IO: Fix bug exporting dupli parent/child relations

Exporting a scene to USD or Alembic would fail when there are multiple
duplicates of parent & child objects, duplicated by the same object. For
example, this happens when such a hierarchy of objects is contained in a
collection, and that collection is instanced multiple times by mesh
vertices. The problem here is that the 'parent' pointer of each
duplicated object points to the real parent; Blender would not figure
out properly which duplicated parent should be used.

This is now resolved by keeping track of the persistent ID of each
duplicated instance, which makes it possible to reconstruct the
parent-child relations of duplicated objects. This does use up some
memory for each dupli, so it could be heavy to export a Spring scene
(with all the pebbles and leaves), but it's only a small addition on top
of the USD/Alembic writer objects that have to be created anyway. At
least with this patch, they're created correctly.

Code-wise, the following changes are made:

- The export graph (that maps export parent to its export children) used
  to have as its key (Object, Duplicator). This is insufficient to
  correctly distinguish between multiple duplis of the same object by
  the same duplicator, so this is now extended to (Object, Duplicator,
  Persistent ID). To make this possible, new classes `ObjectIdentifier`
  and `PersistentID` are introduced.
- Finding the parent of a duplicated object is done via its persistent
  ID. In Python notation, the code first tries to find the parent
  instance where `child_persistent_id[1:] == parent_persistent_id[1:]`.
  If that fails, the dupli with persistent ID `child_persistent_id[1:]`
  is used as parent.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8233

1 files changed, 61 insertions, 0 deletions

diff --git a/source/blender/io/common/IO_dupli_persistent_id.hh b/source/blender/io/common/IO_dupli_persistent_id.hh
new file mode 100644
index 00000000000..dc70c1cdf31
--- /dev/null
+++ b/source/blender/io/common/IO_dupli_persistent_id.hh
@@ -0,0 +1,61 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+#ifndef __IO_COMMON_DUPLI_PERSISTENT_ID_H__
+#define __IO_COMMON_DUPLI_PERSISTENT_ID_H__
+
+#include "BKE_duplilist.h"
+
+#include "DNA_object_types.h" /* For MAX_DUPLI_RECUR */
+
+#include <array>
+#include <optional>
+#include <ostream>
+
+namespace blender::io {
+
+/* Wrapper for DupliObject::persistent_id that can act as a map key. */
+class PersistentID {
+ protected:
+  constexpr static int array_length_ = MAX_DUPLI_RECUR;
+  typedef std::array<int, array_length_> PIDArray;
+  PIDArray persistent_id_;
+
+  explicit PersistentID(const PIDArray &persistent_id_values);
+
+ public:
+  PersistentID();
+  explicit PersistentID(const DupliObject *dupli_ob);
+
+  /* Return true iff the persistent IDs are the same, ignoring the first digit. */
+  bool is_from_same_instancer_as(const PersistentID &other) const;
+
+  /* Construct the persistent ID of this instance's instancer. */
+  PersistentID instancer_pid() const;
+
+  friend bool operator==(const PersistentID &persistent_id_a, const PersistentID &persistent_id_b);
+  friend bool operator<(const PersistentID &persistent_id_a, const PersistentID &persistent_id_b);
+  friend std::ostream &operator<<(std::ostream &os, const PersistentID &persistent_id);
+
+ private:
+  void copy_values_from(const PIDArray &persistent_id_values);
+};
+
+}  // namespace blender::io
+
+#endif  // __IO_COMMON_DUPLI_PARENT_FINDER_H__