fix[frontend]: fix computation of finite axis length for PolySlab

Author: Gregory Roberts

CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fixed issue with `CustomMedium` gradients where other frequencies would wrongly contribute to the gradient.
+- Fixed bug when computing `PolySlab` bounds in plotting functions.
 
 ## [2.8.3] - 2025-04-24
 

tests/test_components/test_geometry.py

@@ -307,6 +307,37 @@ def test_polyslab_bounds():
         td.PolySlab(vertices=((0, 0), (1, 0), (1, 1)), slab_bounds=(0.5, -0.5), axis=2)
 
 
+@pytest.mark.parametrize("axis", (0, 1, 2))
+def test_polyslab_inf_to_finite_bounds(axis):
+    """Test that finite_length_axis for PolySlab first clips at LARGE_NUMBER and then computes the length."""
+    axis_bound = 20
+    ps_low_inf = td.PolySlab(
+        axis=axis,
+        slab_bounds=[-td.inf, axis_bound],
+        vertices=[[0, 0], [2.5, 1], [2, 3], [0.5, 4], [-1.5, 2.5]],
+    )
+    ps_high_inf = td.PolySlab(
+        axis=axis,
+        slab_bounds=[-axis_bound, td.inf],
+        vertices=[[0, 0], [2.5, 1], [2, 3], [0.5, 4], [-1.5, 2.5]],
+    )
+    ps_inf = td.PolySlab(
+        axis=axis,
+        slab_bounds=[-td.inf, td.inf],
+        vertices=[[0, 0], [2.5, 1], [2, 3], [0.5, 4], [-1.5, 2.5]],
+    )
+
+    assert ps_low_inf.finite_length_axis == (
+        LARGE_NUMBER + axis_bound
+    ), "Unexpected finite length for polyslab axis with -inf bound"
+    assert ps_high_inf.finite_length_axis == (
+        LARGE_NUMBER + axis_bound
+    ), "Unexpected finite length for polyslab axis with inf bound"
+    assert (
+        ps_inf.finite_length_axis == 2 * LARGE_NUMBER
+    ), "Unexpected finite length for polyslab axis with two inf bounds"
+
+
 def test_validate_polyslab_vertices_valid():
     with pytest.raises(pydantic.ValidationError):
         POLYSLAB.copy(update=dict(vertices=(1, 2, 3)))

tidy3d/components/geometry/polyslab.py

@@ -388,6 +388,16 @@ def length_axis(self) -> float:
         zmin, zmax = self.slab_bounds
         return zmax - zmin
 
+    @property
+    def finite_length_axis(self) -> float:
+        """Gets the length of the PolySlab along the out of plane dimension.
+        First clips the slab bounds to LARGE_NUMBER and then returns difference.
+        """
+        zmin, zmax = self.slab_bounds
+        zmin = max(zmin, -LARGE_NUMBER)
+        zmax = min(zmax, LARGE_NUMBER)
+        return zmax - zmin
+
     @cached_property
     def reference_polygon(self) -> np.ndarray:
         """The polygon at the reference plane.