Cleanup: USD: various non-functional changes

Tidy up:
- Use blender::Map and blender::Vector for newly created `prim_map`
- Leave bread-crumb comment for moving map creation outside of loop
- Expand UI tooltip for newly added export option

Pull Request: https://projects.blender.org/blender/blender/pulls/131770

source/blender/editors/io/io_usd.cc

@@ -854,9 +854,9 @@ void WM_OT_usd_export(wmOperatorType *ot)
                   "merge_parent_xform",
                   false,
                   "Merge parent Xform",
-                  "Merge USD primitives with their Xform parent if possible: "
-                  "USD does not allow nested UsdGeomGprim. Intermediary Xform will "
-                  "be defined to keep the USD file valid.");
+                  "Merge USD primitives with their Xform parent if possible. USD does not allow "
+                  "nested UsdGeomGprims, intermediary Xform prims will be defined to keep the USD "
+                  "file valid when encountering object hierarchies.");
 }
 
 /* ====== USD Import ====== */

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

@@ -343,7 +343,6 @@ static void import_startjob(void *customdata, wmJobWorkerStatus *worker_status)
   /* Setup parenthood and read actual object data. */
   i = 0;
   for (USDPrimReader *reader : archive->readers()) {
-
     if (!reader) {
       continue;
     }
@@ -355,11 +354,12 @@ static void import_startjob(void *customdata, wmJobWorkerStatus *worker_status)
 
     reader->read_object_data(data->bmain, 0.0);
 
-    data->prim_map[reader->object_prim_path()].push_back(RNA_id_pointer_create(&ob->id));
-
+    /* TODO: Move this outside the loop once when we support reading object data in parallel. */
+    data->prim_map.lookup_or_add_default(reader->object_prim_path())
+        .append(RNA_id_pointer_create(&ob->id));
     if (ob->data) {
-      data->prim_map[reader->data_prim_path()].push_back(
-          RNA_id_pointer_create(static_cast<ID *>(ob->data)));
+      data->prim_map.lookup_or_add_default(reader->data_prim_path())
+          .append(RNA_id_pointer_create(static_cast<ID *>(ob->data)));
     }
 
     USDPrimReader *parent = reader->parent();
@@ -384,7 +384,7 @@ static void import_startjob(void *customdata, wmJobWorkerStatus *worker_status)
       [&](const std::string &path, const std::string &name) {
         Material *mat = data->settings.mat_name_to_mat.lookup_default(name, nullptr);
         if (mat) {
-          data->prim_map[path].push_back(RNA_id_pointer_create(&mat->id));
+          data->prim_map.lookup_or_add_default(path).append(RNA_id_pointer_create(&mat->id));
         }
       });
 

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

@@ -145,7 +145,7 @@ struct USDSceneImportContext {
     if (!prim_map_dict) {
       prim_map_dict = new boost::python::dict;
 
-      for (auto &[path, ids] : prim_map) {
+      prim_map.foreach_item([&](const std::string &path, const Vector<PointerRNA> &ids) {
         if (!prim_map_dict->has_key(path)) {
           (*prim_map_dict)[path] = boost::python::list();
         }
@@ -155,7 +155,7 @@ struct USDSceneImportContext {
         for (auto &ptr_rna : ids) {
           list.append(ptr_rna);
         }
-      }
+      });
     }
 
     return *prim_map_dict;

source/blender/io/usd/intern/usd_hook.hh

@@ -3,17 +3,21 @@
  * SPDX-License-Identifier: GPL-2.0-or-later */
 #pragma once
 
+#include "BLI_map.hh"
+#include "BLI_vector.hh"
+
+#include "RNA_types.hh"
+
 #include <pxr/usd/usd/common.h>
 #include <pxr/usd/usdShade/material.h>
 
 struct Depsgraph;
 struct Material;
-struct PointerRNA;
 struct ReportList;
 
 namespace blender::io::usd {
 
-using ImportedPrimMap = std::map<std::string, std::vector<PointerRNA>>;
+using ImportedPrimMap = Map<std::string, Vector<PointerRNA>>;
 
 /** Ensure classes and type converters necessary for invoking import and export hooks
  * are registered. */

source/blender/io/usd/usd.hh

@@ -136,6 +136,7 @@ struct USDExportParams {
   bool only_deform_bones = false;
 
   bool convert_world_material = true;
+  bool merge_parent_xform = false;
 
   bool use_instancing = false;
   bool export_custom_properties = true;
@@ -168,8 +169,6 @@ struct USDExportParams {
   char collection[MAX_IDPROP_NAME] = "";
   char custom_properties_namespace[MAX_IDPROP_NAME] = "";
 
-  bool merge_parent_xform = false;
-
   /** Communication structure between the wmJob management code and the worker code. Currently used
    * to generate safely reports from the worker thread. */
   wmJobWorkerStatus *worker_status = nullptr;