"{unary_union([normalize(b) for b in buffers])=}")
except Exception as e:
    print(f"Exception after normalize: {e}")

try:
    print(f"{unary_union([orient_polygons(b, exterior_cw=True) for b in buffers])=}")
except Exception as e:
    print(f"Exception after orient_polygons with exterior_cw=True: {e}")

# orient_polygons with exterior_cw=False does work though
print(f"{unary_union([orient_polygons(b) for b in buffers])=}")

# Running with geosop gives the same error:
#    Run-time exception: AssertionFailedException: found two shells in EdgeRing list
geosop = "C:\\Tools\\miniforge3\\envs\\pysnippets\\Library\\bin\\geosop.exe"
wkt = shapely.GeometryCollection(buffers).wkt
cmdline = f'{geosop} -a "{wkt}" unaryUnion'
print(f"Running command: {cmdline}")
os.system(cmdline)

# Visiualize the input
plotting.plot_polygon(buffer_0, color='red')
plotting.plot_polygon(buffer_1, color='green')
plotting.plot_polygon(buffer_2, color='blue')
plt.show()

Output:

all([b.is_valid for b in buffers])=True
Exception: AssertionFailedException: found two shells in EdgeRing list
Exception after normalize: AssertionFailedException: found two shells in EdgeRing list
Exception after orient_polygons with exterior_cw=True: AssertionFailedException: found two shells in EdgeRing list  
unary_union([orient_polygons(b) for b in buffers])=<POLYGON ((-8.061 -2.664, -8.195 -33.711, -11.243 -33.698, -11.109 -2.651, -...>
Running command: C:\Tools\miniforge3\envs\pysnippets\Library\bin\geosop.exe -a "GEOMETRYCOLLECTION (POLYGON ((-8.060879443418989 -2.6638274935170196, -8.194659017192153 -33.71089267415941, -11.242630721688515 -33.69775918301853, -11.10885114791535 -2.6506940023761434, -8.060879443418989 -2.6638274935170196)), POLYGON ((-84.63155622050482 -2.3338899473058037, -84.61842272936394 0.7140817571905578, -8.047745952278113 0.3841442109793418, -8.06087944341899 -2.6638274935170196, -84.63155622050482 -2.3338899473058037)), POLYGON ((-84.63155622050482 -2.3338899473058037, -84.61842272936394 0.7140817571905576, -11.095717656774472 0.3972777021202178, -11.108851147915349 -2.6506940023761434, -84.63155622050482 -2.3338899473058037)))" unaryUnion
Run-time exception: AssertionFailedException: found two shells in EdgeRing list