Empty material slots and meshes with no materials are now handled correctly.

* Meshes without materials will be treated as though they have a single empty material slot.
* Empty material slots are now treated as a "None" material on export instead of being disallowed.
* The "None" material slot is appended to the end of the material list unless otherwise specified through manual material ordering.

io_scene_psk_psa/psk/builder.py

@@ -73,16 +73,14 @@ def _get_mesh_export_space_matrix(node: ObjectNode | None, export_space: str) ->
 def _get_material_name_indices(obj: Object, material_names: list[str]) -> Iterable[int]:
     """
     Returns the index of the material in the list of material names.
-    If the material is not found, the index 0 is returned.
+    If the material is not found or the slot is empty, the index of 'None' is returned.
     """
     for material_slot in obj.material_slots:
-        if material_slot.material is None:
+        try:
+            material_name = material_slot.material.name if material_slot.material is not None else 'None'
+            yield material_names.index(material_name)
+        except ValueError:
             yield 0
-        else:
-            try:
-                yield material_names.index(material_slot.material.name)
-            except ValueError:
-                yield 0
 
 
 def build_psk(context: Context, input_objects: PskInputObjects, options: PskBuildOptions) -> PskBuildResult:
@@ -109,15 +107,34 @@ def build_psk(context: Context, input_objects: PskInputObjects, options: PskBuil
     psk.bones = [bone.psx_bone for bone in psx_bone_create_result.bones]
 
     # Materials
+    mesh_objects = [dfs_object.obj for dfs_object in input_objects.mesh_dfs_objects]
+    
     match options.material_order_mode:
         case 'AUTOMATIC':
-            mesh_objects = [dfs_object.obj for dfs_object in input_objects.mesh_dfs_objects]
             materials = list(get_materials_for_mesh_objects(context.evaluated_depsgraph_get(), mesh_objects))
         case 'MANUAL':
             # The material name list may contain materials that are not on the mesh objects.
             # Therefore, we can take the material_name_list as gospel and simply use it as a lookup table.
             # If a look-up fails, replace it with an empty material.
             materials = [bpy.data.materials.get(x, None) for x in options.material_name_list]
+            
+            # Check if any mesh needs a None material (has no slots or empty slots)
+            needs_none_material = False
+            for mesh_object in mesh_objects:
+                evaluated_mesh_object = mesh_object.evaluated_get(context.evaluated_depsgraph_get())
+                if len(evaluated_mesh_object.material_slots) == 0:
+                    needs_none_material = True
+                    break
+                for material_slot in evaluated_mesh_object.material_slots:
+                    if material_slot.material is None:
+                        needs_none_material = True
+                        break
+                if needs_none_material:
+                    break
+            
+            # Append None at the end if needed and not already present
+            if needs_none_material and None not in materials:
+                materials.append(None)
         case _:
             assert False, f'Invalid material order mode: {options.material_order_mode}'
 
@@ -130,11 +147,7 @@ def build_psk(context: Context, input_objects: PskInputObjects, options: PskBuil
                                                                                 material.psk.mesh_triangle_bit_flags)
         psk.materials.append(psk_material)
 
-    # TODO: This wasn't left in a good state. We should detect if we need to add a "default" material.
-    #  This can be done by checking if there is an empty material slot on any of the mesh objects, or if there are
-    #  no material slots on any of the mesh objects.
-    #  If so, it should be added to the end of the list of materials, and its index should mapped to a None value in the
-    #  material indices list.
+    # Ensure at least one material exists
     if len(psk.materials) == 0:
         # Add a default material if no materials are present.
         psk_material = Psk.Material()
@@ -177,8 +190,12 @@ def build_psk(context: Context, input_objects: PskInputObjects, options: PskBuil
         material_indices = list(_get_material_name_indices(obj, material_names))
 
         if len(material_indices) == 0:
-            # Add a default material if no materials are present.
-            material_indices = [0]
+            # If the mesh has no material slots, map to the 'None' material index
+            try:
+                none_material_index = material_names.index('None')
+            except ValueError:
+                none_material_index = 0
+            material_indices = [none_material_index]
 
         # Store the reference to the evaluated object and data so that we can clean them up later.
         evaluated_mesh_object = None

io_scene_psk_psa/psk/export/operators.py

@@ -29,7 +29,7 @@ def populate_material_name_list(depsgraph: Depsgraph, mesh_objects: Iterable[Obj
     material_list.clear()
     for index, material in enumerate(materials):
         m = material_list.add()
-        m.material_name = material.name
+        m.material_name = material.name if material is not None else 'None'
         m.index = index
 
 
@@ -173,19 +173,32 @@ class PSK_OT_material_list_name_move_down(Operator):
         return {'FINISHED'}
 
 
-def get_sorted_materials_by_names(materials: Iterable[Material], material_names: list[str]) -> list[Material]:
+def get_sorted_materials_by_names(materials: Iterable[Material | None], material_names: list[str]) -> list[Material | None]:
     """
     Sorts the materials by the order of the material names list. Any materials not in the list will be appended to the
-    end of the list in the order they are found.
+    end of the list in the order they are found. None materials (representing empty material slots) are always
+    appended at the very end.
 
-    @param materials: A list of materials to sort
+    @param materials: A list of materials to sort (can include None)
     @param material_names: A list of material names to sort by
-    @return: A sorted list of materials
+    @return: A sorted list of materials (with None at the end if present)
     """
+    materials = list(materials)
+    has_none = None in materials
+    materials = [m for m in materials if m is not None]
+    
     materials_in_collection = [m for m in materials if m.name in material_names]
     materials_not_in_collection = [m for m in materials if m.name not in material_names]
     materials_in_collection = sorted(materials_in_collection, key=lambda x: material_names.index(x.name))
-    return materials_in_collection + materials_not_in_collection
+    
+    result: list[Material | None] = []
+    result.extend(materials_in_collection)
+    result.extend(materials_not_in_collection)
+    
+    if has_none:
+        result.append(None)
+    
+    return result
 
 
 def get_psk_build_options_from_property_group(scene: Scene, pg: PskExportMixin) -> PskBuildOptions:

io_scene_psk_psa/shared/helpers.py

@@ -1,7 +1,7 @@
 import bpy
 from collections import Counter
 from typing import Iterable, cast as typing_cast
-from bpy.types import Armature, AnimData, Collection, Context, Object, ArmatureModifier, SpaceProperties, PropertyGroup
+from bpy.types import Armature, AnimData, Collection, Context, Object, ArmatureModifier, SpaceProperties, PropertyGroup, Material
 from mathutils import Matrix, Vector, Quaternion as BpyQuaternion
 from psk_psa_py.shared.data import PsxBone, Quaternion, Vector3
 
@@ -619,16 +619,29 @@ class PskInputObjects(object):
 
 
 def get_materials_for_mesh_objects(depsgraph: Depsgraph, mesh_objects: Iterable[Object]):
-    yielded_materials = set()
+    '''
+    Yields unique materials used by the given mesh objects.
+    If any mesh has no material slots or any empty material slots, None is yielded at the end.
+    '''
+    yielded_materials: Set[Material] = set()
+    has_none_material = False
     for mesh_object in mesh_objects:
         evaluated_mesh_object = mesh_object.evaluated_get(depsgraph)
-        for i, material_slot in enumerate(evaluated_mesh_object.material_slots):
-            material = material_slot.material
-            if material is None:
-                raise RuntimeError(f'Material slots cannot be empty. ({mesh_object.name}, index {i})')
-            if material not in yielded_materials:
-                yielded_materials.add(material)
-                yield material
+        # Check if mesh has no material slots or any empty material slots
+        if len(evaluated_mesh_object.material_slots) == 0:
+            has_none_material = True
+        else:
+            for material_slot in evaluated_mesh_object.material_slots:
+                material = material_slot.material
+                if material is None:
+                    has_none_material = True
+                else:
+                    if material not in yielded_materials:
+                        yielded_materials.add(material)
+                        yield material
+    # Yield None at the end if any mesh had no material slots or empty material slots
+    if has_none_material:
+        yield None
 
 
 def get_mesh_objects_for_collection(collection: Collection) -> Iterable[DfsObject]: