Cleanup: USD export, refactor mesh instancing

Extract the mesh instancing code from the mesh writing function into a
generic 'mark as instance' function on the abstract USD writer. This will
help in supporting non-mesh instances.

No functional changes.

source/blender/io/usd/intern/usd_writer_abstract.cc

@@ -21,6 +21,8 @@
 
 #include <pxr/base/tf/stringUtils.h>
 
+#include "BLI_assert.h"
+
 /* TfToken objects are not cheap to construct, so we do it once. */
 namespace usdtokens {
 // Materials
@@ -128,6 +130,31 @@ void USDAbstractWriter::write_visibility(const HierarchyContext &context,
   usd_value_writer_.SetAttribute(attr_visibility, pxr::VtValue(visibility), timecode);
 }
 
+/* Reference the original data instead of writing a copy. */
+bool USDAbstractWriter::mark_as_instance(HierarchyContext &context, const pxr::UsdPrim &prim)
+{
+  BLI_assert(context.is_instance());
+
+  if (context.export_path == context.original_export_path) {
+    printf("USD ref error: export path is reference path: %s\n", context.export_path.c_str());
+    BLI_assert(!"USD reference error");
+    return false;
+  }
+
+  pxr::SdfPath ref_path(context.original_export_path);
+  if (!prim.GetReferences().AddInternalReference(ref_path)) {
+    /* See this URL for a description fo why referencing may fail"
+     * https://graphics.pixar.com/usd/docs/api/class_usd_references.html#Usd_Failing_References
+     */
+    printf("USD Export warning: unable to add reference from %s to %s, not instancing object\n",
+           context.export_path.c_str(),
+           context.original_export_path.c_str());
+    return false;
+  }
+
+  return true;
+}
+
 }  // namespace usd
 }  // namespace io
 }  // namespace blender

source/blender/io/usd/intern/usd_writer_abstract.h

@@ -76,6 +76,10 @@ class USDAbstractWriter : public AbstractHierarchyWriter {
   void write_visibility(const HierarchyContext &context,
                         const pxr::UsdTimeCode timecode,
                         pxr::UsdGeomImageable &usd_geometry);
+
+  /* Turn `prim` into an instance referencing `context.original_export_path`.
+   * Return true when the instancing was succesful, false otherwise. */
+  virtual bool mark_as_instance(HierarchyContext &context, const pxr::UsdPrim &prim);
 };
 
 }  // namespace usd

source/blender/io/usd/intern/usd_writer_mesh.cc

@@ -160,29 +160,18 @@ void USDGenericMeshWriter::write_mesh(HierarchyContext &context, Mesh *mesh)
   get_geometry_data(mesh, usd_mesh_data);
 
   if (usd_export_context_.export_params.use_instancing && context.is_instance()) {
-    // This object data is instanced, just reference the original instead of writing a copy.
-    if (context.export_path == context.original_export_path) {
-      printf("USD ref error: export path is reference path: %s\n", context.export_path.c_str());
-      BLI_assert(!"USD reference error");
-      return;
-    }
-    pxr::SdfPath ref_path(context.original_export_path);
-    if (!usd_mesh.GetPrim().GetReferences().AddInternalReference(ref_path)) {
-      /* See this URL for a description fo why referencing may fail"
-       * https://graphics.pixar.com/usd/docs/api/class_usd_references.html#Usd_Failing_References
-       */
-      printf("USD Export warning: unable to add reference from %s to %s, not instancing object\n",
-             context.export_path.c_str(),
-             context.original_export_path.c_str());
+    if (!mark_as_instance(context, usd_mesh.GetPrim())) {
       return;
     }
+
     /* The material path will be of the form </_materials/{material name}>, which is outside the
-    sub-tree pointed to by ref_path. As a result, the referenced data is not allowed to point out
-    of its own sub-tree. It does work when we override the material with exactly the same path,
-    though.*/
+     * sub-tree pointed to by ref_path. As a result, the referenced data is not allowed to point
+     * out of its own sub-tree. It does work when we override the material with exactly the same
+     * path, though.*/
     if (usd_export_context_.export_params.export_materials) {
       assign_materials(context, usd_mesh, usd_mesh_data.face_groups);
     }
+
     return;
   }