Add Flat/Pretty style toggle and dual-branch external style management

(cherry picked from commit c77a28c862)

src/bonsai/bonsai/bim/handler.py

@@ -418,9 +418,10 @@ def get_user(ifc: ifcopenshell.file) -> Union[ifcopenshell.entity_instance, None
 
 
 def viewport_shading_changed_callback(area: bpy.types.Area) -> None:
-    shading = area.spaces.active.shading.type
-    if shading == "RENDERED":
-        tool.Style.get_style_props().active_style_type = "External"
+    shading_type = area.spaces.active.shading.type
+    tool.Style.restore_material_style_types(shading_type)
+    if shading_type == "SOLID":
+        area.spaces.active.shading.color_type = "MATERIAL"
 
 
 def subscribe_to_viewport_shading_changes():

src/bonsai/bonsai/bim/import_ifc.py

@@ -133,10 +133,6 @@ class MaterialCreator:
                 if shape_has_openings and coords.is_a("IfcIndexedTextureMap"):
                     continue
                 tool.Loader.load_indexed_map(coords, self.mesh)
-            elif tool.Style.get_texture_style(material):
-                # No explicit coordinate mapping (e.g. IFC2X3 has no IsMappedBy,
-                # and IFC4 COORD uses generated UVs). Bake XY→UV as fallback.
-                tool.Loader.load_generated_uv_map(self.mesh)
 
     def assign_material_slots_to_faces(self) -> None:
         if not self.mesh["ios_materials"]:

src/bonsai/bonsai/bim/module/project/operator.py

@@ -1299,6 +1299,10 @@ class LoadProjectElements(bpy.types.Operator):
         tool.Project.set_default_modeling_dimensions()
         tool.Root.reload_grid_decorator()
         bonsai.bim.handler.refresh_ui_data()
+        for screen in bpy.data.screens:
+            for area in screen.areas:
+                if area.type == "VIEW_3D":
+                    bonsai.bim.handler.viewport_shading_changed_callback(area)
         return {"FINISHED"}
 
     def get_decomposition_elements(self) -> set[ifcopenshell.entity_instance]:

src/bonsai/bonsai/bim/module/style/__init__.py

@@ -45,7 +45,9 @@ classes = (
     operator.SelectByStyle,
     operator.SelectStyleInStylesUI,
     operator.SetAssetMaterialToExternalStyle,
+    operator.SuggestShadeFromExternalStyle,
     operator.UnlinkStyle,
+    operator.TogglePreferIfcShading,
     operator.UpdateCurrentStyle,
     operator.UpdateStyleColours,
     operator.UpdateStyleTextures,

src/bonsai/bonsai/bim/module/style/operator.py

@@ -16,6 +16,7 @@
 # You should have received a copy of the GNU General Public License
 # along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
 
+import colorsys
 import os
 from pathlib import Path
 from typing import Any, Union
@@ -238,13 +239,15 @@ class UpdateCurrentStyle(bpy.types.Operator):
             if not isinstance(obj.data, (bpy.types.Mesh, bpy.types.Curve)):
                 continue
             for mat in obj.data.materials:
-                if (
-                    mat
-                    and mat not in updated_materials
-                    and (msprops_ := tool.Style.get_material_style_props(mat)).ifc_definition_id != 0
-                ):
-                    msprops_.active_style_type = current_style_type
-                    updated_materials.add(mat)
+                if not mat:
+                    continue
+                msprops_ = tool.Style.get_material_style_props(mat)
+                if msprops_.ifc_definition_id == 0:
+                    continue
+                if mat in updated_materials:
+                    continue
+                msprops_.active_style_type = current_style_type
+                updated_materials.add(mat)
         return {"FINISHED"}
 
 
@@ -457,10 +460,14 @@ class ActivateExternalStyle(bpy.types.Operator):
             self.report({"ERROR"}, f"Error loading external style for \"{material.name}\" - {db['msg']}")
             return {"CANCELLED"}
 
-        self.copy_material_attributes(db["data_block"], material)
+        ext_mat = db["data_block"]
+        self.copy_material_attributes(ext_mat, material)
         if tool.Style.get_use_nodes(material):
-            tool.Blender.copy_node_graph(material, db["data_block"])
-        bpy.data.materials.remove(db["data_block"])
+            if material.get("bim_dual_branch"):
+                tool.Style.update_external_branch(material, ext_mat)
+            else:
+                tool.Style.setup_dual_branch(material, ext_mat)
+        bpy.data.materials.remove(ext_mat)
         return {"FINISHED"}
 
     def copy_material_attributes(self, source, target):
@@ -503,6 +510,218 @@ class ActivateExternalStyle(bpy.types.Operator):
             set_prop(prop_name)
 
 
+class TogglePreferIfcShading(bpy.types.Operator):
+    bl_idname = "bim.toggle_prefer_ifc_shading"
+    bl_label = "Toggle Flat/Pretty"
+    bl_description = (
+        "Toggle between Flat (IFC-native shading) and Pretty (external .blend style) for ALL styles.\n\n"
+        "SHIFT+CLICK to apply to this style only"
+    )
+    bl_options = {"REGISTER", "UNDO"}
+    material_name: bpy.props.StringProperty(name="Material Name", default="", options={"SKIP_SAVE"})
+    single_only: bpy.props.BoolProperty(name="Single Only", default=False, options={"SKIP_SAVE"})
+
+    def invoke(self, context, event):
+        if event.shift:
+            self.single_only = True
+        return self.execute(context)
+
+    def execute(self, context):
+        wm = context.window_manager
+        space = tool.Blender.get_view3d_space()
+        is_solid = space and space.shading.type == "SOLID"
+
+        if is_solid:
+            if space.shading.color_type == "TEXTURE":
+                space.shading.color_type = "MATERIAL"
+            else:
+                meshes_needing_uv = []
+                for obj in bpy.context.scene.objects:
+                    if not isinstance(obj.data, bpy.types.Mesh):
+                        continue
+                    for slot in obj.material_slots:
+                        mat = slot.material
+                        if not mat or not tool.Blender.get_ifc_definition_id(mat):
+                            continue
+                        style_elements = tool.Style.get_style_elements(mat)
+                        if style_elements.get("IfcSurfaceStyleWithTextures") and not obj.data.uv_layers:
+                            meshes_needing_uv.append(obj.data)
+                            break
+                wm.progress_begin(0, max(len(meshes_needing_uv), 1))
+                try:
+                    for i, mesh in enumerate(meshes_needing_uv):
+                        tool.Loader.load_generated_uv_map(mesh)
+                        wm.progress_update(i)
+                finally:
+                    wm.progress_end()
+                space.shading.color_type = "TEXTURE"
+            return {"FINISHED"}
+
+        if self.single_only:
+            mat = bpy.data.materials.get(self.material_name)
+            if not mat:
+                return {"CANCELLED"}
+            msprops = tool.Style.get_material_style_props(mat)
+            msprops.prefer_ifc_shading = not msprops.prefer_ifc_shading
+        else:
+            # Default: apply to all IFC materials
+            source_mat = bpy.data.materials.get(self.material_name)
+            new_value = not source_mat.BIMStyleProperties.prefer_ifc_shading if source_mat else True
+            ifc_mats = [m for m in bpy.data.materials if tool.Blender.get_ifc_definition_id(m)]
+            for mat in ifc_mats:
+                tool.Style.get_material_style_props(mat).prefer_ifc_shading = new_value
+        return {"FINISHED"}
+
+
+class SuggestShadeFromExternalStyle(bpy.types.Operator, tool.Ifc.Operator):
+    bl_idname = "bim.suggest_shade_from_external_style"
+    bl_label = "Suggest Shade from External Style"
+    bl_description = (
+        "Generate a Shade style (Surface Colour + Transparency) from the external .blend style.\n\n"
+        "ALT+CLICK to apply to all styles with an external .blend style"
+    )
+    bl_options = {"REGISTER", "UNDO"}
+    material_name: bpy.props.StringProperty(name="Material Name", default="", options={"SKIP_SAVE"})
+    all_styles: bpy.props.BoolProperty(name="All Styles", default=False, options={"SKIP_SAVE"})
+    value_offset: bpy.props.FloatProperty(
+        name="Value",
+        description="Offset added to the colour's value (-1 = fully dark, 0 = unchanged, +1 = fully light)",
+        default=0.0,
+        min=-1.0,
+        max=1.0,
+        step=1,
+        precision=2,
+        options={"SKIP_SAVE"},
+    )
+    saturation_factor: bpy.props.FloatProperty(
+        name="Saturation",
+        description="Scale applied to the colour's saturation (0 = greyscale, 1 = unchanged, >1 = more saturated)",
+        default=1.0,
+        min=0.0,
+        max=2.0,
+        step=1,
+        precision=2,
+        options={"SKIP_SAVE"},
+    )
+
+    def invoke(self, context, event):
+        if event.alt:
+            self.all_styles = True
+        return context.window_manager.invoke_props_dialog(self)
+
+    def draw(self, context):
+        layout = self.layout
+        layout.prop(self, "value_offset", slider=True)
+        layout.prop(self, "saturation_factor", slider=True)
+        if self.all_styles:
+            layout.label(text="Will apply to all external styles", icon="INFO")
+
+    def _execute(self, context):
+        if self.all_styles:
+            candidates = [
+                (mat, tool.Style.get_style_elements(mat))
+                for mat in bpy.data.materials
+                if tool.Blender.get_ifc_definition_id(mat)
+            ]
+            candidates = [(mat, se) for mat, se in candidates if tool.Style.has_blender_external_style(se)]
+            wm = context.window_manager
+            wm.progress_begin(0, max(len(candidates), 1))
+            count = 0
+            color_cache: dict[tuple[str, str, str], tuple | None] = {}
+            try:
+                for i, (mat, style_elements) in enumerate(candidates):
+                    wm.progress_update(i)
+                    if self._apply_to_material(
+                        mat, style_elements, self.value_offset, self.saturation_factor, color_cache
+                    ):
+                        count += 1
+            finally:
+                wm.progress_end()
+            self.report({"INFO"}, f"Shade style generated for {count} style(s).")
+        else:
+            mat = bpy.data.materials.get(self.material_name)
+            if not mat:
+                return {"CANCELLED"}
+            style_elements = tool.Style.get_style_elements(mat)
+            if not tool.Style.has_blender_external_style(style_elements):
+                self.report({"ERROR"}, "No external .blend style assigned. Please assign an external style first.")
+                return {"CANCELLED"}
+            self._apply_to_material(mat, style_elements, self.value_offset, self.saturation_factor)
+        props = tool.Style.get_style_props()
+        if props.is_editing:
+            core.load_styles(tool.Style, style_type=props.style_type)
+
+    def _apply_to_material(
+        self,
+        material: bpy.types.Material,
+        style_elements: dict,
+        value_offset: float = 0.0,
+        saturation_factor: float = 1.0,
+        color_cache: "dict[tuple[str, str, str], tuple | None] | None" = None,
+    ) -> bool:
+        external_style = style_elements["IfcExternallyDefinedSurfaceStyle"]
+        style_path = Path(tool.Ifc.resolve_uri(external_style.Location))
+        data_block_type, data_block = external_style.Identification.split("/")
+
+        cache_key = (str(style_path), data_block_type, data_block)
+        if color_cache is not None and cache_key in color_cache:
+            cached = color_cache[cache_key]
+            if cached is None:
+                return False  # previously failed for this path
+            surface_colour, transparency = cached
+        else:
+            try:
+                db = tool.Blender.append_data_block(str(style_path), data_block_type, data_block)
+            except OSError as e:
+                self.report({"WARNING"}, f'Could not open blend file for "{material.name}": {e}')
+                if color_cache is not None:
+                    color_cache[cache_key] = None
+                return False
+            if not db["data_block"]:
+                self.report({"WARNING"}, f'Could not load external style for "{material.name}": {db["msg"]}')
+                if color_cache is not None:
+                    color_cache[cache_key] = None
+                return False
+
+            ext_mat = db["data_block"]
+            surface_colour, transparency = tool.Style.get_representative_material_color(ext_mat)
+            bpy.data.materials.remove(ext_mat)
+            if color_cache is not None:
+                color_cache[cache_key] = (surface_colour, transparency)
+
+        if value_offset != 0.0 or saturation_factor != 1.0:
+            h, s, v = colorsys.rgb_to_hsv(*surface_colour)
+            v = max(0.0, min(1.0, v + value_offset))
+            s = max(0.0, min(1.0, s * saturation_factor))
+            surface_colour = colorsys.hsv_to_rgb(h, s, v)
+
+        ifc_style = tool.Ifc.get_entity(material)
+        attributes: dict = {
+            "SurfaceColour": {
+                "Name": None,
+                "Red": surface_colour[0],
+                "Green": surface_colour[1],
+                "Blue": surface_colour[2],
+            },
+        }
+        if tool.Ifc.get_schema() != "IFC2X3":
+            attributes["Transparency"] = transparency
+
+        shading_style = style_elements.get("IfcSurfaceStyleShading")
+        if shading_style:
+            tool.Ifc.run("style.edit_surface_style", style=shading_style, attributes=attributes)
+        else:
+            tool.Ifc.run(
+                "style.add_surface_style",
+                style=ifc_style,
+                ifc_class="IfcSurfaceStyleShading",
+                attributes=attributes,
+            )
+        material.diffuse_color = (*surface_colour, 1.0 - transparency)
+        tool.Style.sync_flat_branch_shading(material, surface_colour, transparency)
+        return True
+
+
 class DisableEditingStyles(bpy.types.Operator):
     bl_idname = "bim.disable_editing_styles"
     bl_options = {"REGISTER", "UNDO"}

src/bonsai/bonsai/bim/module/style/prop.py

@@ -372,6 +372,16 @@ def update_shading_style(self: "BIMStyleProperties", context: bpy.types.Context)
         tool.Style.switch_shading(blender_material, self.active_style_type)
 
 
+def update_prefer_ifc_shading(self: "BIMStyleProperties", context: bpy.types.Context) -> None:
+    style_elements = tool.Style.get_style_elements(self.id_data)
+    has_external = tool.Style.has_blender_external_style(style_elements)
+    if self.prefer_ifc_shading or not has_external:
+        self.active_style_type = "Shading"
+    else:
+        self.active_style_type = "External"
+    self.id_data.update_tag()
+
+
 class BIMStyleProperties(PropertyGroup):
     ifc_definition_id: IntProperty(name="IFC Definition ID")
     active_style_type: EnumProperty(
@@ -381,9 +391,19 @@ class BIMStyleProperties(PropertyGroup):
         default="Shading",
         update=update_shading_style,
     )
+    prefer_ifc_shading: BoolProperty(
+        name="Flat / Pretty",
+        description=(
+            "Toggle between Flat (IFC-native shading) and Pretty (external .blend style). "
+            "When set to Flat, viewport switches to Material Preview or Rendered will not activate the external style."
+        ),
+        default=False,
+        update=update_prefer_ifc_shading,
+    )
     is_renaming: BoolProperty(description="Used to prevent triggering handler callback.", default=False)
 
     if TYPE_CHECKING:
         ifc_definition_id: int
         active_style_type: tool.Style.StyleType
+        prefer_ifc_shading: bool
         is_renaming: bool

src/bonsai/bonsai/bim/module/style/ui.py

@@ -109,6 +109,11 @@ class BIM_PT_styles(Panel):
                 op = row.operator("bim.update_current_style", icon="FILE_REFRESH", text="")
                 op.style_id = style.ifc_definition_id
 
+            if active_style and self.props.style_type == "IfcSurfaceStyle":
+                if material := style.blender_material:
+                    msprops = tool.Style.get_material_style_props(material)
+                    self.draw_style_status_row(material, msprops)
+
             if self.props.style_type == "IfcSurfaceStyle":
                 self.layout.label(text="Surface Style Element:")
                 col = self.layout.column(align=True)
@@ -160,6 +165,87 @@ class BIM_PT_styles(Panel):
                 edit_label = "Save Lighting Style"
             self.draw_edit_ui(edit_label)
 
+    def draw_style_status_row(self, material: bpy.types.Material, msprops) -> None:
+        space = tool.Blender.get_view3d_space()
+        box = self.layout.box()
+
+        obj = bpy.context.active_object
+
+        parts = []
+        if space:
+            shading_type = space.shading.type
+            shading_labels = {
+                "SOLID": "Solid",
+                "MATERIAL": "Material Preview",
+                "RENDERED": "Rendered",
+                "WIREFRAME": "Wireframe",
+            }
+            parts.append(f"Viewport: {shading_labels.get(shading_type, shading_type)}")
+        else:
+            parts.append("No 3D viewport")
+            shading_type = None
+
+        if obj:
+            obj_has_uv = isinstance(obj.data, bpy.types.Mesh) and bool(obj.data.uv_layers)
+            uv_label = "UV \u2713" if obj_has_uv else "UV \u2717"
+            parts.append(f"Selected Object: {obj.name} {uv_label}")
+        else:
+            parts.append("Selected Object: None")
+
+        if shading_type == "SOLID":
+            is_flat = space.shading.color_type != "TEXTURE"
+            mode_label = "Flat"
+            dep_label = "Shade"
+            if not is_flat:
+                mode_label = "Pretty"
+                dep_label = "Texture \u2192 Shade"
+        elif shading_type in ("MATERIAL", "RENDERED"):
+            is_flat = msprops.prefer_ifc_shading
+            mode_label = "Flat"
+            dep_label = "Render+Texture \u2192 Render \u2192 Shade"
+            if not is_flat:
+                mode_label = "Pretty"
+                dep_label = "External \u2192 Render+Texture \u2192 Render \u2192 Shade"
+        else:
+            is_flat = False
+            mode_label = ""
+            dep_label = ""
+
+        row1 = box.row(align=True)
+        row1.label(text=" | ".join(parts))
+
+        if mode_label:
+            row2 = box.row(align=True)
+            row2.label(text=f"Current Mode: {mode_label} \u2014 {dep_label}")
+
+        row3 = box.row(align=True)
+        row3.alignment = "RIGHT"
+        op = row3.operator("bim.suggest_shade_from_external_style", text="", icon="BRUSHES_ALL")
+        op.material_name = material.name
+        op = row3.operator("bim.toggle_prefer_ifc_shading", text="", icon="UV_SYNC_SELECT")
+        op.material_name = material.name
+
+    @staticmethod
+    def _get_shader_label(material: bpy.types.Material, msprops) -> str:
+        space = tool.Blender.get_view3d_space()
+        if space and space.shading.type == "SOLID":
+            return "Not Applicable"
+        if msprops.active_style_type == "External":
+            return "External (.blend)"
+        if not material.node_tree:
+            return "Flat colour"
+        nodes = material.node_tree.nodes
+        has_mix = any(n.type == "MIX_SHADER" for n in nodes)
+        if has_mix:
+            return "Emission (Flat)"
+        has_principled = any(n.type == "BSDF_PRINCIPLED" for n in nodes)
+        has_teximage = any(n.type == "TEX_IMAGE" and any(o.links for o in n.outputs) for n in nodes)
+        if has_principled and has_teximage:
+            return "BSDF + Textures"
+        if has_principled:
+            return "Principled BSDF"
+        return "Flat colour"
+
     def draw_surface_style_shading(self):
         row = self.layout.row()
         row.prop(self.props, "surface_colour")

src/bonsai/bonsai/tool/blender.py

@@ -803,6 +803,40 @@ class Blender(bonsai.core.tool.Blender):
         # restore shader editor settings
         shader_editor.pin = previous_pin_setting
 
+    @classmethod
+    def copy_node_graph_additive(
+        cls, material_to: bpy.types.Material, material_from: bpy.types.Material
+    ) -> bpy.types.ShaderNodeOutputMaterial | None:
+        """Paste nodes from material_from alongside the existing nodes in material_to.
+
+        Unlike copy_node_graph this does NOT clear the existing node tree first.
+        Returns the OUTPUT_MATERIAL node that was added from material_from, or None.
+        """
+        temp_override = cls.get_shader_editor_context()
+        shader_editor = temp_override["space"]
+
+        before_names = {n.name for n in material_to.node_tree.nodes}
+
+        previous_pin_setting = shader_editor.pin
+        shader_editor.pin = True
+        shader_editor.node_tree = material_from.node_tree
+
+        for node in material_from.node_tree.nodes:
+            node.select = True
+        with bpy.context.temp_override(**temp_override):
+            bpy.ops.node.clipboard_copy()
+
+        shader_editor.node_tree = material_to.node_tree
+        with bpy.context.temp_override(**temp_override):
+            bpy.ops.node.clipboard_paste(offset=(0, 0))
+
+        shader_editor.pin = previous_pin_setting
+
+        for node in material_to.node_tree.nodes:
+            if node.name not in before_names and node.type == "OUTPUT_MATERIAL":
+                return node
+        return None
+
     @classmethod
     def get_material_node(
         cls, blender_material: bpy.types.Material, node_type: str, kwargs: Optional[dict] = {}

src/bonsai/bonsai/tool/style.py

@@ -592,6 +592,146 @@ class Style(bonsai.core.tool.Style):
         external_style = style_elements.get("IfcExternallyDefinedSurfaceStyle", None)
         return bool(external_style and external_style.Location and external_style.Location.endswith(".blend"))
 
+    @classmethod
+    def _color_from_principled(cls, node: bpy.types.Node) -> tuple[tuple[float, float, float], float]:
+        color = cls._resolve_color_socket(node.inputs["Base Color"])
+        alpha_socket = node.inputs["Alpha"]
+        alpha_source = cls._upstream_color_source(alpha_socket)
+        if alpha_source and alpha_source[0] == "IMAGE":
+            pixels = alpha_source[1].pixels[:]
+            n = len(pixels) // 4
+            step = max(1, n // 4096)
+            a_sum = sum(pixels[i * 4 + 3] for i in range(0, n, step))
+            count = len(range(0, n, step)) or 1
+            transparency = 1.0 - (a_sum / count)
+        else:
+            transparency = 1.0 - alpha_socket.default_value
+        return color, transparency
+
+    @classmethod
+    def _color_from_shader_socket(
+        cls, socket: bpy.types.NodeSocket, seen: set[str] | None = None
+    ) -> tuple[tuple[float, float, float], float] | None:
+        if seen is None:
+            seen = set()
+        for link in socket.links:
+            node = link.from_node
+            if node.name in seen:
+                continue
+            seen.add(node.name)
+            if node.type == "BSDF_PRINCIPLED":
+                return cls._color_from_principled(node)
+            if node.type in ("BSDF_DIFFUSE", "DIFFUSE_BSDF"):
+                return cls._resolve_color_socket(node.inputs["Color"]), 0.0
+            if node.type == "BSDF_GLASS":
+                return cls._resolve_color_socket(node.inputs["Color"]), 0.0
+            if node.type in ("MIX_SHADER", "ADD_SHADER"):
+                for inp in node.inputs:
+                    if inp.type == "SHADER" and inp.is_linked:
+                        result = cls._color_from_shader_socket(inp, seen)
+                        if result:
+                            return result
+        return None
+
+    @classmethod
+    def get_representative_material_color(
+        cls, material: bpy.types.Material
+    ) -> tuple[tuple[float, float, float], float]:
+        if material.node_tree:
+            nodes = material.node_tree.nodes
+            output_node = next(
+                (n for n in nodes if n.type == "OUTPUT_MATERIAL" and n.is_active_output), None
+            ) or next((n for n in nodes if n.type == "OUTPUT_MATERIAL"), None)
+            if output_node:
+                result = cls._color_from_shader_socket(output_node.inputs["Surface"])
+                if result:
+                    return result
+            # Fallback: scan all shader nodes if no output node or graph traversal found nothing
+            for node in nodes:
+                if node.type == "BSDF_PRINCIPLED":
+                    return cls._color_from_principled(node)
+            for node in nodes:
+                if node.type in ("BSDF_DIFFUSE", "DIFFUSE_BSDF"):
+                    return cls._resolve_color_socket(node.inputs["Color"]), 0.0
+            for node in nodes:
+                if node.type == "BSDF_GLASS":
+                    return cls._resolve_color_socket(node.inputs["Color"]), 0.0
+        color = tuple(material.diffuse_color[:3])
+        transparency = 1.0 - material.diffuse_color[3]
+        return color, transparency
+
+    @classmethod
+    def _collect_upstream_sources(cls, socket: bpy.types.NodeSocket, seen: set[str]) -> list[tuple[str, object]]:
+        """Recursively collect all upstream colour/image sources reachable from *socket*."""
+        results = []
+        for link in socket.links:
+            node = link.from_node
+            if node.name in seen:
+                continue
+            seen.add(node.name)
+            if node.type == "TEX_IMAGE":
+                results.append(("IMAGE", node.image))
+            elif node.type == "VALTORGB":
+                results.append(("COLORRAMP", node))
+            else:
+                for inp in node.inputs:
+                    if inp.is_linked:
+                        results.extend(cls._collect_upstream_sources(inp, seen))
+        return results
+
+    @classmethod
+    def _upstream_color_source(
+        cls, socket: bpy.types.NodeSocket, seen: set[str] | None = None
+    ) -> tuple[str, object] | None:
+        sources = cls._collect_upstream_sources(socket, set() if seen is None else seen)
+        # Prefer a concrete image texture over a colour ramp (which may be greyscale/procedural).
+        for s in sources:
+            if s[0] == "IMAGE":
+                return s
+        for s in sources:
+            if s[0] == "COLORRAMP":
+                return s
+        return None
+
+    @classmethod
+    def _resolve_color_socket(cls, socket: bpy.types.NodeSocket) -> tuple[float, float, float]:
+        source = cls._upstream_color_source(socket)
+        if source is None:
+            return tuple(socket.default_value[:3])
+        kind, obj = source
+        if kind == "IMAGE":
+            return cls._average_image_color(obj)
+        if kind == "COLORRAMP":
+            return cls._average_colorramp_color(obj)
+        return tuple(socket.default_value[:3])
+
+    @staticmethod
+    def _average_image_color(image: bpy.types.Image) -> tuple[float, float, float]:
+        pixels = image.pixels[:]
+        n = len(pixels) // 4
+        if n == 0:
+            return (0.5, 0.5, 0.5)
+        step = max(1, n // 4096)
+        r_sum = g_sum = b_sum = 0.0
+        count = 0
+        for i in range(0, n, step):
+            base = i * 4
+            r_sum += pixels[base]
+            g_sum += pixels[base + 1]
+            b_sum += pixels[base + 2]
+            count += 1
+        return (r_sum / count, g_sum / count, b_sum / count)
+
+    @staticmethod
+    def _average_colorramp_color(node: bpy.types.Node) -> tuple[float, float, float]:
+        elements = node.color_ramp.elements
+        if not elements:
+            return (0.5, 0.5, 0.5)
+        r = sum(e.color[0] for e in elements) / len(elements)
+        g = sum(e.color[1] for e in elements) / len(elements)
+        b = sum(e.color[2] for e in elements) / len(elements)
+        return (r, g, b)
+
     @classmethod
     def is_editing_styles(cls) -> bool:
         props = cls.get_style_props()
@@ -669,9 +809,179 @@ class Style(bonsai.core.tool.Style):
         props = cls.get_material_style_props(blender_material)
         props.active_style_type = props.active_style_type
 
+    @classmethod
+    def get_branch_outputs(
+        cls, material: bpy.types.Material
+    ) -> tuple[bpy.types.ShaderNode | None, bpy.types.ShaderNode | None]:
+        """Return (external_output_node, flat_output_node), or (None, None) if not dual-branch."""
+        if not material.node_tree:
+            return None, None
+        ext = material.node_tree.nodes.get("BIM_Output_External")
+        fast = material.node_tree.nodes.get("BIM_Output_Flat")
+        return ext, fast
+
+    @classmethod
+    def _remove_external_branch(cls, material: bpy.types.Material) -> None:
+        """Remove all nodes reachable from BIM_Output_External (walks links backwards)."""
+        if not material.node_tree:
+            return
+        nodes = material.node_tree.nodes
+        output = nodes.get("BIM_Output_External")
+        if not output:
+            return
+        to_remove: set[str] = set()
+        stack = [output]
+        while stack:
+            node = stack.pop()
+            if node.name in to_remove:
+                continue
+            to_remove.add(node.name)
+            for inp in node.inputs:
+                for link in inp.links:
+                    stack.append(link.from_node)
+        for name in list(to_remove):
+            n = nodes.get(name)
+            if n:
+                nodes.remove(n)
+
+    @classmethod
+    def _build_flat_branch_nodes(cls, material: bpy.types.Material) -> bpy.types.ShaderNode:
+        """Add a Principled BSDF flat-branch to material's existing node tree.
+
+        Reads IfcSurfaceStyleRendering or IfcSurfaceStyleShading from the linked IFC entity.
+        Defaults to a white BSDF when no IFC shading data is available.
+        Returns the new Material Output node (named BIM_Output_Flat, is_active_output=False).
+        """
+        from mathutils import Vector
+
+        style_elements = cls.get_style_elements(material)
+        nodes = material.node_tree.nodes
+        links = material.node_tree.links
+
+        bsdf = nodes.new("ShaderNodeBsdfPrincipled")
+        bsdf.location = Vector((10, -600))
+        output = nodes.new("ShaderNodeOutputMaterial")
+        output.name = "BIM_Output_Flat"
+        output.location = Vector((300, -600))
+        output.is_active_output = False
+        links.new(bsdf.outputs["BSDF"], output.inputs["Surface"])
+
+        rendering_style = None
+        shading_only = None
+        for surface_style in style_elements.values():
+            if surface_style.is_a() == "IfcSurfaceStyleShading":
+                shading_only = surface_style
+            elif surface_style.is_a("IfcSurfaceStyleRendering"):
+                rendering_style = surface_style
+                shading_only = None
+
+        if rendering_style:
+            d = tool.Loader.surface_style_to_dict(rendering_style)
+            if d.get("DiffuseColour"):
+                ctype, cval = d["DiffuseColour"]
+                if ctype == "IfcColourRgb":
+                    bsdf.inputs["Base Color"].default_value = cval + (1,)
+                    solid_color = cval
+                else:
+                    cval = tuple(v * cval for v in d["SurfaceColour"])
+                    bsdf.inputs["Base Color"].default_value = cval + (1,)
+                    solid_color = cval
+            else:
+                r, g, b = d["SurfaceColour"]
+                bsdf.inputs["Base Color"].default_value = (r, g, b, 1.0)
+                solid_color = (r, g, b)
+            if d.get("SpecularColour"):
+                ctype, cval = d["SpecularColour"]
+                if ctype == "IfcNormalisedRatioMeasure":
+                    bsdf.inputs["Metallic"].default_value = cval
+            if d.get("SpecularHighlight"):
+                bsdf.inputs["Roughness"].default_value = d["SpecularHighlight"]
+            transparency = d.get("Transparency") or 0.0
+            bsdf.inputs["Alpha"].default_value = 1 - transparency
+            if transparency > 0:
+                material.blend_method = "BLEND"
+            material.diffuse_color = solid_color + (1.0 - transparency,)
+        elif shading_only:
+            d = tool.Loader.surface_style_to_dict(shading_only)
+            r, g, b = d["SurfaceColour"]
+            alpha = 1 - (d.get("Transparency") or 0.0)
+            bsdf.inputs["Base Color"].default_value = (r, g, b, 1.0)
+            bsdf.inputs["Alpha"].default_value = alpha
+            if alpha < 1.0:
+                material.blend_method = "BLEND"
+            material.diffuse_color = (r, g, b, alpha)
+        # else: leave default white Principled BSDF
+        return output
+
+    @classmethod
+    def setup_dual_branch(cls, material: bpy.types.Material, ext_material: bpy.types.Material) -> bool:
+        """Build a dual-branch node tree: flat branch from IFC data + external branch from ext_material.
+
+        Clears any existing nodes and builds both branches from scratch.
+        External branch output (BIM_Output_External) is set active — Pretty mode.
+        Flat branch output (BIM_Output_Flat) is inactive — Flat mode.
+        Returns True on success; False if no shader editor is available (falls back to single-branch).
+        """
+        cls.set_use_nodes(material, True)
+        for n in material.node_tree.nodes[:]:
+            material.node_tree.nodes.remove(n)
+
+        cls._build_flat_branch_nodes(material)
+
+        ext_output = tool.Blender.copy_node_graph_additive(material, ext_material)
+        if not ext_output:
+            # No shader editor available: fall back to single-branch
+            tool.Blender.copy_node_graph(material, ext_material)
+            return False
+
+        ext_output.name = "BIM_Output_External"
+        ext_output.is_active_output = True
+        material["bim_dual_branch"] = True
+        return True
+
+    @classmethod
+    def update_external_branch(cls, material: bpy.types.Material, ext_material: bpy.types.Material) -> None:
+        """Replace the external-branch nodes of an already dual-branch material."""
+        cls._remove_external_branch(material)
+        ext_output = tool.Blender.copy_node_graph_additive(material, ext_material)
+        if ext_output:
+            ext_output.name = "BIM_Output_External"
+            ext_output.is_active_output = True
+            fast = material.node_tree.nodes.get("BIM_Output_Flat")
+            if fast:
+                fast.is_active_output = False
+
+    @classmethod
+    def sync_flat_branch_shading(
+        cls, material: bpy.types.Material, surface_colour: tuple[float, float, float], transparency: float
+    ) -> None:
+        """Update the flat-branch Principled BSDF with new shading values.
+
+        Call this after creating or editing IfcSurfaceStyleShading so the flat branch
+        stays in sync without requiring a full setup_dual_branch rebuild.
+        """
+        if not material.node_tree:
+            return
+        fast_output = material.node_tree.nodes.get("BIM_Output_Flat")
+        if not fast_output:
+            return
+        for link in fast_output.inputs["Surface"].links:
+            if link.from_node.type == "BSDF_PRINCIPLED":
+                bsdf = link.from_node
+                r, g, b = surface_colour
+                bsdf.inputs["Base Color"].default_value = (r, g, b, 1.0)
+                bsdf.inputs["Alpha"].default_value = 1.0 - transparency
+                break
+
     @classmethod
     def switch_shading(cls, blender_material: bpy.types.Material, style_type: StyleType) -> None:
         if style_type == "External":
+            ext, fast = cls.get_branch_outputs(blender_material)
+            if ext and fast:
+                ext.is_active_output = True
+                fast.is_active_output = False
+                blender_material.update_tag()
+                return
             try:
                 bpy.ops.bim.activate_external_style(material_name=blender_material.name)
             except RuntimeError as error:
@@ -679,21 +989,41 @@ class Style(bonsai.core.tool.Style):
                     return
                 raise error
         elif style_type == "Shading":
+            ext, fast = cls.get_branch_outputs(blender_material)
+            if ext and fast:
+                fast.is_active_output = True
+                ext.is_active_output = False
+                blender_material.update_tag()
+                return
             style_elements = tool.Style.get_style_elements(blender_material)
             rendering_style = None
             texture_style = None
+            shading_only_style = None
 
             for surface_style in style_elements.values():
                 if surface_style.is_a() == "IfcSurfaceStyleShading":
+                    shading_only_style = surface_style
                     tool.Loader.create_surface_style_shading(blender_material, surface_style)
                 elif surface_style.is_a("IfcSurfaceStyleRendering"):
                     rendering_style = surface_style
+                    shading_only_style = None  # rendering overrides shading-only path
                     tool.Loader.create_surface_style_rendering(blender_material, surface_style)
                 elif surface_style.is_a("IfcSurfaceStyleWithTextures"):
                     texture_style = surface_style
 
             if rendering_style and texture_style:
                 tool.Loader.create_surface_style_with_textures(blender_material, rendering_style, texture_style)
+            elif shading_only_style and not rendering_style:
+                # create a minimal Principled BSDF so Material Preview/Rendered shows the colour instead of white.
+                tool.Style.set_use_nodes(blender_material, True)
+                tool.Loader.restart_material_node_tree(blender_material)
+                bsdf = tool.Blender.get_material_node(blender_material, "BSDF_PRINCIPLED")
+                if bsdf:
+                    r, g, b, a = blender_material.diffuse_color
+                    bsdf.inputs["Base Color"].default_value = (r, g, b, 1)
+                    bsdf.inputs["Alpha"].default_value = a
+                    if a < 1.0:
+                        blender_material.blend_method = "BLEND"
         else:
             assert False, f"Invalid style type found: {style_type}"
 
@@ -739,3 +1069,30 @@ class Style(bonsai.core.tool.Style):
         elements = ifcopenshell.util.element.get_elements_by_style(tool.Ifc.get(), style)
         objects = [tool.Ifc.get_object(e) for e in elements]
         tool.Geometry.reload_representation(objects)
+
+    @classmethod
+    def restore_material_style_types(cls, shading_type: str) -> bool:
+        """Set each IFC material's active_style_type to the richest available for the given viewport mode.
+
+        In SOLID mode all materials use "Shading".
+        In MATERIAL_PREVIEW / RENDERED, materials with an external .blend style use "External"
+        unless prefer_ifc_shading is set on that material.
+
+        Returns True if any IFC material has IfcSurfaceStyleWithTextures (used to decide color_type).
+        """
+        has_any_textures = False
+        for material in bpy.data.materials:
+            if not tool.Blender.get_ifc_definition_id(material):
+                continue
+            props = cls.get_material_style_props(material)
+            style_elements = cls.get_style_elements(material)
+            if style_elements.get("IfcSurfaceStyleWithTextures"):
+                has_any_textures = True
+            if shading_type == "SOLID":
+                props.active_style_type = "Shading"
+            else:  # MATERIAL_PREVIEW or RENDERED
+                if cls.has_blender_external_style(style_elements) and not props.prefer_ifc_shading:
+                    props.active_style_type = "External"
+                else:
+                    props.active_style_type = "Shading"
+        return has_any_textures