See #1227. Fix slicing normal calculation for AXIS3 elements.

src/bonsai/bonsai/tool/loader.py

@@ -1037,13 +1037,17 @@ class Loader(bonsai.core.tool.Loader):
         bm.from_mesh(mesh)
         prev_co = None
         # no = Vector((0.0, 1.0, 0.0))
-        no = cls.get_extrusion_vector(element).normalized()
+        if not usage:
-        if usage and usage.LayerSetDirection == "AXIS2":
+            sense_factor = 1  # Assume the extrusion vector points in the direction sense
+            no = cls.get_extrusion_vector(element).normalized()
+            co = Vector((0.0, 0.0, offset))
+        elif usage.LayerSetDirection == "AXIS2":
             co = Vector((0.0, offset, 0.0))
             no = no.cross(Vector([1.0, 0.0, 0.0]))
-        else:
+        elif usage.LayerSetDirection == "AXIS3":
-            sense_factor = 1  # If it isn't AXIS2, then the normal points in the direction sense
             co = Vector((0.0, 0.0, offset))
+            no = cls.get_extrusion_vector(element).normalized()
+            no = Vector([0.0, 0.0, 1.0 if no.z > 0 else -1.0])
         # Cache this
         body = ifcopenshell.util.representation.get_context(tool.Ifc.get(), "Model", "Body", "MODEL_VIEW")
         styles = {}

src/ifcopenshell-python/ifcopenshell/util/representation.py

@@ -463,10 +463,19 @@ def get_material_style(
                             return style
 
 
-def get_reference_line(wall: ifcopenshell.entity_instance, fallback_length: float = 1.0):
+def get_reference_line(wall: ifcopenshell.entity_instance, fallback_length: float = 1.0) -> list[npt.NDArray]:
     """Fetch the reference axis that goes in the +X direction
 
+    A base line will then be offset from this reference line based on the
+    material usage. From that base line, the layer thicknesses will offset
+    again, and be extruded to form the body representation.
+
     :param wall: ifcopenshell.entity_instance
+    :param fallback_length: If there is no reference axis, assume it starts at
+        the object placement (i.e. 0.0, 0.0) and extends for this fallback
+        length along the +X axis.
+    :return: A list of two 2D coordinates representing the start and end of the
+        axis. The axis always goes in the +X direction.
     """
     if axis := ifcopenshell.util.representation.get_representation(wall, "Plan", "Axis", "GRAPH_VIEW"):
         for item in ifcopenshell.util.representation.resolve_representation(axis).Items: