Stationing referent can optionally be located relative to the basis_curve (default) or the alignment curve

(cherry picked from commit b5c1b81ede)

src/ifcopenshell-python/ifcopenshell/api/alignment/add_stationing_referent.py

@@ -34,6 +34,7 @@ def add_stationing_referent(
     distance_along: float,
     station: float,
     incoming_station: Optional[float] = None,
+    on_basis_curve: Optional[bool] = None,
 ) -> entity_instance:
     """
     Adds an IfcReferent to the alignment that defines the stationing system.
@@ -43,6 +44,7 @@ def add_stationing_referent(
     :param distance_along: distance along the alignment basis curve
     :param station: station value
     :param incoming_station: station value of the incoming segment, only set to specify a station equation
+    :param on_basis_curve: whether the referent is positioned on the basis curve or the alignment curve, if None the function will default to the basis curve
     :return: referent
 
     Example:
@@ -53,11 +55,14 @@ def add_stationing_referent(
         ifcopenshell.api.alignment.add_stationing_referent(model,name="1+00.0",alignment=alignment,distance_along=0.0,station=100.0)
     """
 
-    basis_curve = ifcopenshell.api.alignment.get_basis_curve(alignment)
+    if on_basis_curve is None:
+        on_basis_curve = True
+
+    curve = ifcopenshell.api.alignment.get_basis_curve(alignment) if on_basis_curve else ifcopenshell.api.alignment.get_curve(alignment)
 
     object_placement = None
     representation = None
-    if basis_curve and basis_curve.is_a("IfcCompositeCurve") and 0 < len(basis_curve.Segments):
+    if curve and curve.is_a("IfcCompositeCurve") and 0 < len(curve.Segments):
         object_placement = file.createIfcLinearPlacement(
             RelativePlacement=file.createIfcAxis2PlacementLinear(
                 Location=file.createIfcPointByDistanceExpression(
@@ -65,7 +70,7 @@ def add_stationing_referent(
                     OffsetLateral=None,
                     OffsetVertical=None,
                     OffsetLongitudinal=None,
-                    BasisCurve=basis_curve,
+                    BasisCurve=curve,
                 )
             ),
         )

src/ifcopenshell-python/test/api/alignment/test_add_stationing_referent.py

@@ -0,0 +1,115 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2025 Thomas Krijnen <thomas@aecgeeks.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+
+import ifcopenshell.api.alignment
+import ifcopenshell.api.context
+import ifcopenshell.api.unit
+import ifcopenshell.util.element
+
+
+def _create_test_file():
+    file = ifcopenshell.file(schema="IFC4X3")
+    project = file.createIfcProject(GlobalId=ifcopenshell.guid.new(), Name="Test")
+    length = ifcopenshell.api.unit.add_si_unit(file, unit_type="LENGTHUNIT")
+    ifcopenshell.api.unit.assign_unit(file, units=[length])
+    geometric_representation_context = ifcopenshell.api.context.add_context(file, context_type="Model")
+    axis_model_representation_subcontext = ifcopenshell.api.context.add_context(
+        file,
+        context_type="Model",
+        context_identifier="Axis",
+        target_view="MODEL_VIEW",
+        parent=geometric_representation_context,
+    )
+    return file
+
+
+def _create_test_alignment_with_vertical(file):
+    # include_vertical=True so that get_curve() (IfcGradientCurve, on the "Axis" representation)
+    # and get_basis_curve() (IfcCompositeCurve, on the "FootPrint" representation) are different
+    # entities, letting the on_basis_curve option be observed.
+    alignment = ifcopenshell.api.alignment.create(file, "TestAlignment", include_vertical=True, start_station=0.0)
+    assert ifcopenshell.api.alignment.get_basis_curve(alignment).is_a("IfcCompositeCurve")
+    assert ifcopenshell.api.alignment.get_curve(alignment).is_a("IfcGradientCurve")
+    assert ifcopenshell.api.alignment.get_basis_curve(alignment) != ifcopenshell.api.alignment.get_curve(alignment)
+    return alignment
+
+
+def _assert_common_referent_asserts(referent, name, station):
+    assert referent.is_a("IfcReferent")
+    assert referent.PredefinedType == "STATION"
+    assert referent.Name == name
+    assert ifcopenshell.util.element.get_pset(element=referent, name="Pset_Stationing")
+    assert ifcopenshell.util.element.get_pset(element=referent, name="Pset_Stationing", prop="Station") == station
+    assert referent.ObjectPlacement != None
+
+
+def test_add_stationing_referent_on_basis_curve_none_defaults_to_basis_curve():
+    # on_basis_curve=None should behave the same as on_basis_curve=True
+    file = _create_test_file()
+    alignment = _create_test_alignment_with_vertical(file)
+
+    referent = ifcopenshell.api.alignment.add_stationing_referent(
+        file, "1+00.000", alignment, distance_along=100.0, station=100.0, on_basis_curve=None
+    )
+
+    _assert_common_referent_asserts(referent, "1+00.000", 100.0)
+
+    assert referent.ObjectPlacement.is_a("IfcLinearPlacement")
+    assert referent.ObjectPlacement.RelativePlacement.Location.BasisCurve == ifcopenshell.api.alignment.get_basis_curve(
+        alignment
+    )
+
+
+def test_add_stationing_referent_on_basis_curve_true():
+    file = _create_test_file()
+    alignment = _create_test_alignment_with_vertical(file)
+
+    referent = ifcopenshell.api.alignment.add_stationing_referent(
+        file, "1+00.000", alignment, distance_along=100.0, station=100.0, on_basis_curve=True
+    )
+
+    _assert_common_referent_asserts(referent, "1+00.000", 100.0)
+
+    assert referent.ObjectPlacement.is_a("IfcLinearPlacement")
+    assert referent.ObjectPlacement.RelativePlacement.Location.BasisCurve == ifcopenshell.api.alignment.get_basis_curve(
+        alignment
+    )
+
+
+def test_add_stationing_referent_on_basis_curve_false():
+    # with a vertical layout present, on_basis_curve=False positions the referent on the
+    # alignment curve (IfcGradientCurve) rather than on the basis curve (IfcCompositeCurve).
+    file = _create_test_file()
+    alignment = _create_test_alignment_with_vertical(file)
+
+    referent = ifcopenshell.api.alignment.add_stationing_referent(
+        file, "1+00.000", alignment, distance_along=100.0, station=100.0, on_basis_curve=False
+    )
+
+    _assert_common_referent_asserts(referent, "1+00.000", 100.0)
+
+    assert referent.ObjectPlacement.is_a("IfcLinearPlacement")
+    basis_curve = referent.ObjectPlacement.RelativePlacement.Location.BasisCurve
+    assert basis_curve == ifcopenshell.api.alignment.get_curve(alignment)
+    assert basis_curve != ifcopenshell.api.alignment.get_basis_curve(alignment)
+
+
+test_add_stationing_referent_on_basis_curve_none_defaults_to_basis_curve()
+test_add_stationing_referent_on_basis_curve_true()
+test_add_stationing_referent_on_basis_curve_false()