fix(FXC-5154): add function to compute min radius

Author: marc-flex

tidy3d/components/tcad/simulation/heat_charge.py

@@ -125,12 +125,45 @@
 # define some limits for transient heat simulations
 TRANSIENT_HEAT_MAX_STEPS = 1000
 
-# OpenCASCADE minimum tolerance for cylinder radii
-OPENCASCADE_CYLINDER_RADIUS_TOL = 1e-6
+# Minimum tolerance for cylinder radii
+CYLINDER_RADIUS_TOL = 1e-6
 # Minimum radius as fraction of the larger radius (for tapered cylinders)
 MIN_CYLINDER_RADIUS_FRACTION = 0.01
 
 
+def _get_cylinder_radii_with_meshing_tol(geometry: Cylinder) -> tuple[float, float]:
+    """Get cylinder radii clamped to the minimum meshing tolerance.
+
+    OpenCASCADE has a minimum tolerance for radii; this function clamps
+    small or negative values to a small positive value to ensure valid
+    geometry that can be meshed. The minimum is set relative to the larger
+    radius to ensure meshability while still creating a reasonably sharp
+    tip for tapered cylinders.
+
+    Parameters
+    ----------
+    geometry : Cylinder
+        The cylinder geometry to get radii from.
+
+    Returns
+    -------
+    tuple[float, float]
+        ``(r1, r2)`` -- bottom and top radii, clamped to the minimum allowed radius.
+    """
+    r1 = geometry.radius_bottom
+    r2 = geometry.radius_top
+
+    min_radius = max(
+        CYLINDER_RADIUS_TOL,
+        MIN_CYLINDER_RADIUS_FRACTION * max(abs(r1), abs(r2)),
+    )
+
+    r1 = max(r1, min_radius)
+    r2 = max(r2, min_radius)
+
+    return r1, r2
+
+
 class TCADAnalysisTypes(str, Enum):
     """Enumeration of the types of simulations currently supported"""
 
@@ -376,31 +409,27 @@ def _warn_small_cylinder_radius(cls, val: tuple[Structure, ...]) -> tuple[Struct
                     r_top = base_geometry.radius_top
                     is_tapered = not np.isclose(r_bottom, r_top)
 
-                    # Compute minimum allowed radius (matches backend heat_mesh.py logic)
-                    min_radius = max(
-                        OPENCASCADE_CYLINDER_RADIUS_TOL,
-                        MIN_CYLINDER_RADIUS_FRACTION * max(abs(r_bottom), abs(r_top)),
-                    )
+                    r1, r2 = _get_cylinder_radii_with_meshing_tol(base_geometry)
 
-                    # Warn if radii are below minimum
+                    # Warn if radii were clamped
                     if is_tapered:
-                        if r_bottom < min_radius:
+                        if r1 > r_bottom:
                             log.warning(
                                 f"Cylinder 'radius_bottom' ({r_bottom:.3e}) is below the minimum "
-                                f"radius for meshing ({min_radius:.3e}). The sidewall angle may be "
+                                f"radius for meshing ({r1:.3e}). The sidewall angle may be "
                                 f"too steep. Will be clamped to minimum radius or mesh size, whichever is larger."
                             )
-                        if r_top < min_radius:
+                        if r2 > r_top:
                             log.warning(
                                 f"Cylinder 'radius_top' ({r_top:.3e}) is below the minimum "
-                                f"radius for meshing ({min_radius:.3e}). The sidewall angle may be "
+                                f"radius for meshing ({r2:.3e}). The sidewall angle may be "
                                 f"too steep. Will be clamped to minimum radius or mesh size, whichever is larger."
                             )
                     else:
-                        if r_bottom < min_radius:
+                        if r1 > r_bottom:
                             log.warning(
                                 f"Cylinder 'radius' ({r_bottom:.3e}) is below the minimum "
-                                f"radius for meshing ({min_radius:.3e}). "
+                                f"radius for meshing ({r1:.3e}). "
                                 f"Will be clamped to minimum radius or mesh size, whichever is larger."
                             )
         return val