Store item_ids in imported meshes and set them on toggling edit mode

It will be used later to add more IFC capabilities to EDIT mode.

Example - https://imgur.com/a/HAkjoCY

src/blenderbim/blenderbim/bim/import_ifc.py

@@ -1387,6 +1387,7 @@ class IfcImporter:
                 #
                 # we do `.tolist()` because Blender can't assign `np.int32` to it's custom attributes
                 mesh["ios_edges"] = list(set(tuple(e) for e in ifcopenshell.util.shape.get_edges(geometry).tolist()))
+                mesh["ios_item_ids"] = ifcopenshell.util.shape.get_representation_item_ids(geometry).tolist()
 
                 mesh.vertices.add(num_vertices)
                 mesh.vertices.foreach_set("co", verts)

src/blenderbim/blenderbim/bim/module/geometry/operator.py

@@ -1651,6 +1651,7 @@ class OverrideModeSetEdit(bpy.types.Operator):
                         should_sync_changes_first=False,
                         apply_openings=False,
                     )
+                tool.Geometry.apply_item_ids_as_vertex_groups(obj)
                 tool.Geometry.dissolve_triangulated_edges(obj)
                 obj.data.BIMMeshProperties.mesh_checksum = tool.Geometry.get_mesh_checksum(obj.data)
             else:

src/blenderbim/blenderbim/tool/geometry.py

@@ -37,6 +37,7 @@ import blenderbim.core.system
 import blenderbim.core.geometry
 import blenderbim.tool as tool
 import blenderbim.bim.import_ifc
+from collections import defaultdict
 from math import radians, pi
 from mathutils import Vector, Matrix
 from blenderbim.bim.ifc import IfcStore
@@ -173,6 +174,39 @@ class Geometry(blenderbim.core.tool.Geometry):
             bm.free()
             del obj.data["ios_edges"]
 
+    @classmethod
+    def apply_item_ids_as_vertex_groups(cls, obj: bpy.types.Object) -> None:
+        """Save mesh-object item_ids as vertex groups in format 'ios_item_id_xxxx'.
+
+        Since ios_item_ids are item ids for original faces (triangulated),
+        this method should be used before `dissolve_triangulated_edges`."""
+
+        mesh = obj.data
+        assert isinstance(mesh, bpy.types.Mesh)
+        # I guess, they're already applied.
+        if "ios_item_ids" not in mesh:
+            return
+
+        # Just to be safe.
+        if "ios_edges" not in mesh:
+            raise Exception("Triangulated edges are already dissolved, cannot aply item ids.")
+
+        polygon_verts = np.empty(len(mesh.polygons) * 3, dtype="I")
+        mesh.polygons.foreach_get("vertices", polygon_verts)
+        polygon_verts = polygon_verts.reshape(-1, 3)
+
+        ios_item_ids: list[int] = mesh["ios_item_ids"]
+        vertices_by_item_ids = defaultdict(list[int])
+        for i, item_id in enumerate(ios_item_ids):
+            # .tolist() as VertexGroup.add() is not ready for uints.
+            vertices_by_item_ids[item_id].extend(polygon_verts[i].tolist())
+
+        for item_id, verts in vertices_by_item_ids.items():
+            vg = obj.vertex_groups.new(name=f"ios_item_id_{item_id}")
+            vg.add(verts, weight=1.0, type="ADD")
+
+        del mesh["ios_item_ids"]
+
     @classmethod
     def does_representation_id_exist(cls, representation_id: int) -> bool:
         try:

src/blenderbim/test/tool/test_geometry.py

@@ -727,3 +727,35 @@ class TestRemoveRepresentationItemFromShapeAspect(NewFile):
         subject.remove_representation_items_from_shape_aspect([items[1]], shape_aspect1)
         representation = shape_aspect1.ShapeRepresentations[0]
         assert set(representation.Items) == {items[0]}
+
+
+class TestApplyItemIdsAsVertexGroups(NewFile):
+    def test_run(self):
+        bpy.ops.mesh.primitive_cube_add(size=10, location=(0, 0, 4))
+        obj = bpy.data.objects["Cube"]
+        bpy.ops.object.editmode_toggle()
+        bpy.ops.mesh.quads_convert_to_tris()
+        bpy.ops.mesh.duplicate_move()
+        bpy.ops.object.editmode_toggle()
+
+        mesh = obj.data
+        assert isinstance(mesh, bpy.types.Mesh)
+        assert len(mesh.polygons) == 24
+
+        mesh["ios_edges"] = []  # Method requirement.
+        ios_item_ids = (95,) * 12 + (45,) * 12
+        mesh["ios_item_ids"] = ios_item_ids
+        unique = tuple(dict.fromkeys(ios_item_ids))
+        ios_item_ids_unique = [unique.index(i) for i in ios_item_ids]
+
+        tool.Geometry.apply_item_ids_as_vertex_groups(obj)
+
+        vertex_group_names = [vg.name for vg in obj.vertex_groups]
+        assert vertex_group_names == [f"ios_item_id_{i}" for i in unique]
+
+        verts = mesh.vertices
+        for i, polygon in enumerate(mesh.polygons):
+            for vi in polygon.vertices:
+                vert = verts[vi]
+                groups = [g.group for g in vert.groups]
+                assert groups == [ios_item_ids_unique[i]]

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

@@ -275,6 +275,11 @@ def get_faces(geometry: ShapeType) -> npt.NDArray[np.int32]:
     return np.array([[faces[i], faces[i + 1], faces[i + 2]] for i in range(0, len(faces), 3)])
 
 
+def get_representation_item_ids(geometry: ShapeType) -> npt.NDArray[np.int32]:
+    """Get representation item ids for the geometry faces."""
+    return np.frombuffer(geometry.item_ids_buffer, dtype="i")
+
+
 def get_shape_vertices(shape: ShapeType, geometry: ShapeType) -> npt.NDArray[np.float64]:
     """Get the shape's vertices as a numpy array