USD: add USD importer

This is an initial implementation of a USD importer.

This work is comprised of Tangent Animation's open source USD importer,
combined with features @makowalski had implemented.

The design is very similar to the approach taken in the Alembic
importer. The core functionality resides in a collection of "reader"
classes, each of which is responsible for converting an instance of a
USD prim to the corresponding Blender Object representation.

The flow of control for the conversion can be followed in the
`import_startjob()` and `import_endjob()` functions in `usd_capi.cc`.
The `USDStageReader` class is responsible for traversing the USD stage
and instantiating the appropriate readers.

Reviewed By: sybren, HooglyBoogly

Differential Revision: https://developer.blender.org/D10700

1 files changed, 52 insertions, 0 deletions

diff --git a/source/blender/io/usd/intern/usd_reader_geom.h b/source/blender/io/usd/intern/usd_reader_geom.h
new file mode 100644
index 00000000000..99e22248f2b
--- /dev/null
+++ b/source/blender/io/usd/intern/usd_reader_geom.h
@@ -0,0 +1,52 @@
+/*
+ * 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) 2021 Tangent Animation.
+ * All rights reserved.
+ */
+#pragma once
+
+#include "usd.h"
+#include "usd_reader_xform.h"
+
+struct Mesh;
+
+namespace blender::io::usd {
+
+class USDGeomReader : public USDXformReader {
+
+ public:
+  USDGeomReader(const pxr::UsdPrim &prim,
+                const USDImportParams &import_params,
+                const ImportSettings &settings)
+      : USDXformReader(prim, import_params, settings)
+  {
+  }
+
+  virtual Mesh *read_mesh(struct Mesh *existing_mesh,
+                          double motionSampleTime,
+                          int read_flag,
+                          const char **err_str) = 0;
+
+  virtual bool topology_changed(Mesh * /* existing_mesh */, double /* motionSampleTime */)
+  {
+    return true;
+  }
+
+  void add_cache_modifier();
+  void add_subdiv_modifier();
+};
+
+}  // namespace blender::io::usd