Fix #104669: Incorrect material assignment of imported FBX with duplicate materials

The initial fix made for #103976 incorrectly identified the issue as
being part of the exporter and changed the exporter to work with the
bugged behaviour of the importer. This patch fixes the issue on the
importer side.

Unlike Blender, Materials always belong to the FBX Model (Object
equivalent), whereas Blender defaults to the Materials belonging to the
Mesh (FBX Geometry equivalent). When a Mesh was used by multiple
Objects, to prevent appending the Materials of each FBX Model to the
Mesh, a set of already used Materials was kept and non-unique Materials
would be skipped. However, skipping non-unique Materials would mean the
material indices of a Mesh no longer match its Materials, causing the
material indices to appear offset or invalid.

This patch changes the import of Meshes containing duplicate Materials.
The duplicates are now added to the Mesh in their original order
instead of being skipped.

This patch changes the import of Meshes used by more than one Object
whereby the imported Objects have different Materials. After the
Materials from the first imported Object are appended to the Mesh, any
different Materials from other imported Objects at the same indices have
their Material Slot linked to the Object with the differing Material set
in that Material Slot. If other imported Objects have more Materials
than the number of existing materials on the Mesh, the additional
Materials are appended to the Mesh. Previously, Materials that were
already present in the Mesh would be skipped and new Materials would be
appended to the end of the Mesh's Materials.

Meshes containing duplicate materials that were exported by Blender 3.5
and earlier would export duplicate materials despite only one of the
duplicates being used by the exported mesh (fixed in #104667). All the
duplicates in these FBX files are now imported with this change.

Due to the previous incorrect attempt fix for #103976, FBX with
duplicate materials exported by Blender 3.5 have incorrect material
indices (fixed in #104667). The importer issue fixed by this patch
combined with the exporter issue would cause such incorrectly exported
FBX files to import as if they had been exported correctly in the first
place. Now that the importer issue is fixed, this is no longer the case
and the importer will now show that those Blender 3.5 exported FBX with
duplicate materials have incorrect material indices.

Pull Request: https://projects.blender.org/blender/blender-addons/pulls/104683

io_scene_fbx/import_fbx.py

@@ -3310,33 +3310,64 @@ def load(operator, context, filepath="",
 
     def _():
         # link Material's to Geometry (via Model's)
-        for fbx_uuid, fbx_item in fbx_table_nodes.items():
-            fbx_obj, blen_data = fbx_item
-            if fbx_obj.id != b'Geometry':
+        processed_meshes = set()
+        for helper_uuid, helper_node in fbx_helper_nodes.items():
+            obj = helper_node.bl_obj
+            if not obj or obj.type != 'MESH':
                 continue
 
-            mesh = fbx_table_nodes.get(fbx_uuid, (None, None))[1]
+            # Get the Mesh corresponding to the Geometry used by this Model.
+            mesh = obj.data
+            processed_meshes.add(mesh)
 
-            # can happen in rare cases
-            if mesh is None:
+            # Get the Materials from the Model's connections.
+            material_connections = connection_filter_reverse(helper_uuid, b'Material')
+            if not material_connections:
                 continue
 
-            # In Blender, we link materials to data, typically (meshes), while in FBX they are linked to objects...
-            # So we have to be careful not to re-add endlessly the same material to a mesh!
-            # This can easily happen with 'baked' dupliobjects, see T44386.
-            # TODO: add an option to link materials to objects in Blender instead?
-            done_materials = set()
+            mesh_mats = mesh.materials
+            num_mesh_mats = len(mesh_mats)
 
-            for (fbx_lnk, fbx_lnk_item, fbx_lnk_type) in connection_filter_forward(fbx_uuid, b'Model'):
-                # link materials
-                fbx_lnk_uuid = elem_uuid(fbx_lnk)
-                for (fbx_lnk_material, material, fbx_lnk_material_type) in connection_filter_reverse(fbx_lnk_uuid, b'Material'):
-                    if material not in done_materials:
-                        mesh.materials.append(material)
-                        done_materials.add(material)
+            if num_mesh_mats == 0:
+                # This is the first (or only) model to use this Geometry. This is the most common case when importing.
+                # All the Materials can trivially be appended to the Mesh's Materials.
+                mats_to_append = material_connections
+                mats_to_compare = ()
+            elif num_mesh_mats == len(material_connections):
+                # Another Model uses the same Geometry and has already appended its Materials to the Mesh. This is the
+                # second most common case when importing.
+                # It's also possible that a Model could share the same Geometry and have the same number of Materials,
+                # but have different Materials, though this is less common.
+                # The Model Materials will need to be compared with the Mesh Materials at the same indices to check if
+                # they are different.
+                mats_to_append = ()
+                mats_to_compare = material_connections
+            else:
+                # Under the assumption that only used Materials are connected to the Model, the number of Materials of
+                # each Model using a specific Geometry should be the same, otherwise the Material Indices of the
+                # Geometry will be out-of-bounds of the Materials of at least one of the Models using that Geometry.
+                # We wouldn't expect this case to happen, but there's nothing to say it can't.
+                # We'll handle a differing number of Materials by appending any extra Materials and comparing the rest.
+                mats_to_append = material_connections[num_mesh_mats:]
+                mats_to_compare = material_connections[:num_mesh_mats]
 
-            # We have to validate mesh polygons' ma_idx, see T41015!
-            # Some FBX seem to have an extra 'default' material which is not defined in FBX file.
+            for _fbx_lnk_material, material, _fbx_lnk_material_type in mats_to_append:
+                mesh_mats.append(material)
+
+            mats_to_compare_and_slots = zip(mats_to_compare, obj.material_slots)
+            for (_fbx_lnk_material, material, _fbx_lnk_material_type), mat_slot in mats_to_compare_and_slots:
+                if material != mat_slot.material:
+                    # Material Slots default to being linked to the Mesh, so a previously processed Object is also using
+                    # this Mesh, but the Mesh uses a different Material for this Material Slot.
+                    # To have a different Material for this Material Slot on this Object only, the Material Slot must be
+                    # linked to the Object rather than the Mesh.
+                    # TODO: add an option to link all materials to objects in Blender instead?
+                    mat_slot.link = 'OBJECT'
+                    mat_slot.material = material
+
+        # We have to validate mesh polygons' ma_idx, see #41015!
+        # Some FBX seem to have an extra 'default' material which is not defined in FBX file.
+        for mesh in processed_meshes:
             if mesh.validate_material_indices():
                 print("WARNING: mesh '%s' had invalid material indices, those were reset to first material" % mesh.name)
     _(); del _