fix styles update on material assignment/unassignment #4843

src/blenderbim/blenderbim/bim/module/model/product.py

@@ -18,6 +18,7 @@
 
 import bpy
 import mathutils
+import numpy as np
 from functools import reduce
 import ifcopenshell
 import ifcopenshell.api
@@ -39,7 +40,7 @@ from mathutils import Vector, Matrix
 from bpy_extras.object_utils import AddObjectHelper
 from . import prop
 import json
-from typing import Any, Union
+from typing import Any, Union, Optional
 
 
 class EnableAddType(bpy.types.Operator, tool.Ifc.Operator):
@@ -513,88 +514,19 @@ def regenerate_profile_usage(usecase_path, ifc_file, settings):
 
 
 def ensure_material_assigned(usecase_path: str, ifc_file: ifcopenshell.file, settings: dict[str, Any]) -> None:
-    if usecase_path == "material.assign_material":
-        if not settings.get("material", None):
-            return
-        elements = settings["products"]
-    else:
-        elements = []
-        for rel in ifc_file.by_type("IfcRelAssociatesMaterial"):
-            if rel.RelatingMaterial == settings["material"] or [
-                e for e in ifc_file.traverse(rel.RelatingMaterial) if e == settings["material"]
-            ]:
-                elements.extend(rel.RelatedObjects)
+    elements = settings["products"]
 
-    update_blender_ifc_materials(elements)
+    for element in elements[:]:
+        if element.is_a("IfcElementType"):
+            elements.extend(tool.Model.get_occurrences_without_material_override(element))
+
+    tool.Model.apply_ifc_material_changes(elements, assigned_material=settings["material"])
 
 
 def ensure_material_unassigned(usecase_path: str, ifc_file: ifcopenshell.file, settings: dict[str, Any]) -> None:
     elements = settings["products"]
-    if elements[0].is_a("IfcElementType"):
-        elements.extend(ifcopenshell.util.element.get_types(elements[0]))
-    update_blender_ifc_materials(elements)
 
-
-def update_blender_ifc_materials(elements: list[ifcopenshell.entity_instance]) -> None:
-    """update mesh blender materials that have ifc material connected to them
-    by replacing them with `blender_material`"""
-    # since different elements can share meshes (e.g. occurrecnes without openings)
-    # we need to make sure not to affect them accidentally
-    meshes_users: dict[bpy.types.Mesh, set[bpy.types.Object]] = dict()
-    for obj in bpy.data.objects:
-        if not obj.data:
-            continue
-        meshes_users.setdefault(obj.data, set()).add(obj)
-
-    objects: set[bpy.types.Object] = set()
-    for element in elements:
-        obj: bpy.types.Object = tool.Ifc.get_object(element)
-        if not obj or not obj.data:
-            continue
-        objects.add(obj)
-
-    meshes: set[bpy.types.Mesh] = {obj.data for obj in objects}
-
-    for mesh in meshes:
-        mesh_users = meshes_users[mesh]
-        if not mesh_users.issubset(objects):
-            continue
-
-        # NOTE: we need `obj` as removing materials and appending them to `mesh.materials`
-        # will mess up mesh faces material indices
-
-        # NOTE: we make an assumption here that all mesh users
-        # have the same material - they either inherit it from the type
-        # or type doesn't have a material.
-        #
-        # If we add option to UI to add materials overriding type materials
-        # then this assumption won't be safe anymore
-
-        obj = next(iter(mesh_users))
-        element = tool.Ifc.get_entity(obj)
-        current_material = ifcopenshell.util.element.get_material(element)
-        if current_material:
-            current_material = tool.Ifc.get_object(current_material)
-
-        material_replaced = False
-
-        for material_slot in obj.material_slots:
-            material = material_slot.material
-            if material is None:
-                continue
-            ifc_material = tool.Ifc.get_entity(material)
-            # it's blender material for style, so ignore it
-            if not ifc_material:
-                continue
-            if ifc_material == current_material:
-                continue
-            material_slot.material = current_material
-            material_replaced = True
-
-        if not material_replaced and current_material:
-            mesh.materials.append(current_material)
-
-        # clear empty slots
-        for i, material in reversed(list(enumerate(mesh.materials[:]))):
-            if material is None:
-                mesh.materials.pop(index=i)
+    for element in elements[:]:
+        if element.is_a("IfcElementType"):
+            elements.extend(tool.Model.get_occurrences_without_material_override(element))
+    tool.Model.apply_ifc_material_changes(elements)

src/blenderbim/blenderbim/core/geometry.py

@@ -128,8 +128,9 @@ def switch_representation(
     if not current_obj_data and geometry.is_text_literal(representation):
         return
 
-    has_openings = apply_openings and getattr(entity, "HasOpenings", None)
-    if has_openings:
+    use_immediate_repr = apply_openings and getattr(entity, "HasOpenings", None)
+    use_immediate_repr = use_immediate_repr or geometry.has_material_style_override(entity)
+    if use_immediate_repr:
         # if it has openings make sure to switch to element's mapped representation
         representation = geometry.unresolve_type_representation(representation, entity)
     else:
@@ -145,7 +146,7 @@ def switch_representation(
     else:
         new_repr_data = old_repr_data
 
-    geometry.change_object_data(obj, new_repr_data, is_global=is_global and not has_openings)
+    geometry.change_object_data(obj, new_repr_data, is_global=is_global and not use_immediate_repr)
     geometry.record_object_materials(obj)
 
     # we assume that all the occurences and the type have the same representation context active

src/blenderbim/blenderbim/tool/geometry.py

@@ -508,6 +508,18 @@ class Geometry(blenderbim.core.tool.Geometry):
             return bool(obj.data.splines)
         return False
 
+    @classmethod
+    def has_material_style_override(cls, element: ifcopenshell.entity_instance) -> bool:
+        if element.is_a("IfcTypeProduct"):
+            return False
+        own_material = ifcopenshell.util.element.get_material(element, should_inherit=False)
+        if own_material:
+            inherited_style = cls.get_inherited_material_style(element)
+            style = tool.Material.get_style(own_material) if own_material else None
+            if inherited_style != style:
+                return True
+        return False
+
     @classmethod
     def import_representation(cls, obj, representation, apply_openings=True):
         logger = logging.getLogger("ImportIFC")
@@ -802,17 +814,29 @@ class Geometry(blenderbim.core.tool.Geometry):
         meshes_to_objects = {(obj := tool.Ifc.get_object(element)).data: obj for element in elements}
 
         for obj in meshes_to_objects.values():
-            representation = tool.Ifc.get().by_id(obj.data.BIMMeshProperties.ifc_definition_id)
-            blenderbim.core.geometry.switch_representation(
-                tool.Ifc,
-                tool.Geometry,
-                obj=obj,
-                representation=representation,
-                should_reload=True,
-                is_global=True,
-                should_sync_changes_first=False,
-                apply_openings=True,
-            )
+            cls._reload_representation(obj)
+
+    @classmethod
+    def _reload_representation(cls, obj: bpy.types.Object) -> None:
+        """Reload representation only for this object.
+
+        Be careful as this method won't reload representation for related objects
+        that use the same representation but have different meshes
+        (e.g. because of the openings).
+        In the most cases just use reload_representation
+        as it will handle those complications by itself.
+        """
+        representation = tool.Ifc.get().by_id(obj.data.BIMMeshProperties.ifc_definition_id)
+        blenderbim.core.geometry.switch_representation(
+            tool.Ifc,
+            tool.Geometry,
+            obj=obj,
+            representation=representation,
+            should_reload=True,
+            is_global=True,
+            should_sync_changes_first=False,
+            apply_openings=True,
+        )
 
     @classmethod
     def remove_representation_item(cls, representation_item):
@@ -1010,3 +1034,64 @@ class Geometry(blenderbim.core.tool.Geometry):
             if (result := obj.BIMObjectProperties.blender_offset_type) == "NONE":
                 result = obj.BIMObjectProperties.blender_offset_type = "OBJECT_PLACEMENT"
             return result
+
+    @classmethod
+    def has_geometry_without_styles(cls, mesh: bpy.types.Mesh) -> bool:
+        """Check if mesh has geometry without styles.
+
+        Detects geometry without styles based on how
+        MaterialCreator works - will check if either
+        mesh has no material slots or has an empty material slot.
+        """
+        return not mesh.materials or any(m is None for m in mesh.materials)
+
+    @classmethod
+    def get_representation_styles(
+        cls, representation: ifcopenshell.entity_instance
+    ) -> set[ifcopenshell.entity_instance]:
+        """Return a set of styles assigned to the representation directly."""
+
+        styles = set()
+
+        # Get all stylable representation items.
+        items = []
+        for item in representation.Items:
+            if item.is_a("IfcMappedItem"):
+                items.extend(item.MappingSource.MappedRepresentation.Items)
+            if item.is_a("IfcBooleanResult"):
+                operand = item.FirstOperand
+                while True:
+                    items.append(operand)
+                    if operand.is_a("IfcBooleanResult"):
+                        operand = operand.FirstOperand
+                    else:
+                        break
+            items.append(item)
+
+        for item in items:
+            if not item.StyledByItem:
+                continue
+            current_styles = list(item.StyledByItem[0].Styles)
+            while current_styles:
+                style = current_styles.pop()
+                if style.is_a("IfcPresentationStyle"):
+                    styles.add(style)
+                elif style.is_a("IfcPresentationStyleAssignment"):
+                    current_styles.extend(style.Styles)
+
+        return styles
+
+    @classmethod
+    def get_inherited_material_style(
+        cls, element: ifcopenshell.entity_instance
+    ) -> Union[ifcopenshell.entity_instance, None]:
+        if element.is_a("IfcTypeProduct"):
+            return
+        element_type = ifcopenshell.util.element.get_type(element)
+        if not element_type:
+            return
+        materials = ifcopenshell.util.element.get_materials(element_type)
+        if not materials:
+            return
+        material_style = tool.Material.get_style(materials[0])
+        return material_style

src/blenderbim/blenderbim/tool/model.py

@@ -1255,3 +1255,144 @@ class Model(blenderbim.core.tool.Model):
         if fillings:
             with bpy.context.temp_override(selected_objects=list(fillings.values())):
                 bpy.ops.bim.recalculate_fill()
+
+    @classmethod
+    def apply_ifc_material_changes(
+        cls,
+        elements: list[ifcopenshell.entity_instance],
+        assigned_material: Optional[ifcopenshell.entity_instance] = None,
+    ) -> None:
+        """Update mesh blender materials for provided elements after material assignment/unassignment.
+
+        `assigned_material` argument is there just to indicate whether we apply material changes
+        after material assignment or material unassignment.
+        """
+
+        style_object = None
+        if assigned_material:
+            # NOTE: currently only IfcMaterials are supported
+            # for anyone else we just switch representation.
+            if not assigned_material.is_a("IfcMaterial"):
+                tool.Geometry.reload_representation([tool.Ifc.get_object(e) for e in elements])
+                return
+
+        # Since different elements can share meshes (e.g. occurrences without openings)
+        # we need to make sure not to process them multiple times.
+        meshes_users: dict[bpy.types.Mesh, set[bpy.types.Object]] = dict()
+        for obj in bpy.data.objects:
+            if not obj.data:
+                continue
+            meshes_users.setdefault(obj.data, set()).add(obj)
+
+        objects: set[bpy.types.Object] = set()
+        for element in elements:
+            obj: bpy.types.Object = tool.Ifc.get_object(element)
+            if not obj or not obj.data:
+                continue
+            objects.add(obj)
+
+        meshes: set[bpy.types.Mesh] = {obj.data for obj in objects}
+
+        for mesh in meshes:
+            mesh_users = meshes_users[mesh]
+
+            if not mesh_users.issubset(objects):
+                # It's unsafe to make changes to the mesh
+                # as it's used by objects unrelated to the current change.
+                # E.g. material with a style was assigned to a particular occurrence
+                # and this change shouldn't be applied to other occurrences and type.
+                objs_to_reload = mesh_users.intersection(objects)
+                for obj in objs_to_reload:
+                    tool.Geometry._reload_representation(obj)
+                continue
+
+            obj = next(iter(mesh_users))
+            element = tool.Ifc.get_entity(obj)
+            own_material = ifcopenshell.util.element.get_material(element, should_inherit=False)
+            inherited_mstyle = tool.Geometry.get_inherited_material_style(element)
+
+            if assigned_material:
+                if own_material:
+                    ms2 = tool.Material.get_style(own_material)
+                    ms1 = inherited_mstyle
+                else:
+                    ms1 = None
+                    ms2 = inherited_mstyle
+
+                cls.replace_material_style(mesh, obj, ms1, ms2)
+
+            else:  # Material unnassignment.
+                if not tool.Geometry.has_geometry_without_styles(mesh):
+                    tool.Geometry._reload_representation(obj)
+                    continue
+
+                if not inherited_mstyle:
+                    continue
+
+                cls.replace_material_style(mesh, obj, None, inherited_mstyle)
+
+    @classmethod
+    def get_occurrences_without_material_override(
+        cls, element_type: ifcopenshell.entity_instance
+    ) -> list[ifcopenshell.entity_instance]:
+        occurrences = [
+            e
+            for e in ifcopenshell.util.element.get_types(element_type)
+            if not ifcopenshell.util.element.get_material(e, should_inherit=False)
+        ]
+        return occurrences
+
+    @classmethod
+    def replace_material_style(
+        cls,
+        mesh: bpy.types.Mesh,
+        obj: bpy.types.Object,
+        mstyle1: Union[ifcopenshell.entity_instance, None],
+        mstyle2: Union[ifcopenshell.entity_instance, None],
+    ) -> None:
+        if mstyle1 == mstyle2:
+            return
+
+        # Get Blender materials.
+        mbstyle1, mbstyle2 = None, None
+        if mstyle1:
+            mbstyle1 = tool.Ifc.get_object(mstyle1)
+            assert isinstance(mbstyle1, bpy.types.Material)
+        if mstyle2:
+            mbstyle2 = tool.Ifc.get_object(mstyle2)
+            assert isinstance(mbstyle2, bpy.types.Material)
+
+        # Copy data to the list as mesh.materials doesn't allow to search for None.
+        materials: list[Union[bpy.types.Material, None]] = mesh.materials[:]
+        # Material style is overridden by representation item, nothing to change.
+        if mesh.materials and mbstyle1 not in materials:
+            return
+
+        i1 = None
+        if mbstyle1 is None:
+            if not materials:
+                mesh.materials.append(None)
+                i1 = 0
+            else:
+                i1 = materials.index(None)
+        else:
+            rep = tool.Geometry.get_active_representation(obj)
+            assert rep  # Type checker.
+            rep_styles = tool.Geometry.get_representation_styles(rep)
+            if mbstyle1 in rep_styles:
+                tool.Geometry._reload_representation(obj)
+                return
+            else:
+                i1 = materials.index(mbstyle1)
+
+        if mbstyle2 in materials:
+            i2 = materials.index(mbstyle2)
+            # Reassign faces.
+            buffer = np.empty(len(mesh.polygons), dtype=np.int32)
+            mesh.polygons.foreach_get("material_index", buffer)
+            buffer[buffer == i1] = i2
+            mesh.polygons.foreach_set("material_index", buffer)
+
+            mesh.materials.pop(index=i1)
+        else:
+            mesh.materials[i1] = mbstyle2

src/blenderbim/test/tool/test_model.py

@@ -18,13 +18,18 @@
 
 import bpy
 import ifcopenshell
+import ifcopenshell.api.material
+import ifcopenshell.api.style
+import ifcopenshell.util.element
+import ifcopenshell.util.representation
+import ifcopenshell.util.unit
 import blenderbim.core.tool
 import blenderbim.tool as tool
 import numpy as np
 import json
 from test.bim.bootstrap import NewFile
 from blenderbim.tool.model import Model as subject
-from ifcopenshell.util.shape_builder import V
+from ifcopenshell.util.shape_builder import V, ShapeBuilder
 
 
 class TestImplementsTool(NewFile):
@@ -438,3 +443,212 @@ class TestUsingArrays(NewFile):
             element = tool.Ifc.get_entity(obj)
             pset = ifcopenshell.util.element.get_pset(element, "BBIM_Array")
             assert pset is None, (obj, pset)
+
+
+class TestApplyIfcMaterialChanges(NewFile):
+    def get_used_styles(self, obj: bpy.types.Object) -> set[ifcopenshell.entity_instance]:
+        ifc_file = tool.Ifc.get()
+        return {ifc_file.by_id(s.material.BIMMaterialProperties.ifc_style_id) for s in obj.material_slots if s.material}
+
+    def get_mesh(self, obj: bpy.types.Object) -> bpy.types.Mesh:
+        mesh = obj.data
+        assert isinstance(mesh, bpy.types.Mesh)
+        return mesh
+
+    def setup_test(self, and_elements: bool = True) -> None:
+        bpy.context.scene.BIMProjectProperties.template_file = "0"
+        bpy.context.scene.unit_settings.length_unit = "MILLIMETERS"
+        bpy.ops.bim.create_project()
+        ifc_file = tool.Ifc.get()
+
+        # Setup materials and styles.
+        context = ifcopenshell.util.representation.get_context(ifc_file, "Model", "Body", "MODEL_VIEW")
+        assert context  # Type checker.
+
+        red_material = ifcopenshell.api.material.add_material(ifc_file, "Red Material")
+        bpy.ops.bim.load_styles(style_type="IfcSurfaceStyle")
+        bpy.ops.bim.enable_adding_presentation_style()
+        bpy.data.scenes["Scene"].BIMStylesProperties.style_name = "Red"
+        bpy.ops.bim.add_presentation_style()
+        red_style = next((i for i in ifc_file.by_type("IfcSurfaceStyle") if i.Name == "Red"))
+        ifcopenshell.api.style.assign_material_style(ifc_file, red_material, red_style, context)
+
+        blue_material = ifcopenshell.api.material.add_material(ifc_file, "Blue Material")
+        bpy.ops.bim.enable_adding_presentation_style()
+        bpy.data.scenes["Scene"].BIMStylesProperties.style_name = "Blue"
+        bpy.ops.bim.add_presentation_style()
+        blue_style = next((i for i in ifc_file.by_type("IfcSurfaceStyle") if i.Name == "Blue"))
+        ifcopenshell.api.style.assign_material_style(ifc_file, blue_material, blue_style, context)
+
+        bpy.ops.bim.enable_adding_presentation_style()
+        bpy.data.scenes["Scene"].BIMStylesProperties.style_name = "Green"
+        bpy.ops.bim.add_presentation_style()
+
+        if and_elements:
+            self.setup_elements()
+
+    def setup_elements(self) -> None:
+        ifc_file = tool.Ifc.get()
+        blue_material = next((i for i in ifc_file.by_type("IfcMaterial") if i.Name == "Blue Material"))
+        blue_style = tool.Material.get_style(blue_material)
+
+        # Element type.
+        bpy.ops.mesh.primitive_cube_add(size=10, location=(0, 0, 4))
+        element_type_obj = bpy.data.objects["Cube"]
+        bpy.ops.bim.assign_class(ifc_class="IfcActuatorType", predefined_type="ELECTRICACTUATOR", userdefined_type="")
+        element_type = tool.Ifc.get_entity(element_type_obj)
+
+        # Setup occurrences.
+        relating_type_id = element_type.id()
+        bpy.ops.bim.add_constr_type_instance(relating_type_id=relating_type_id)
+        simple = bpy.context.active_object
+        simple.name = "Simple"
+
+        # Occurrence with an opening.
+        bpy.ops.bim.add_constr_type_instance(relating_type_id=relating_type_id)
+        with_opening = bpy.context.active_object
+        with_opening.name = "With Opening"
+        bpy.ops.bim.add_potential_opening()
+        tool.Blender.set_objects_selection(
+            bpy.context, active_object=with_opening, selected_objects=[with_opening, bpy.data.objects["Opening"]]
+        )
+        bpy.ops.bim.add_opening()
+
+        # Occurrence with a material override.
+        bpy.ops.bim.add_constr_type_instance(relating_type_id=relating_type_id)
+        with_material = bpy.context.active_object
+        with_material.name = "With Material"
+        tool.Blender.set_objects_selection(bpy.context, active_object=with_material, selected_objects=[with_material])
+
+        ifcopenshell.api.material.assign_material(
+            ifc_file, products=[tool.Ifc.get_entity(with_material)], material=blue_material
+        )
+
+        assert self.get_used_styles(element_type_obj) == set()
+        for element in ifc_file.by_type("IfcActuator"):
+            obj = tool.Ifc.get_object(element)
+            expected = {blue_style} if obj.name == "With Material" else set()
+            assert self.get_used_styles(obj) == expected
+
+    def test_element_type_and_occurrences(self):
+        self.setup_test()
+        ifc_file = tool.Ifc.get()
+        element_type = next(ifc_file.by_type("IfcActuatorType").__iter__())
+        red_material = next((i for i in ifc_file.by_type("IfcMaterial") if i.Name == "Red Material"))
+        red_style = tool.Material.get_style(red_material)
+        blue_style = next((i for i in ifc_file.by_type("IfcSurfaceStyle") if i.Name == "Blue"))
+
+        ifcopenshell.api.material.assign_material(ifc_file, material=red_material, products=[element_type])
+        assert self.get_used_styles(tool.Ifc.get_object(element_type)) == {red_style}
+        for element in ifc_file.by_type("IfcActuator"):
+            obj = tool.Ifc.get_object(element)
+            expected = {blue_style} if obj.name == "With Material" else {red_style}
+            assert self.get_used_styles(obj) == expected
+
+        ifcopenshell.api.material.unassign_material(ifc_file, products=[element_type])
+        assert self.get_used_styles(tool.Ifc.get_object(element_type)) == set()
+        for element in ifc_file.by_type("IfcActuator"):
+            obj = tool.Ifc.get_object(element)
+            expected = {blue_style} if obj.name == "With Material" else set()
+            assert self.get_used_styles(obj) == expected
+
+    def test_dont_override_exisiting_styles(self):
+        self.setup_test()
+        ifc_file = tool.Ifc.get()
+        element_type = next(ifc_file.by_type("IfcActuatorType").__iter__())
+        red_material = next((i for i in ifc_file.by_type("IfcMaterial") if i.Name == "Red Material"))
+        green_style = next((i for i in ifc_file.by_type("IfcSurfaceStyle") if i.Name == "Green"))
+
+        # Occurrence with a style.
+        element_type_obj = tool.Ifc.get_object(element_type)
+        tool.Blender.set_objects_selection(
+            bpy.context, active_object=element_type_obj, selected_objects=[element_type_obj]
+        )
+        bpy.ops.bim.assign_style_to_selected(style_id=green_style.id())
+
+        ifcopenshell.api.material.assign_material(ifc_file, material=red_material, products=[element_type])
+        assert self.get_used_styles(tool.Ifc.get_object(element_type)) == {green_style}
+        for element in ifc_file.by_type("IfcActuator"):
+            obj = tool.Ifc.get_object(element)
+            assert self.get_used_styles(obj) == {green_style}
+
+        ifcopenshell.api.material.unassign_material(ifc_file, products=[element_type])
+        assert self.get_used_styles(tool.Ifc.get_object(element_type)) == {green_style}
+        for element in ifc_file.by_type("IfcActuator"):
+            obj = tool.Ifc.get_object(element)
+            assert self.get_used_styles(obj) == {green_style}
+
+    def test_assign_material_to_representation_that_has_2_items_and_1_item_has_a_style(self):
+        self.setup_test(and_elements=False)
+        ifc_file = tool.Ifc.get()
+        red_material = next((i for i in ifc_file.by_type("IfcMaterial") if i.Name == "Red Material"))
+        red_style = tool.Material.get_style(red_material)
+        green_style = next((i for i in ifc_file.by_type("IfcSurfaceStyle") if i.Name == "Green"))
+
+        bpy.ops.mesh.primitive_cube_add(size=10, location=(0, 0, 4))
+        obj = bpy.data.objects["Cube"]
+        bpy.ops.bim.assign_class(ifc_class="IfcActuator", predefined_type="ELECTRICACTUATOR", userdefined_type="")
+        element = tool.Ifc.get_entity(obj)
+        builder = ShapeBuilder(ifc_file)
+
+        # Change representation that consists of 2 rep items:
+        # 1 with style and other without.
+        rep = tool.Geometry.get_active_representation(obj)
+        assert rep
+        cube = rep.Items[0]
+        cube2 = builder.deep_copy(cube)
+        rep.Items = [cube, cube2]
+        tool.Style.assign_style_to_representation_item(cube, green_style)
+        tool.Geometry._reload_representation(obj)
+
+        def get_material_indices(mesh: bpy.types.Mesh) -> np.ndarray:
+            buffer = np.empty(len(mesh.polygons), dtype=np.int32)
+            mesh.polygons.foreach_get("material_index", buffer)
+            return buffer
+
+        mesh = self.get_mesh(obj)
+        assert len(mesh.materials) == 2
+        assert set(mesh.materials) == {bpy.data.materials["Green"], None}
+
+        ifcopenshell.api.material.assign_material(ifc_file, products=[element], material=red_material)
+        assert self.get_used_styles(obj) == {green_style, red_style}
+        ifcopenshell.api.material.unassign_material(ifc_file, products=[element])
+        mesh = self.get_mesh(obj)
+        assert len(mesh.materials) == 2
+        assert set(mesh.materials) == {bpy.data.materials["Green"], None}
+
+        # Test that if style is the same it would just reuse it.
+        tool.Style.assign_style_to_representation_item(cube, red_style)
+        tool.Geometry._reload_representation(obj)
+        mesh = self.get_mesh(obj)
+        assert len(mesh.materials) == 2
+        assert set(mesh.materials) == {bpy.data.materials["Red"], None}
+
+        ifcopenshell.api.material.assign_material(ifc_file, products=[element], material=red_material)
+        assert mesh.materials[:] == [bpy.data.materials["Red"]]
+        # All polygons are just reassigned to the existing material.
+        assert set(get_material_indices(mesh)) == {mesh.materials.find("Red")}
+
+        ifcopenshell.api.material.unassign_material(ifc_file, products=[element])
+        mesh = self.get_mesh(obj)
+        assert len(mesh.materials) == 2
+        assert set(mesh.materials) == {bpy.data.materials["Red"], None}
+        assert set(get_material_indices(mesh)) == {0, 1}
+
+    def test_assign_unassign_overriding_occurrence_material(self):
+        self.setup_test(and_elements=True)
+        ifc_file = tool.Ifc.get()
+        element_type = next(ifc_file.by_type("IfcActuatorType").__iter__())
+        red_material = next((i for i in ifc_file.by_type("IfcMaterial") if i.Name == "Red Material"))
+        no_style_material = ifcopenshell.api.material.add_material(ifc_file, "No Style")
+        obj = bpy.data.objects["Simple"]
+        element = tool.Ifc.get_entity(obj)
+
+        ifcopenshell.api.material.assign_material(ifc_file, material=red_material, products=[element_type])
+
+        # Override type material.
+        ifcopenshell.api.material.assign_material(ifc_file, material=no_style_material, products=[element])
+        assert self.get_mesh(obj).materials[:] == []
+
+        ifcopenshell.api.material.unassign_material(ifc_file, products=[element])
+        assert self.get_mesh(obj).materials[:] == [bpy.data.materials["Red"]]