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FIX: contourf nested contours #2225

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Aug 4, 2023
Merged

FIX: contourf nested contours #2225

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15 changes: 10 additions & 5 deletions lib/cartopy/mpl/patch.py
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Original file line number Diff line number Diff line change
Expand Up @@ -168,15 +168,20 @@ def path_to_geos(path, force_ccw=False):
geom = sgeom.LineString(path_verts)

# If geom is a Polygon and is contained within the last geom in
# collection, add it to its list of internal polygons, otherwise
# simply append it as a new external geom.
# collection, it usually needs to be an interior to that geom (e.g. a
# lake within a land mass). Sometimes there is a further geom within
# this interior (e.g. an island in a lake, or some instances of
# contours). This needs to be a new external geom in the collection.
if geom.is_empty:
pass
elif (len(collection) > 0 and
isinstance(collection[-1][0], sgeom.Polygon) and
isinstance(geom, sgeom.Polygon) and
collection[-1][0].contains(geom.exterior)):
collection[-1][1].append(geom.exterior)
if any(internal.contains(geom) for internal in collection[-1][1]):
collection.append((geom, []))
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@rcomer rcomer Aug 4, 2023 •
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Although I’ve tested this across quite a few plots, I’m now questioning whether it is quite right. If we append the geom from the third level down to collection, could the next geom be at the second level down and so would need to check against something earlier than collection[-1]?

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@greglucas greglucas Aug 4, 2023 •
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Hm... that is a good question. I wonder if you could somehow keep track of the "last" geometry in the nested collections and sort of recursively drill down/out from there?

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I was thinking of this as a tree problem with the path segments ordered in a "depth first" way. However looking at matplotlib/matplotlib#25247, I think the paths that used to be separate are now just concatenated together. So anything at the second level has to be directly preceded by either something else at the second level or something at the first level, and so my above hypothetical can't happen for contours at least.

else:
collection[-1][1].append(geom)
elif isinstance(geom, sgeom.Point):
other_result_geoms.append(geom)
else:
Expand All @@ -188,8 +193,8 @@ def path_to_geos(path, force_ccw=False):
geom_collection = []
for external_geom, internal_polys in collection:
if internal_polys:
# XXX worry about islands within lakes
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I assume that XXX is the same as TODO and I'm taking a punt that this is now covered.

geom = sgeom.Polygon(external_geom.exterior, internal_polys)
exteriors = [geom.exterior for geom in internal_polys]
geom = sgeom.Polygon(external_geom.exterior, exteriors)
else:
geom = external_geom

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16 changes: 16 additions & 0 deletions lib/cartopy/tests/mpl/test_patch.py
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Expand Up @@ -43,3 +43,19 @@ def test_polygon_with_interior_and_singularity(self):
geoms = cpatch.path_to_geos(p)
assert [type(geom) for geom in geoms] == [sgeom.Polygon, sgeom.Point]
assert len(geoms[0].interiors) == 1

def test_nested_polygons(self):
# A geometry with three nested squares.
vertices = [[0, 0], [0, 10], [10, 10], [10, 0], [0, 0],
[2, 2], [2, 8], [8, 8], [8, 2], [2, 2],
[4, 4], [4, 6], [6, 6], [6, 4], [4, 4]]
codes = [1, 2, 2, 2, 79, 1, 2, 2, 2, 79, 1, 2, 2, 2, 79]
p = Path(vertices, codes=codes)
geoms = cpatch.path_to_geos(p)

# The first square makes the first geometry with the second square as
# its interior. The third square is its own geometry with no interior.
assert len(geoms) == 2
assert all(type(geom) == sgeom.Polygon for geom in geoms)
assert len(geoms[0].interiors) == 1
assert len(geoms[1].interiors) == 0