Alembic: new exporter based on the USD exporter structure

The Alembic exporter has been restructured by leverages the
`AbstractHierarchyIterator` introduced by the USD exporter. The produced
Alembic files have not changed much (details below), as the Alembic
writing code has simply been moved from the old exporter to the new. How
the export hierarchy is handled changed a lot, though, and also the way
in which transforms are computed. As a result, T71395 is fixed.

Differences between the old and new exporter, in terms of the produced
Alembic file:
- Duplicated objects now have a unique numerical suffix.
- Matrices are computed differently, namely by simply computing the
  evaluated transform of the object relative to the evaluated transform
  of its export-parent. This fixes {T71395}, but otherwise should
  produce the same result as before (but with simpler code).

Compared to the old Alembic exporter, Subdivision modifiers are now
disabled in a cleaner, more efficient way (they are disabled when
exporting with the "Apply Subdivisions" option is unchecked). Previously
the exporter would move to a new frame, disable the modifier, evaluate
the object, and enable the modifier again. This is now done before
exporting starts, and modifiers are only restored when exporting ends.

Some issues with the old Alembic exporter that have NOT been fixed in
this patch:
- Exporting NURBS patches and curves (see T49114 for example).
- Exporting flattened hierarchy in combination with dupli-objects. This
  seems to be broken in the old Alembic exporter as well, but nobody
  reported this yet.

Differential Revision: https://developer.blender.org/D7664

Reviewed By: Sergey

1 files changed, 87 insertions, 0 deletions

diff --git a/source/blender/io/alembic/exporter/abc_archive.h b/source/blender/io/alembic/exporter/abc_archive.h
new file mode 100644
index 00000000000..43d0acf2520
--- /dev/null
+++ b/source/blender/io/alembic/exporter/abc_archive.h
@@ -0,0 +1,87 @@
+/*
+ * 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.
+ */
+
+/** \file
+ * \ingroup Alembic
+ */
+
+#pragma once
+
+#include "ABC_alembic.h"
+#include "IO_abstract_hierarchy_iterator.h"
+
+#include <Alembic/Abc/OArchive.h>
+#include <Alembic/Abc/OTypedScalarProperty.h>
+
+#include <fstream>
+#include <set>
+#include <string>
+
+struct Main;
+struct Scene;
+
+namespace blender {
+namespace io {
+namespace alembic {
+
+/* Container for an Alembic archive and time sampling info.
+ *
+ * Constructor arguments are used to create the correct output stream and to set the archive's
+ * metadata. */
+class ABCArchive {
+ public:
+  typedef std::set<double> Frames;
+
+  Alembic::Abc::OArchive *archive;
+
+  ABCArchive(const Main *bmain,
+             const Scene *scene,
+             AlembicExportParams params,
+             std::string filename);
+  ~ABCArchive();
+
+  uint32_t time_sampling_index_transforms() const;
+  uint32_t time_sampling_index_shapes() const;
+
+  Frames::const_iterator frames_begin() const;
+  Frames::const_iterator frames_end() const;
+  size_t total_frame_count() const;
+
+  bool is_xform_frame(double frame) const;
+  bool is_shape_frame(double frame) const;
+
+  ExportSubset export_subset_for_frame(double frame) const;
+
+  void update_bounding_box(const Imath::Box3d &bounds);
+
+ private:
+  std::ofstream abc_ostream_;
+  uint32_t time_sampling_index_transforms_;
+  uint32_t time_sampling_index_shapes_;
+
+  Frames xform_frames_;
+  Frames shape_frames_;
+  Frames export_frames_;
+
+  Alembic::Abc::OBox3dProperty abc_archive_bbox_;
+};
+
+}  // namespace alembic
+}  // namespace io
+}  // namespace blender