fix bug in 2D material subdivision where invalid shapely geometry was…

… produced

CHANGELOG.md

@@ -36,6 +36,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix numerical precision issue in `FieldProjectionCartesianMonitor`.
 - Bug where lumped elements in the `Simulation` were being overwritten by the `TerminalComponentModeler`.
 - Bug in `Simulation.subsection` where lumped elements were not being correctly removed.
+- Bug when adding 2D structures to the `Simulation` that are composed of multiple overlapping polygons.
 
 ## [2.7.2] - 2024-08-07
 

tests/test_components/test_geometry.py

@@ -14,6 +14,7 @@
 from tidy3d.components.geometry.base import Planar
 from tidy3d.components.geometry.mesh import AREA_SIZE_THRESHOLD
 from tidy3d.components.geometry.utils import flatten_groups, traverse_geometries
+from tidy3d.components.geometry.utils_2d import subdivide
 from tidy3d.constants import LARGE_NUMBER
 from tidy3d.exceptions import SetupError, Tidy3dKeyError, ValidationError
 
@@ -956,3 +957,20 @@ def test_update_from_bounds():
     for geom2d in geometries:
         with pytest.raises(NotImplementedError):
             geom_update = geom2d._update_from_bounds(bounds=new_bounds, axis=axis)
+
+
+def test_subdivide():
+    # Test the functionality that subdivides structures with Medium2D into partitions,
+    # where each partition is paired with homogeneous medium above and below
+    box = td.Box(size=(1, 1, 0))
+    overlap_box = td.Box(size=(2, 0.5, 0), center=(0.5, 0, 0))
+    # These overlapping boxes have their left edge coincident, and the second box has a smaller left edge.
+    # This results in an invalid geometry for MultiPolygon which must be fixed by applying a union
+    # operation in ``subdivide``. This should fix other types of invalid geometries as well.
+    overlapping_boxes = td.GeometryGroup(geometries=(box, overlap_box))
+
+    background_structure = td.Structure(medium=td.Medium(), geometry=td.Box(size=(10, 10, 10)))
+    subdivisions = subdivide(
+        geom=overlapping_boxes, axis=2, axis_dl=1e-5, structures=[background_structure]
+    )
+    assert len(subdivisions) == 1

tidy3d/components/geometry/utils_2d.py

@@ -122,15 +122,13 @@ def to_multipolygon(shapely_geometry) -> shapely.MultiPolygon:
     plane = {coord: center}
 
     # Convert input geometry into MultiPolygon shapely geometry and track the original structure that references the media properties
-    geom_shapely = Geometry.evaluate_inf_shape(
-        shapely.MultiPolygon(geom.intersections_plane(**plane))
-    )
+    geom_shapely = Geometry.evaluate_inf_shape(shapely.union_all(geom.intersections_plane(**plane)))
 
     plane[coord] = center + check_delta[1]
     above_shapely = [
         (
             Geometry.evaluate_inf_shape(
-                shapely.MultiPolygon(structure.geometry.intersections_plane(**plane))
+                shapely.union_all(structure.geometry.intersections_plane(**plane))
             ),
             structure,
         )
@@ -141,7 +139,7 @@ def to_multipolygon(shapely_geometry) -> shapely.MultiPolygon:
     below_shapely = [
         [
             Geometry.evaluate_inf_shape(
-                shapely.MultiPolygon(structure.geometry.intersections_plane(**plane))
+                shapely.union_all(structure.geometry.intersections_plane(**plane))
             ),
             structure,
         ]