USD: Introducing a simple USD Exporter

This commit introduces the first version of an exporter to Pixar's
Universal Scene Description (USD) format.

Reviewed By: sergey, LazyDodo

Differential Revision: https://developer.blender.org/D6287

- The USD libraries are built by `make deps`, but not yet built by
  install_deps.sh.
- Only experimental support for instancing; by default all duplicated
  objects are made real in the USD file. This is fine for exporting a
  linked-in posed character, not so much for thousands of pebbles etc.
- The way materials and UV coordinates and Normals are exported is going
  to change soon.
- This patch contains LazyDodo's fixes for building on Windows in D5359.

== Meshes ==

USD seems to support neither per-material nor per-face-group
double-sidedness, so we just use the flag from the first non-empty
material slot. If there is no material we default to double-sidedness.

Each UV map is stored on the mesh in a separate primvar. Materials can
refer to these UV maps, but this is not yet exported by Blender. The
primvar name is the same as the UV Map name. This is to allow the
standard name "st" for texture coordinates by naming the UV Map as such,
without having to guess which UV Map is the "standard" one.

Face-varying mesh normals are written to USD. When the mesh has custom
loop normals those are written. Otherwise the poly flag `ME_SMOOTH` is
inspected to determine the normals.

The UV maps and mesh normals take up a significant amount of space, so
exporting them is optional. They're still enabled by default, though.
For comparison: a shot of Spring (03_035_A) is 1.2 GiB when exported
with UVs and normals, and 262 MiB without. We probably have room for
optimisation of written UVs and normals.

The mesh subdivision scheme isn't using the default value 'Catmull
Clark', but uses 'None', indicating we're exporting a polygonal mesh.
This is necessary for USD to understand our normals; otherwise the mesh
is always rendered smooth. In the future we may want to expose this
choice of subdivision scheme to the user, or auto-detect it when we
actually support exporting pre-subdivision meshes.

A possible optimisation could be to inspect whether all polygons are
smooth or flat, and mark the USD mesh as such. This can be added when
needed.

== Animation ==

Mesh and transform animation are now written when passing
`animation=True` to the export operator. There is no inspection of
whether an object is actually animated or not; USD can handle
deduplication of static values for us.

The administration of which timecode to use for the export is left to
the file-format-specific concrete subclasses of
`AbstractHierarchyIterator`; the abstract iterator itself doesn't know
anything about the passage of time. This will allow subclasses for the
frame-based USD format and time-based Alembic format.

== Support for simple preview materials ==

Very simple versions of the materials are now exported, using only the
viewport diffuse RGB, metallic, and roughness.

When there are multiple materials, the mesh faces are stored as geometry
subset and each material is assigned to the appropriate subset. If there
is only one material this is skipped.

The first material if any) is always applied to the mesh itself
(regardless of the existence of geometry subsets), because the Hydra
viewport doesn't support materials on subsets. See
https://github.com/PixarAnimationStudios/USD/issues/542 for more info.

Note that the geometry subsets are not yet time-sampled, so it may break
when an animated mesh changes topology.

Materials are exported as a flat list under a top-level '/_materials'
namespace. This inhibits instancing of the objects using those
materials, so this is subject to change.

== Hair ==

Only the parent strands are exported, and only with a constant colour.
No UV coordinates, no information about the normals.

== Camera ==

Only perspective cameras are supported for now.

== Particles ==

Particles are only written when they are alive, which means that they
are always visible (there is currently no code that deals with marking
them as invisible outside their lifespan).

Particle-system-instanced objects are exported by suffixing the object
name with the particle's persistent ID, giving each particle XForm a
unique name.

== Instancing/referencing ==

This exporter has experimental support for instancing/referencing.

Dupli-object meshes are now written to USD as references to the original
mesh. This is still very limited in correctness, as there are issues
referencing to materials from a referenced mesh.

I am still committing this, as it gives us a place to start when
continuing the quest for proper instancing in USD.

== Lights ==

USD does not directly support spot lights, so those aren't exported yet.
It's possible to add this in the future via the UsdLuxShapingAPI. The
units used for the light intensity are also still a bit of a mystery.

== Fluid vertex velocities ==

Currently only fluid simulations (not meshes in general) have explicit
vertex velocities. This is the most important case for exporting
velocities, though, as the baked mesh changes topology all the time, and
thus computing the velocities at import time in a post-processing step
is hard.

== The Building Process ==

- USD is built as monolithic library, instead of 25 smaller libraries.
  We were linking all of them as 'whole archive' anyway, so this doesn't
  affect the final file size. It does, however, make life easier with
  respect to linking order, and handling upstream changes.
- The JSON files required by USD are installed into datafiles/usd; they
  are required on every platform. Set the `PXR_PATH_DEBUG` to any value
  to have the USD library print the paths it uses to find those files.
- USD is patched so that it finds the aforementioned JSON files in a path
  that we pass to it from Blender.
- USD is patched to have a `PXR_BUILD_USD_TOOLS` CMake option to disable
  building the tools in its `bin` directory. This is sent as a pull
  request at https://github.com/PixarAnimationStudios/USD/pull/1048

5 files changed, 487 insertions, 0 deletions

diff --git a/tests/gtests/CMakeLists.txt b/tests/gtests/CMakeLists.txt
index 54a1ee41198..7da65bcc8b9 100644
--- a/tests/gtests/CMakeLists.txt
+++ b/tests/gtests/CMakeLists.txt
@@ -19,4 +19,7 @@ if(WITH_GTESTS)
   if(WITH_ALEMBIC)
     add_subdirectory(alembic)
   endif()
+  if(WITH_USD)
+    add_subdirectory(usd)
+  endif()
 endif()
diff --git a/tests/gtests/usd/CMakeLists.txt b/tests/gtests/usd/CMakeLists.txt
new file mode 100644
index 00000000000..e2768509ec4
--- /dev/null
+++ b/tests/gtests/usd/CMakeLists.txt
@@ -0,0 +1,87 @@
+# ***** BEGIN GPL LICENSE BLOCK *****
+#
+# 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) 2019, Blender Foundation
+# All rights reserved.
+# ***** END GPL LICENSE BLOCK *****
+
+# This suppresses the warning "This file includes at least one deprecated or antiquated
+# header which may be removed without further notice at a future date", which is caused
+# by the USD library including <ext/hash_set> on Linux. This has been reported at:
+# https://github.com/PixarAnimationStudios/USD/issues/1057.
+if(UNIX AND NOT APPLE)
+  add_definitions(-D_GLIBCXX_PERMIT_BACKWARD_HASH)
+endif()
+if(WIN32)
+  add_definitions(-DNOMINMAX)
+endif()
+add_definitions(-DPXR_STATIC)
+
+set(INC
+  .
+  ..
+  ../../../source/blender/blenlib
+  ../../../source/blender/blenkernel
+  ../../../source/blender/usd
+  ../../../source/blender/makesdna
+  ../../../source/blender/depsgraph
+  ${USD_INCLUDE_DIRS}
+  ${BOOST_INCLUDE_DIR}
+  ${TBB_INCLUDE_DIR}
+)
+
+set(LIB
+  bf_blenloader_test
+  bf_blenloader
+
+  # Should not be needed but gives windows linker errors if the ocio libs are linked before this:
+  bf_intern_opencolorio
+  bf_gpu
+
+  bf_usd
+)
+
+include_directories(${INC})
+
+setup_libdirs()
+get_property(BLENDER_SORTED_LIBS GLOBAL PROPERTY BLENDER_SORTED_LIBS_PROP)
+
+set(SRC
+  abstract_hierarchy_iterator_test.cc
+  hierarchy_context_order_test.cc
+)
+
+if(UNIX AND NOT APPLE)
+  # TODO(Sybren): This unit test has only been tested on Linux, and should possibly be
+  # restructured to support other platforms as well.
+  list(APPEND SRC usd_stage_creation_test.cc)
+endif()
+
+
+if(WITH_BUILDINFO)
+  list(APPEND SRC "$<TARGET_OBJECTS:buildinfoobj>")
+endif()
+
+BLENDER_SRC_GTEST_EX(
+  NAME usd
+  SRC "${SRC}"
+  EXTRA_LIBS "${LIB}"
+  COMMAND_ARGS
+    --test-assets-dir "${CMAKE_SOURCE_DIR}/../lib/tests"
+    --test-blender-executable-dir "${EXECUTABLE_OUTPUT_PATH}"
+)
+
+setup_liblinks(usd_test)
diff --git a/tests/gtests/usd/abstract_hierarchy_iterator_test.cc b/tests/gtests/usd/abstract_hierarchy_iterator_test.cc
new file mode 100644
index 00000000000..e87ef547052
--- /dev/null
+++ b/tests/gtests/usd/abstract_hierarchy_iterator_test.cc
@@ -0,0 +1,202 @@
+/*
+ * 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) 2019 Blender Foundation.
+ * All rights reserved.
+ */
+#include "blenloader/blendfile_loading_base_test.h"
+#include "intern/abstract_hierarchy_iterator.h"
+
+extern "C" {
+#include "BLI_math.h"
+#include "DEG_depsgraph.h"
+#include "DNA_object_types.h"
+}
+
+#include <map>
+#include <set>
+
+/* Mapping from ID.name to set of export hierarchy path. Duplicated objects can be exported
+ * multiple times, hence the set. */
+typedef std::map<std::string, std::set<std::string>> created_writers;
+
+using namespace USD;
+
+class TestHierarchyWriter : public AbstractHierarchyWriter {
+ public:
+  created_writers &writers_map;
+
+  TestHierarchyWriter(created_writers &writers_map) : writers_map(writers_map)
+  {
+  }
+
+  void write(HierarchyContext &context) override
+  {
+    const char *id_name = context.object->id.name;
+    created_writers::mapped_type &writers = writers_map[id_name];
+
+    BLI_assert(writers.find(context.export_path) == writers.end());
+    writers.insert(context.export_path);
+  }
+};
+
+void debug_print_writers(const char *label, const created_writers &writers_map)
+{
+  printf("%s:\n", label);
+  for (auto idname_writers : writers_map) {
+    printf("    %s:\n", idname_writers.first.c_str());
+    for (const std::string &export_path : idname_writers.second) {
+      printf("      - %s\n", export_path.c_str());
+    }
+  }
+}
+
+class TestingHierarchyIterator : public AbstractHierarchyIterator {
+ public: /* Public so that the test cases can directly inspect the created writers. */
+  created_writers transform_writers;
+  created_writers data_writers;
+  created_writers hair_writers;
+  created_writers particle_writers;
+
+ public:
+  explicit TestingHierarchyIterator(Depsgraph *depsgraph) : AbstractHierarchyIterator(depsgraph)
+  {
+  }
+  virtual ~TestingHierarchyIterator()
+  {
+  }
+
+ protected:
+  AbstractHierarchyWriter *create_transform_writer(const HierarchyContext *context) override
+  {
+    return new TestHierarchyWriter(transform_writers);
+  }
+  AbstractHierarchyWriter *create_data_writer(const HierarchyContext *context) override
+  {
+    return new TestHierarchyWriter(data_writers);
+  }
+  AbstractHierarchyWriter *create_hair_writer(const HierarchyContext *context) override
+  {
+    return new TestHierarchyWriter(hair_writers);
+  }
+  AbstractHierarchyWriter *create_particle_writer(const HierarchyContext *context) override
+  {
+    return new TestHierarchyWriter(particle_writers);
+  }
+
+  void delete_object_writer(AbstractHierarchyWriter *writer) override
+  {
+    delete writer;
+  }
+};
+
+class USDHierarchyIteratorTest : public BlendfileLoadingBaseTest {
+ protected:
+  TestingHierarchyIterator *iterator;
+
+  virtual void SetUp()
+  {
+    BlendfileLoadingBaseTest::SetUp();
+    iterator = nullptr;
+  }
+
+  virtual void TearDown()
+  {
+    iterator_free();
+    BlendfileLoadingBaseTest::TearDown();
+  }
+
+  /* Create a test iterator. */
+  void iterator_create()
+  {
+    iterator = new TestingHierarchyIterator(depsgraph);
+  }
+  /* Free the test iterator if it is not nullptr. */
+  void iterator_free()
+  {
+    if (iterator == nullptr) {
+      return;
+    }
+    delete iterator;
+    iterator = nullptr;
+  }
+};
+
+TEST_F(USDHierarchyIteratorTest, ExportHierarchyTest)
+{
+  /* Load the test blend file. */
+  if (!blendfile_load("usd/usd_hierarchy_export_test.blend")) {
+    return;
+  }
+  depsgraph_create(DAG_EVAL_RENDER);
+  iterator_create();
+
+  iterator->iterate_and_write();
+
+  // Mapping from object name to set of export paths.
+  created_writers expected_transforms = {
+      {"OBCamera", {"/Camera"}},
+      {"OBDupli1", {"/Dupli1"}},
+      {"OBDupli2", {"/ParentOfDupli2/Dupli2"}},
+      {"OBGEO_Ear_L",
+       {"/Dupli1/GEO_Head-0/GEO_Ear_L-1",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/GEO_Ear_L",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/GEO_Ear_L-1"}},
+      {"OBGEO_Ear_R",
+       {"/Dupli1/GEO_Head-0/GEO_Ear_R-2",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/GEO_Ear_R",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/GEO_Ear_R-2"}},
+      {"OBGEO_Head",
+       {"/Dupli1/GEO_Head-0",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0"}},
+      {"OBGEO_Nose",
+       {"/Dupli1/GEO_Head-0/GEO_Nose-3",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/GEO_Nose",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/GEO_Nose-3"}},
+      {"OBGround plane", {"/Ground plane"}},
+      {"OBOutsideDupliGrandParent", {"/Ground plane/OutsideDupliGrandParent"}},
+      {"OBOutsideDupliParent", {"/Ground plane/OutsideDupliGrandParent/OutsideDupliParent"}},
+      {"OBParentOfDupli2", {"/ParentOfDupli2"}}};
+  EXPECT_EQ(expected_transforms, iterator->transform_writers);
+
+  created_writers expected_data = {
+      {"OBCamera", {"/Camera/Camera"}},
+      {"OBGEO_Ear_L",
+       {"/Dupli1/GEO_Head-0/GEO_Ear_L-1/Ear",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/GEO_Ear_L/Ear",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/GEO_Ear_L-1/Ear"}},
+      {"OBGEO_Ear_R",
+       {"/Dupli1/GEO_Head-0/GEO_Ear_R-2/Ear",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/GEO_Ear_R/Ear",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/GEO_Ear_R-2/Ear"}},
+      {"OBGEO_Head",
+       {"/Dupli1/GEO_Head-0/Face",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/Face",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/Face"}},
+      {"OBGEO_Nose",
+       {"/Dupli1/GEO_Head-0/GEO_Nose-3/Nose",
+        "/Ground plane/OutsideDupliGrandParent/OutsideDupliParent/GEO_Head/GEO_Nose/Nose",
+        "/ParentOfDupli2/Dupli2/GEO_Head-0/GEO_Nose-3/Nose"}},
+      {"OBGround plane", {"/Ground plane/Plane"}},
+      {"OBParentOfDupli2", {"/ParentOfDupli2/Icosphere"}},
+  };
+
+  EXPECT_EQ(expected_data, iterator->data_writers);
+
+  // The scene has no hair or particle systems.
+  EXPECT_EQ(0, iterator->hair_writers.size());
+  EXPECT_EQ(0, iterator->particle_writers.size());
+}
diff --git a/tests/gtests/usd/hierarchy_context_order_test.cc b/tests/gtests/usd/hierarchy_context_order_test.cc
new file mode 100644
index 00000000000..ce3b43484e7
--- /dev/null
+++ b/tests/gtests/usd/hierarchy_context_order_test.cc
@@ -0,0 +1,123 @@
+/*
+ * 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) 2019 Blender Foundation.
+ * All rights reserved.
+ */
+#include "intern/abstract_hierarchy_iterator.h"
+
+#include "testing/testing.h"
+
+extern "C" {
+#include "BLI_utildefines.h"
+}
+
+using namespace USD;
+
+class HierarchyContextOrderTest : public testing::Test {
+};
+
+static Object *fake_pointer(int value)
+{
+  return static_cast<Object *>(POINTER_FROM_INT(value));
+}
+
+TEST_F(HierarchyContextOrderTest, ObjectPointerTest)
+{
+  HierarchyContext ctx_a;
+  ctx_a.object = fake_pointer(1);
+  ctx_a.duplicator = nullptr;
+
+  HierarchyContext ctx_b;
+  ctx_b.object = fake_pointer(2);
+  ctx_b.duplicator = nullptr;
+
+  EXPECT_EQ(true, ctx_a < ctx_b);
+  EXPECT_EQ(false, ctx_b < ctx_a);
+  EXPECT_EQ(false, ctx_a < ctx_a);
+}
+
+TEST_F(HierarchyContextOrderTest, DuplicatorPointerTest)
+{
+  HierarchyContext ctx_a;
+  ctx_a.object = fake_pointer(1);
+  ctx_a.duplicator = fake_pointer(1);
+  ctx_a.export_name = "A";
+
+  HierarchyContext ctx_b;
+  ctx_b.object = fake_pointer(1);
+  ctx_b.duplicator = fake_pointer(1);
+  ctx_b.export_name = "B";
+
+  EXPECT_EQ(true, ctx_a < ctx_b);
+  EXPECT_EQ(false, ctx_b < ctx_a);
+  EXPECT_EQ(false, ctx_a < ctx_a);
+}
+
+TEST_F(HierarchyContextOrderTest, ExportParentTest)
+{
+  HierarchyContext ctx_a;
+  ctx_a.object = fake_pointer(1);
+  ctx_a.export_parent = fake_pointer(1);
+
+  HierarchyContext ctx_b;
+  ctx_b.object = fake_pointer(1);
+  ctx_b.export_parent = fake_pointer(2);
+
+  EXPECT_EQ(true, ctx_a < ctx_b);
+  EXPECT_EQ(false, ctx_b < ctx_a);
+  EXPECT_EQ(false, ctx_a < ctx_a);
+}
+
+TEST_F(HierarchyContextOrderTest, TransitiveTest)
+{
+  HierarchyContext ctx_a;
+  ctx_a.object = fake_pointer(1);
+  ctx_a.export_parent = fake_pointer(1);
+  ctx_a.duplicator = nullptr;
+  ctx_a.export_name = "A";
+
+  HierarchyContext ctx_b;
+  ctx_b.object = fake_pointer(2);
+  ctx_b.export_parent = nullptr;
+  ctx_b.duplicator = fake_pointer(1);
+  ctx_b.export_name = "B";
+
+  HierarchyContext ctx_c;
+  ctx_c.object = fake_pointer(2);
+  ctx_c.export_parent = fake_pointer(2);
+  ctx_c.duplicator = fake_pointer(1);
+  ctx_c.export_name = "C";
+
+  HierarchyContext ctx_d;
+  ctx_d.object = fake_pointer(2);
+  ctx_d.export_parent = fake_pointer(3);
+  ctx_d.duplicator = nullptr;
+  ctx_d.export_name = "D";
+
+  EXPECT_EQ(true, ctx_a < ctx_b);
+  EXPECT_EQ(true, ctx_a < ctx_c);
+  EXPECT_EQ(true, ctx_a < ctx_d);
+  EXPECT_EQ(true, ctx_b < ctx_c);
+  EXPECT_EQ(true, ctx_b < ctx_d);
+  EXPECT_EQ(true, ctx_c < ctx_d);
+
+  EXPECT_EQ(false, ctx_b < ctx_a);
+  EXPECT_EQ(false, ctx_c < ctx_a);
+  EXPECT_EQ(false, ctx_d < ctx_a);
+  EXPECT_EQ(false, ctx_c < ctx_b);
+  EXPECT_EQ(false, ctx_d < ctx_b);
+  EXPECT_EQ(false, ctx_d < ctx_c);
+}
diff --git a/tests/gtests/usd/usd_stage_creation_test.cc b/tests/gtests/usd/usd_stage_creation_test.cc
new file mode 100644
index 00000000000..fcf1e93ea7d
--- /dev/null
+++ b/tests/gtests/usd/usd_stage_creation_test.cc
@@ -0,0 +1,72 @@
+/*
+ * 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) 2019 Blender Foundation.
+ * All rights reserved.
+ */
+#include "testing/testing.h"
+#include <pxr/usd/usd/stage.h>
+
+#include <string>
+
+extern "C" {
+#include "BLI_path_util.h"
+#include "BLI_utildefines.h"
+
+#include "BKE_appdir.h"
+
+/* Workaround to make it possible to pass a path at runtime to USD. See creator.c. */
+void usd_initialise_plugin_path(const char *datafiles_usd_path);
+}
+
+DEFINE_string(test_blender_executable_dir, "", "Blender's installation directory.");
+
+class USDStageCreationTest : public testing::Test {
+};
+
+TEST_F(USDStageCreationTest, JSONFileLoadingTest)
+{
+  std::string filename = "usd-stage-creation-test.usdc";
+
+  if (FLAGS_test_blender_executable_dir.empty()) {
+    FAIL() << "Pass the flag";
+  }
+
+  /* Required on Linux to make BKE_appdir_folder_id() find the datafiles.
+   * Without going to this directory, Blender looks for the datafiles in
+   * .../bin/tests instead of .../bin */
+  const char *blender_executable_dir = FLAGS_test_blender_executable_dir.c_str();
+  if (chdir(blender_executable_dir) < 0) {
+    FAIL() << "unable to change directory to " << FLAGS_test_blender_executable_dir;
+  }
+
+  const char *usd_datafiles_relpath = BKE_appdir_folder_id(BLENDER_DATAFILES, "usd");
+  EXPECT_NE(usd_datafiles_relpath, nullptr) << "Unable to find datafiles/usd";
+
+  char usd_datafiles_abspath[FILE_MAX];
+  BLI_path_join(usd_datafiles_abspath,
+                sizeof(usd_datafiles_abspath),
+                blender_executable_dir,
+                usd_datafiles_relpath,
+                NULL);
+
+  usd_initialise_plugin_path(usd_datafiles_abspath);
+
+  /* Simply the ability to create a USD Stage for a specific filename means that the extension has
+   * been recognised by the USD library, and that a USD plugin has been loaded to write such files.
+   * Practically, this is a test to see whether the USD JSON files can be found and loaded. */
+  pxr::UsdStageRefPtr usd_stage = pxr::UsdStage::CreateNew(filename);
+  EXPECT_TRUE(usd_stage) << "unable to find suitable USD plugin to write " << filename;
+}