Expected behavior and actual behavior.
Holes are not correctly accounted for in shapely's implementation of the polylabel algorithm.
Steps to reproduce the problem.
from shapely.geometry import Polygon
from shapely.algorithms.polylabel import polylabel
poly1=Polygon(shell=[(0,0),(10,0),(10,10),(0,10),(0,0)][::-1], holes=[[(2,2),(6,2),(6,6),(2,6),(2,2)]])
import geopandas
gdf2 = geopandas.GeoDataFrame(geometry=[poly1])
gdf2.plot()
label_location = polylabel(poly1, tolerance=0.05)
print(label_location.x)
print(label_location.y)
The output of this script is:
6.25
3.75
But the correct position is something close to 7.65625, 7.65625. This can be verified by looking at a plot of this polygon
Maybe I am doing something wrong in the definition of the polygon but I have tried many different ways of instantiating this polygon and I always get the same result.
Operating system
Linux
Shapely version and provenance
Tested with shapely version 1.6 and 1.7a3
benediktbrandt
All 3 comments
Could you please verify that the coordinate that https://github.com/mapbox/polylabel itself suggests is different?
kannes
on 6 Feb 2020
Original algorithm in TypeScript:
import polylabel from "polylabel";
let exterior = [[0,0],[10,0],[10,10],[0,10],[0,0]];
let hole = [[2,2],[6,2],[6,6],[2,6],[2,2]];
let polygon = [exterior, hole]
console.log(polylabel(polygon, 0.01));
PS> .\node_modules\.bin\tsc test.ts
PS> node .\test.js
[ 7.65625, 7.65625 ]
Shapely:
>>> from shapely.ops import polylabel
>>> from shapely.geometry import Polygon
>>> exterior = [[0,0],[10,0],[10,10],[0,10],[0,0]]
>>> hole = [[2,2],[6,2],[6,6],[2,6],[2,2]]
>>> p = Polygon(exterior, [hole])
>>> polylabel(p, 0.01).coords[:]
[(6.25, 3.75)]
snorfalorpagus
on 21 Feb 2020
You are correct @benediktbrandt. The Shapely implementation doesn't handle the holes correctly, as it only considers the distance to the exterior of the polygon. I think this is a bug this should be fixed to match the behaviour of the original algorithm.
The conversion of the polygon exterior to a linestring is a little inefficient here too as it will be done for every cell created.
snorfalorpagus
on 21 Feb 2020
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Most helpful comment
You are correct @benediktbrandt. The Shapely implementation doesn't handle the holes correctly, as it only considers the distance to the exterior of the polygon. I think this is a bug this should be fixed to match the behaviour of the original algorithm.
https://github.com/Toblerity/Shapely/blob/cfa66d418fe3afe2fcb48438fcc9156142abaeb8/shapely/algorithms/polylabel.py#L44-L53
The conversion of the polygon exterior to a linestring is a little inefficient here too as it will be done for every cell created.