Dealing with "cracks" in the unary_union of several imprecise Polygons?
Question:
I used shapely.ops.unary_union on a number of 6-sided shapely.geometry.Polygons, and obtained the following shape A:
Note how there are two "cracks" in the upper part. These are not intended, and are presumably caused by some floating-point edge cases.
If you construct another shape B that sits inside of A, and if A happens to intersect one of these "cracks", then A.covers(B) will be False!
In my particular case, this leads to a test suite failure, because A.covers(B) is supposed to be an invariant. Therefore I need to deal with this somehow. Is there some algorithm I can use to "seal" these cracks?
In practice, these "cracks" will not affect the functioning of the application, because we only care about the outer border of A covering B. Thus I am open to solutions that adjust the individual hexagons in order to make the cracks go away by introducing overlaps.
However, I cannot accept the outer border of this shape changing, because that would actually no longer be testing the application as it is intended to be used.
To summarize, I want the outcome to look like this (my hand-edited version):
Answers:
You can fix this by buffering and un-buffering the shape.
Here’s an example of a polygon with a small crack:
from shapely.geometry import Polygon
bad_polygon = Polygon([[0, 0], [1, 0], [1, 0.4999], [0.5, 0.5], [1, 0.5001], [1, 1], [0, 1], [0, 0]])
To fix it, expand the shape slightly, and contract it the same amount, using the buffer() method.
tol = 1e-4
bad_polygon.buffer(tol).buffer(-tol)
The value tol must be at least as large as half the distance across the crack at the crack’s widest point.
I used shapely.ops.unary_union on a number of 6-sided shapely.geometry.Polygons, and obtained the following shape A:
Note how there are two "cracks" in the upper part. These are not intended, and are presumably caused by some floating-point edge cases.
If you construct another shape B that sits inside of A, and if A happens to intersect one of these "cracks", then A.covers(B) will be False!
In my particular case, this leads to a test suite failure, because A.covers(B) is supposed to be an invariant. Therefore I need to deal with this somehow. Is there some algorithm I can use to "seal" these cracks?
In practice, these "cracks" will not affect the functioning of the application, because we only care about the outer border of A covering B. Thus I am open to solutions that adjust the individual hexagons in order to make the cracks go away by introducing overlaps.
However, I cannot accept the outer border of this shape changing, because that would actually no longer be testing the application as it is intended to be used.
To summarize, I want the outcome to look like this (my hand-edited version):
You can fix this by buffering and un-buffering the shape.
Here’s an example of a polygon with a small crack:
from shapely.geometry import Polygon
bad_polygon = Polygon([[0, 0], [1, 0], [1, 0.4999], [0.5, 0.5], [1, 0.5001], [1, 1], [0, 1], [0, 0]])
To fix it, expand the shape slightly, and contract it the same amount, using the buffer() method.
tol = 1e-4
bad_polygon.buffer(tol).buffer(-tol)
The value tol must be at least as large as half the distance across the crack at the crack’s widest point.