LibOverride: Better handling of 'orphaned' Objects in override creation.

One of current annoying limitations of Blender re Collections/Objects is
that objects are forbidden to not be instantiated in at least one
collection.

Code ensuring that as a pst-processing step of override creation/resync
operations would be a bit too eager to add those objects to an external
'ad-hoc' collection, which poses several issues (both in term of keeping
the scene well organized, and related to override hierarchy handling).

So now be very conservative and only generate and use  external 'storage'
collection for those objects when it is absolutely mandatory.

In pratice, it means this should never happen anymore on any decently
organized data source.

1 files changed, 14 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/lib_override.c b/source/blender/blenkernel/intern/lib_override.c
index 84091abd410..6c94eda2ee0 100644
--- a/source/blender/blenkernel/intern/lib_override.c
+++ b/source/blender/blenkernel/intern/lib_override.c
@@ -719,8 +719,19 @@ static void lib_override_library_create_post_process(Main *bmain,
 
       if (BLI_gset_lookup(all_objects_in_scene, ob_new) == NULL) {
         if (id_root != NULL && default_instantiating_collection == NULL) {
-          switch (GS(id_root->name)) {
+          ID *id_ref = id_root->newid != NULL ? id_root->newid : id_root;
+          switch (GS(id_ref->name)) {
             case ID_GR: {
+              /* Adding the object to a specific collection outside of the root overridden one is a
+               * fairly bad idea (it breaks the override hierarchy concept). But htere is no other
+               * way to do this currently (we cannot add new collections to overridden root one,
+               * this is not currently supported).
+               * Since that will be fairly annoying and noisy, only do that in case the override
+               * object is not part of any existing collection (i.e. its user count is 0). In
+               * practice this should never happen I think. */
+              if (ID_REAL_USERS(ob_new) != 0) {
+                continue;
+              }
               default_instantiating_collection = BKE_collection_add(
                   bmain, (Collection *)id_root, "OVERRIDE_HIDDEN");
               /* Hide the collection from viewport and render. */
@@ -731,9 +742,9 @@ static void lib_override_library_create_post_process(Main *bmain,
             case ID_OB: {
               /* Add the other objects to one of the collections instantiating the
                * root object, or scene's master collection if none found. */
-              Object *ob_root = (Object *)id_root;
+              Object *ob_ref = (Object *)id_ref;
               LISTBASE_FOREACH (Collection *, collection, &bmain->collections) {
-                if (BKE_collection_has_object(collection, ob_root) &&
+                if (BKE_collection_has_object(collection, ob_ref) &&
                     BKE_view_layer_has_collection(view_layer, collection) &&
                     !ID_IS_LINKED(collection) && !ID_IS_OVERRIDE_LIBRARY(collection)) {
                   default_instantiating_collection = collection;