scripts/startup/bl_ui/properties_render.py

@@ -883,6 +883,25 @@ class RENDER_PT_simplify_greasepencil(RenderButtonsPanel, Panel, GreasePencilSim
     bl_options = {'DEFAULT_CLOSED'}
 
 
+class RENDER_PT_hydra_debug(RenderButtonsPanel, Panel):
+    bl_label = "Hydra Debug"
+    bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 200
+    COMPAT_ENGINES = {'HYDRA_STORM'}
+
+    @classmethod
+    def poll(cls, context):
+        prefs = context.preferences
+        return (context.engine in cls.COMPAT_ENGINES) and prefs.view.show_developer_ui
+
+    def draw(self, context):
+        layout = self.layout
+        layout.use_property_split = True
+
+        hydra = context.scene.hydra
+        layout.prop(hydra, "export_method")
+
+
 classes = (
     RENDER_PT_context,
     RENDER_PT_eevee_sampling,
@@ -915,6 +934,7 @@ classes = (
     RENDER_PT_opengl_color,
     RENDER_PT_opengl_options,
     RENDER_PT_opengl_film,
+    RENDER_PT_hydra_debug,
     RENDER_PT_color_management,
     RENDER_PT_color_management_curves,
     RENDER_PT_simplify,

source/blender/io/usd/CMakeLists.txt

@@ -97,6 +97,7 @@ set(SRC
   intern/usd_reader_volume.cc
   intern/usd_reader_xform.cc
 
+  usd.hh
   usd.h
 
   intern/usd_asset_utils.h

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

@@ -3,6 +3,7 @@
  * SPDX-License-Identifier: GPL-2.0-or-later */
 
 #include "usd.h"
+#include "usd.hh"
 #include "usd_hierarchy_iterator.h"
 
 #include <pxr/base/plug/registry.h>
@@ -198,67 +199,41 @@ static bool perform_usdz_conversion(const ExportJobData *data)
   return true;
 }
 
-static void export_startjob(void *customdata,
+static pxr::UsdStageRefPtr export_to_stage(const USDExportParams &params,
-                            /* Cannot be const, this function implements wm_jobs_start_callback.
+                                           Depsgraph *depsgraph,
-                             * NOLINTNEXTLINE: readability-non-const-parameter. */
+                                           const char *filepath,
                                            bool *stop,
                                            bool *do_update,
                                            float *progress)
 {
-  ExportJobData *data = static_cast<ExportJobData *>(customdata);
+  pxr::UsdStageRefPtr usd_stage = pxr::UsdStage::CreateNew(filepath);
-  data->export_ok = false;
-  data->start_time = timeit::Clock::now();
-
-  G.is_rendering = true;
-  if (data->wm) {
-    WM_set_locked_interface(data->wm, true);
-  }
-  G.is_break = false;
-
-  /* Construct the depsgraph for exporting. */
-  Scene *scene = DEG_get_input_scene(data->depsgraph);
-  if (data->params.visible_objects_only) {
-    DEG_graph_build_from_view_layer(data->depsgraph);
-  }
-  else {
-    DEG_graph_build_for_all_objects(data->depsgraph);
-  }
-  BKE_scene_graph_update_tagged(data->depsgraph, data->bmain);
-
-  *progress = 0.0f;
-  *do_update = true;
-
-  /* For restoring the current frame after exporting animation is done. */
-  const int orig_frame = scene->r.cfra;
-
-  pxr::UsdStageRefPtr usd_stage = pxr::UsdStage::CreateNew(data->unarchived_filepath);
   if (!usd_stage) {
-    /* This happens when the USD JSON files cannot be found. When that happens,
+    return usd_stage;
-     * the USD library doesn't know it has the functionality to write USDA and
-     * USDC files, and creating a new UsdStage fails. */
-    WM_reportf(RPT_ERROR,
-               "USD Export: unable to find suitable USD plugin to write %s",
-               data->unarchived_filepath);
-    return;
   }
 
-  usd_stage->SetMetadata(pxr::UsdGeomTokens->upAxis, pxr::VtValue(pxr::UsdGeomTokens->z));
+  Scene *scene = DEG_get_input_scene(depsgraph);
+  Main *bmain = DEG_get_bmain(depsgraph);
+
   usd_stage->SetMetadata(pxr::UsdGeomTokens->metersPerUnit, double(scene->unit.scale_length));
   usd_stage->GetRootLayer()->SetDocumentation(std::string("Blender v") +
                                               BKE_blender_version_string());
 
   /* Set up the stage for animated data. */
-  if (data->params.export_animation) {
+  if (params.export_animation) {
     usd_stage->SetTimeCodesPerSecond(FPS);
     usd_stage->SetStartTimeCode(scene->r.sfra);
     usd_stage->SetEndTimeCode(scene->r.efra);
   }
 
-  ensure_root_prim(usd_stage, data->params);
+  /* For restoring the current frame after exporting animation is done. */
+  const int orig_frame = scene->r.cfra;
 
-  USDHierarchyIterator iter(data->bmain, data->depsgraph, usd_stage, data->params);
+  usd_stage->SetMetadata(pxr::UsdGeomTokens->upAxis, pxr::VtValue(pxr::UsdGeomTokens->z));
+  ensure_root_prim(usd_stage, params);
 
-  if (data->params.export_animation) {
+  USDHierarchyIterator iter(bmain, depsgraph, usd_stage, params);
+
+  if (params.export_animation) {
     /* Writing the animated frames is not 100% of the work, but it's our best guess. */
     float progress_per_frame = 1.0f / std::max(1, (scene->r.efra - scene->r.sfra + 1));
 
@@ -270,15 +245,19 @@ static void export_startjob(void *customdata,
       /* Update the scene for the next frame to render. */
       scene->r.cfra = int(frame);
       scene->r.subframe = frame - scene->r.cfra;
-      BKE_scene_graph_update_for_newframe(data->depsgraph);
+      BKE_scene_graph_update_for_newframe(depsgraph);
 
       iter.set_export_frame(frame);
       iter.iterate_and_write();
 
+      if (progress) {
         *progress += progress_per_frame;
+      }
+      if (do_update) {
         *do_update = true;
       }
     }
+  }
   else {
     /* If we're not animating, a single iteration over all objects is enough. */
     iter.iterate_and_write();
@@ -296,14 +275,65 @@ static void export_startjob(void *customdata,
     }
   }
 
-  usd_stage->GetRootLayer()->Save();
-
   /* Finish up by going back to the keyframe that was current before we started. */
   if (scene->r.cfra != orig_frame) {
     scene->r.cfra = orig_frame;
-    BKE_scene_graph_update_for_newframe(data->depsgraph);
+    BKE_scene_graph_update_for_newframe(depsgraph);
   }
 
+  return usd_stage;
+}
+
+pxr::UsdStageRefPtr export_to_stage(const USDExportParams &params,
+                                    Depsgraph *depsgraph,
+                                    const char *filepath)
+{
+  return export_to_stage(params, depsgraph, filepath, nullptr, nullptr, nullptr);
+}
+
+static void export_startjob(void *customdata,
+                            /* Cannot be const, this function implements wm_jobs_start_callback.
+                             * NOLINTNEXTLINE: readability-non-const-parameter. */
+                            bool *stop,
+                            bool *do_update,
+                            float *progress)
+{
+  ExportJobData *data = static_cast<ExportJobData *>(customdata);
+  data->export_ok = false;
+  data->start_time = timeit::Clock::now();
+
+  G.is_rendering = true;
+  if (data->wm) {
+    WM_set_locked_interface(data->wm, true);
+  }
+  G.is_break = false;
+
+  /* Construct the depsgraph for exporting. */
+  if (data->params.visible_objects_only) {
+    DEG_graph_build_from_view_layer(data->depsgraph);
+  }
+  else {
+    DEG_graph_build_for_all_objects(data->depsgraph);
+  }
+  BKE_scene_graph_update_tagged(data->depsgraph, data->bmain);
+
+  *progress = 0.0f;
+  *do_update = true;
+
+  pxr::UsdStageRefPtr usd_stage = export_to_stage(
+      data->params, data->depsgraph, data->unarchived_filepath, stop, do_update, progress);
+  if (!usd_stage) {
+    /* This happens when the USD JSON files cannot be found. When that happens,
+     * the USD library doesn't know it has the functionality to write USDA and
+     * USDC files, and creating a new UsdStage fails. */
+    WM_reportf(RPT_ERROR,
+               "USD Export: unable to find suitable USD plugin to write %s",
+               data->unarchived_filepath);
+    return;
+  }
+
+  usd_stage->GetRootLayer()->Save();
+
   if (data->targets_usdz()) {
     bool usd_conversion_success = perform_usdz_conversion(data);
     if (!usd_conversion_success) {

source/blender/io/usd/usd.hh

@@ -0,0 +1,18 @@
+/* SPDX-FileCopyrightText: 2023 Blender Foundation
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+#pragma once
+
+#include <pxr/usd/usd/stage.h>
+
+struct Depsgraph;
+struct USDExportParams;
+
+namespace blender::io::usd {
+
+pxr::UsdStageRefPtr export_to_stage(const USDExportParams &params,
+                                    Depsgraph *depsgraph,
+                                    const char *filepath);
+
+};

source/blender/makesdna/DNA_scene_defaults.h

@@ -227,6 +227,11 @@
                     SCE_EEVEE_SSR_HALF_RESOLUTION | SCE_EEVEE_SHADOW_SOFT, \
   }
 
+#define _DNA_DEFAULT_SceneHydra \
+  { \
+    .export_method = SCE_HYDRA_EXPORT_HYDRA, \
+  }
+
 #define _DNA_DEFAULT_Scene \
   { \
     .cursor = _DNA_DEFAULT_View3DCursor, \
@@ -240,6 +245,8 @@
     .safe_areas = _DNA_DEFAULT_DisplaySafeAreas, \
  \
     .eevee = _DNA_DEFAULT_SceneEEVEE, \
+ \
+    .hydra = _DNA_DEFAULT_SceneHydra, \
   }
 
 /** \} */

source/blender/makesdna/DNA_scene_types.h

@@ -1858,6 +1858,11 @@ typedef struct SceneGpencil {
   char _pad[4];
 } SceneGpencil;
 
+typedef struct SceneHydra {
+  int export_method;
+  int _pad0;
+} SceneHydra;
+
 /** \} */
 
 /* -------------------------------------------------------------------- */
@@ -2004,6 +2009,7 @@ typedef struct Scene {
   struct SceneDisplay display;
   struct SceneEEVEE eevee;
   struct SceneGpencil grease_pencil_settings;
+  struct SceneHydra hydra;
 } Scene;
 
 /** \} */
@@ -2741,6 +2747,13 @@ enum {
   SCE_DISPLAY_AA_SAMPLES_32 = 32,
 };
 
+/** #SceneHydra->export_method */
+
+enum {
+  SCE_HYDRA_EXPORT_HYDRA = 0,
+  SCE_HYDRA_EXPORT_USD = 1,
+};
+
 /** \} */
 
 #ifdef __cplusplus

source/blender/makesrna/intern/rna_scene.c

@@ -1188,6 +1188,11 @@ static char *rna_SceneGpencil_path(const PointerRNA *UNUSED(ptr))
   return BLI_strdup("grease_pencil_settings");
 }
 
+static char *rna_SceneHydra_path(const PointerRNA * /*ptr*/)
+{
+  return BLI_strdup("hydra");
+}
+
 static int rna_RenderSettings_stereoViews_skip(CollectionPropertyIterator *iter,
                                                void *UNUSED(data))
 {
@@ -7975,6 +7980,36 @@ static void rna_def_scene_gpencil(BlenderRNA *brna)
   RNA_def_property_update(prop, NC_SCENE | ND_RENDER_OPTIONS, NULL);
 }
 
+static void rna_def_scene_hydra(BlenderRNA *brna)
+{
+  StructRNA *srna;
+  PropertyRNA *prop;
+
+  static const EnumPropertyItem hydra_export_method_items[] = {
+      {SCE_HYDRA_EXPORT_HYDRA,
+       "HYDRA",
+       0,
+       "Hydra",
+       "Fast interactive editing through native Hydra integration"},
+      {SCE_HYDRA_EXPORT_USD,
+       "USD",
+       0,
+       "USD",
+       "Export scene through USD file, for accurate comparison with USD file export"},
+      {0, nullptr, 0, nullptr, nullptr},
+  };
+
+  srna = RNA_def_struct(brna, "SceneHydra", nullptr);
+  RNA_def_struct_path_func(srna, "rna_SceneHydra_path");
+  RNA_def_struct_ui_text(srna, "Scene Hydra", "Scene Hydra render engine settings");
+
+  prop = RNA_def_property(srna, "export_method", PROP_ENUM, PROP_NONE);
+  RNA_def_property_enum_items(prop, hydra_export_method_items);
+  RNA_def_property_ui_text(
+      prop, "Export Method", "How to export the Blender scene to the Hydra render engine");
+  RNA_def_property_update(prop, NC_SCENE | ND_RENDER_OPTIONS, nullptr);
+}
+
 void RNA_def_scene(BlenderRNA *brna)
 {
   StructRNA *srna;
@@ -8468,6 +8503,11 @@ void RNA_def_scene(BlenderRNA *brna)
   RNA_def_property_struct_type(prop, "SceneGpencil");
   RNA_def_property_ui_text(prop, "Grease Pencil", "Grease Pencil settings for the scene");
 
+  /* Hydra */
+  prop = RNA_def_property(srna, "hydra", PROP_POINTER, PROP_NONE);
+  RNA_def_property_struct_type(prop, "SceneHydra");
+  RNA_def_property_ui_text(prop, "Hydra", "Hydra settings for the scene");
+
   /* Nestled Data. */
   /* *** Non-Animated *** */
   RNA_define_animate_sdna(false);
@@ -8486,6 +8526,7 @@ void RNA_def_scene(BlenderRNA *brna)
   rna_def_display_safe_areas(brna);
   rna_def_scene_display(brna);
   rna_def_scene_eevee(brna);
+  rna_def_scene_hydra(brna);
   rna_def_view_layer_aov(brna);
   rna_def_view_layer_lightgroup(brna);
   rna_def_view_layer_eevee(brna);

source/blender/render/hydra/CMakeLists.txt

@@ -30,6 +30,7 @@ set(INC
   ../../depsgraph
   ../../blenkernel
   ../../imbuf
+  ../../io/usd
   ../../gpu
   ../../gpu/intern
   ../../python/intern
@@ -65,30 +66,33 @@ set(SRC
   simple_light_task_delegate.cc
   simple_light_task_delegate.h
 
-  scene_delegate/blender_scene_delegate.h
   scene_delegate/blender_scene_delegate.cc
-  scene_delegate/id.h
+  scene_delegate/blender_scene_delegate.h
-  scene_delegate/id.cc
-  scene_delegate/object.h
-  scene_delegate/object.cc
-  scene_delegate/material.h
-  scene_delegate/material.cc
-  scene_delegate/mesh.h
-  scene_delegate/mesh.cc
-  scene_delegate/curves.h
   scene_delegate/curves.cc
-  scene_delegate/mtlx_hydra_adapter.h
+  scene_delegate/curves.h
-  scene_delegate/mtlx_hydra_adapter.cc
+  scene_delegate/id.cc
-  scene_delegate/light.h
+  scene_delegate/id.h
-  scene_delegate/light.cc
-  scene_delegate/world.h
-  scene_delegate/world.cc
-  scene_delegate/instancer.h
-  scene_delegate/instancer.cc
-  scene_delegate/image.h
   scene_delegate/image.cc
-  scene_delegate/volume.h
+  scene_delegate/image.h
+  scene_delegate/instancer.cc
+  scene_delegate/instancer.h
+  scene_delegate/light.cc
+  scene_delegate/light.h
+  scene_delegate/material.cc
+  scene_delegate/material.h
+  scene_delegate/mesh.cc
+  scene_delegate/mesh.h
+  scene_delegate/mtlx_hydra_adapter.cc
+  scene_delegate/mtlx_hydra_adapter.h
+  scene_delegate/object.cc
+  scene_delegate/object.h
+  scene_delegate/settings.h
+  scene_delegate/usd_scene_delegate.cc
+  scene_delegate/usd_scene_delegate.hh
   scene_delegate/volume.cc
+  scene_delegate/volume.h
+  scene_delegate/world.cc
+  scene_delegate/world.h
 )
 
 set(LIB

source/blender/render/hydra/engine.cc

@@ -9,8 +9,11 @@
 #include <pxr/usd/usdGeom/tokens.h>
 
 #include "BLI_path_util.h"
+
 #include "GPU_context.h"
 
+#include "DEG_depsgraph_query.h"
+
 #include "engine.h"
 
 namespace blender::render::hydra {
@@ -47,8 +50,6 @@ Engine::Engine(RenderEngine *bl_engine, const std::string &render_delegate_name)
   render_index_.reset(pxr::HdRenderIndex::New(render_delegate_.Get(), hd_drivers));
   free_camera_delegate_ = std::make_unique<pxr::HdxFreeCameraSceneDelegate>(
       render_index_.get(), pxr::SdfPath::AbsoluteRootPath().AppendElementString("freeCamera"));
-  scene_delegate_ = std::make_unique<BlenderSceneDelegate>(
-      render_index_.get(), pxr::SdfPath::AbsoluteRootPath().AppendElementString("scene"), this);
   render_task_delegate_ = std::make_unique<RenderTaskDelegate>(
       render_index_.get(), pxr::SdfPath::AbsoluteRootPath().AppendElementString("renderTask"));
   if (render_delegate_name == "HdStormRendererPlugin") {
@@ -62,21 +63,44 @@ Engine::Engine(RenderEngine *bl_engine, const std::string &render_delegate_name)
 
 void Engine::sync(Depsgraph *depsgraph, bContext *context)
 {
-  scene_delegate_->populate(depsgraph, context);
+  const Scene *scene = DEG_get_evaluated_scene(depsgraph);
+
+  if (scene->hydra.export_method == SCE_HYDRA_EXPORT_HYDRA) {
+    /* Fast path. */
+    usd_scene_delegate_.reset();
+
+    if (!hydra_scene_delegate_) {
+      pxr::SdfPath scene_path = pxr::SdfPath::AbsoluteRootPath().AppendElementString("scene");
+      hydra_scene_delegate_ = std::make_unique<BlenderSceneDelegate>(
+          render_index_.get(), scene_path, scene_delegate_settings_);
+    }
+    hydra_scene_delegate_->populate(depsgraph, context);
+  }
+  else {
+    /* Slow USD export for reference. */
+    if (hydra_scene_delegate_) {
+      /* Freeing the Hydra scene delegate crashes as something internal to USD
+       * still holds a pointer to it, only clear it instead. */
+      hydra_scene_delegate_->clear();
     }
 
-void Engine::sync_usd(pxr::UsdStageRefPtr stage)
+    if (!usd_scene_delegate_) {
-{
+      pxr::SdfPath scene_path = pxr::SdfPath::AbsoluteRootPath().AppendElementString("usd_scene");
-  if (!usd_delegate_) {
+      usd_scene_delegate_ = std::make_unique<USDSceneDelegate>(
-    usd_delegate_ = std::make_unique<pxr::UsdImagingDelegate>(
+          render_index_.get(), scene_path, scene_delegate_settings_);
-        render_index_.get(), pxr::SdfPath::AbsoluteRootPath().AppendElementString("usd"));
+    }
+    usd_scene_delegate_->populate(depsgraph, context);
   }
-  usd_delegate_->Populate(stage->GetPseudoRoot());
 }
 
 void Engine::set_sync_setting(const std::string &key, const pxr::VtValue &val)
 {
-  scene_delegate_->set_setting(key, val);
+  if (key == "MaterialXFilenameKey") {
+    scene_delegate_settings_.mx_filename_key = pxr::TfToken(val.Get<std::string>());
+  }
+  else {
+    scene_delegate_settings_.render_tokens.add_overwrite(pxr::TfToken(key), val);
+  }
 }
 
 void Engine::set_render_setting(const std::string &key, const pxr::VtValue &val)

source/blender/render/hydra/engine.h

@@ -18,9 +18,11 @@
 #include "CLG_log.h"
 
 #include "render_task_delegate.h"
-#include "scene_delegate/blender_scene_delegate.h"
 #include "simple_light_task_delegate.h"
 
+#include "scene_delegate/blender_scene_delegate.h"
+#include "scene_delegate/usd_scene_delegate.hh"
+
 namespace blender::render::hydra {
 
 extern struct CLG_LogRef *LOG_RENDER_HYDRA;
@@ -31,7 +33,6 @@ class Engine {
   virtual ~Engine() = default;
 
   void sync(Depsgraph *depsgraph, bContext *context);
-  void sync_usd(pxr::UsdStageRefPtr stage);
   virtual void render(Depsgraph *depsgraph) = 0;
 
   void set_sync_setting(const std::string &key, const pxr::VtValue &val);
@@ -42,18 +43,21 @@ class Engine {
  protected:
   float renderer_percent_done();
 
-  RenderEngine *bl_engine_;
+  RenderEngine *bl_engine_ = nullptr;
 
   /* The order is important due to deletion order */
   pxr::HgiUniquePtr hgi_;
   pxr::HdDriver hgi_driver_;
   pxr::HdPluginRenderDelegateUniqueHandle render_delegate_;
   std::unique_ptr<pxr::HdRenderIndex> render_index_;
-  std::unique_ptr<BlenderSceneDelegate> scene_delegate_;
+
+  SceneDelegateSettings scene_delegate_settings_;
+  std::unique_ptr<BlenderSceneDelegate> hydra_scene_delegate_;
+  std::unique_ptr<USDSceneDelegate> usd_scene_delegate_;
+
   std::unique_ptr<RenderTaskDelegate> render_task_delegate_;
   std::unique_ptr<pxr::HdxFreeCameraSceneDelegate> free_camera_delegate_;
   std::unique_ptr<SimpleLightTaskDelegate> simple_light_task_delegate_;
-  std::unique_ptr<pxr::UsdImagingDelegate> usd_delegate_;
   std::unique_ptr<pxr::HdEngine> engine_;
 };
 

source/blender/render/hydra/python.cc

@@ -123,24 +123,6 @@ static PyObject *engine_sync_func(PyObject * /*self*/, PyObject *args)
   Py_RETURN_NONE;
 }
 
-static PyObject *engine_sync_usd_func(PyObject * /*self*/, PyObject *args)
-{
-  PyObject *pyengine, *pystage;
-  if (!PyArg_ParseTuple(args, "OO", &pyengine, &pystage)) {
-    Py_RETURN_NONE;
-  }
-
-  Engine *engine = (Engine *)PyLong_AsVoidPtr(pyengine);
-
-  boost::python::extract<pxr::UsdStageRefPtr> extract(pystage);
-  pxr::UsdStagePtr stage = extract();
-
-  engine->sync_usd(stage);
-
-  CLOG_INFO(LOG_RENDER_HYDRA, 2, "Engine %016llx", engine);
-  Py_RETURN_NONE;
-}
-
 static PyObject *engine_render_func(PyObject * /*self*/, PyObject *args)
 {
   PyObject *pyengine, *pydepsgraph;
@@ -249,7 +231,6 @@ static PyMethodDef methods[] = {
     {"engine_create", engine_create_func, METH_VARARGS, ""},
     {"engine_free", engine_free_func, METH_VARARGS, ""},
     {"engine_sync", engine_sync_func, METH_VARARGS, ""},
-    {"engine_sync_usd", engine_sync_usd_func, METH_VARARGS, ""},
     {"engine_render", engine_render_func, METH_VARARGS, ""},
     {"engine_view_draw", engine_view_draw_func, METH_VARARGS, ""},
     {"engine_set_sync_setting", engine_set_sync_setting_func, METH_VARARGS, ""},

source/blender/render/hydra/scene_delegate/blender_scene_delegate.cc

@@ -28,8 +28,8 @@ bool BlenderSceneDelegate::ShadingSettings::operator==(const ShadingSettings &ot
 
 BlenderSceneDelegate::BlenderSceneDelegate(pxr::HdRenderIndex *parent_index,
                                            pxr::SdfPath const &delegate_id,
-                                           Engine *engine)
+                                           const SceneDelegateSettings &settings)
-    : HdSceneDelegate(parent_index, delegate_id), engine(engine)
+    : HdSceneDelegate(parent_index, delegate_id), settings(settings)
 {
   instancer_data_ = std::make_unique<InstancerData>(this, instancer_prim_id());
 }
@@ -259,16 +259,6 @@ void BlenderSceneDelegate::clear()
   view3d = nullptr;
 }
 
-void BlenderSceneDelegate::set_setting(const std::string &key, const pxr::VtValue &val)
-{
-  if (key == "MaterialXFilenameKey") {
-    settings.mx_filename_key = pxr::TfToken(val.Get<std::string>());
-  }
-  else {
-    settings.render_tokens.add_overwrite(pxr::TfToken(key), val);
-  }
-}
-
 pxr::SdfPath BlenderSceneDelegate::prim_id(ID *id, const char *prefix) const
 {
   /* Making id of object in form like <prefix>_<pointer in 16 hex digits format> */

source/blender/render/hydra/scene_delegate/blender_scene_delegate.h

@@ -17,6 +17,7 @@
 #include "light.h"
 #include "mesh.h"
 #include "object.h"
+#include "settings.h"
 #include "volume.h"
 #include "world.h"
 
@@ -34,11 +35,6 @@ class BlenderSceneDelegate : public pxr::HdSceneDelegate {
   friend MaterialData; /* has access to objects and instancers */
 
  public:
-  struct Settings {
-    pxr::TfToken mx_filename_key;
-    Map<pxr::TfToken, pxr::VtValue> render_tokens;
-  };
-
   struct ShadingSettings {
     bool use_scene_lights = true;
     bool use_scene_world = true;
@@ -51,7 +47,7 @@ class BlenderSceneDelegate : public pxr::HdSceneDelegate {
 
   BlenderSceneDelegate(pxr::HdRenderIndex *parent_index,
                        pxr::SdfPath const &delegate_id,
-                       Engine *engine);
+                       const SceneDelegateSettings &settings);
   ~BlenderSceneDelegate() override = default;
 
   /* Delegate methods */
@@ -77,14 +73,12 @@ class BlenderSceneDelegate : public pxr::HdSceneDelegate {
 
   void populate(Depsgraph *depsgraph, bContext *context);
   void clear();
-  void set_setting(const std::string &key, const pxr::VtValue &val);
 
   Depsgraph *depsgraph = nullptr;
   bContext *context = nullptr;
   View3D *view3d = nullptr;
   Scene *scene = nullptr;
-  Engine *engine;
+  const SceneDelegateSettings &settings;
-  Settings settings;
   ShadingSettings shading_settings;
 
  private:

source/blender/render/hydra/scene_delegate/image.cc

@@ -19,7 +19,7 @@ namespace blender::render::hydra {
 static std::string get_cache_file(const std::string &file_name, bool mkdir = true)
 {
   char dir_path[FILE_MAX];
-  BLI_path_join(dir_path, sizeof(dir_path), BKE_tempdir_session(), "hydra_image_cache");
+  BLI_path_join(dir_path, sizeof(dir_path), BKE_tempdir_session(), "hydra", "image_cache");
   if (mkdir) {
     BLI_dir_create_recursive(dir_path);
   }

source/blender/render/hydra/scene_delegate/light.cc

@@ -142,7 +142,7 @@ pxr::VtValue LightData::get_data(pxr::TfToken const &key) const
     return pxr::VtValue(it->second);
   }
 
-  pxr::VtValue *ret_ptr = scene_delegate_->settings.render_tokens.lookup_ptr(key);
+  const pxr::VtValue *ret_ptr = scene_delegate_->settings.render_tokens.lookup_ptr(key);
   if (ret_ptr) {
     return *ret_ptr;
   }

source/blender/render/hydra/scene_delegate/settings.h

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: Apache-2.0
+ * Copyright 2011-2023 Blender Foundation */
+
+#pragma once
+
+#include <pxr/base/tf/token.h>
+
+#include "BLI_map.hh"
+
+namespace blender::render::hydra {
+
+struct SceneDelegateSettings {
+  pxr::TfToken mx_filename_key;
+  Map<pxr::TfToken, pxr::VtValue> render_tokens;
+};
+
+};  // namespace blender::render::hydra

source/blender/render/hydra/scene_delegate/usd_scene_delegate.cc

@@ -0,0 +1,78 @@
+/* SPDX-License-Identifier: Apache-2.0
+ * Copyright 2011-2022 Blender Foundation */
+
+#include "usd_scene_delegate.hh"
+
+#include "BLI_fileops.h"
+#include "BLI_path_util.h"
+#include "BLI_string.h"
+
+#include "BKE_appdir.h"
+
+#include "DEG_depsgraph_query.h"
+
+#include "usd.h"
+#include "usd.hh"
+
+namespace blender::render::hydra {
+
+USDSceneDelegate::USDSceneDelegate(pxr::HdRenderIndex *render_index,
+                                   pxr::SdfPath const &delegate_id,
+                                   const SceneDelegateSettings &settings)
+    : render_index_(render_index), delegate_id_(delegate_id), settings_(settings)
+{
+  /* Temporary directory to write any additional files to, like image or VDB files. */
+  char unique_name[FILE_MAXFILE];
+  BLI_snprintf(unique_name, sizeof(unique_name), "%p", this);
+
+  char dir_path[FILE_MAX];
+  BLI_path_join(
+      dir_path, sizeof(dir_path), BKE_tempdir_session(), "usd_scene_delegate", unique_name);
+
+  char file_path[FILE_MAX];
+  BLI_path_join(file_path, sizeof(file_path), dir_path, "scene.usdc");
+
+  temp_dir_ = dir_path;
+  temp_file_ = file_path;
+}
+
+USDSceneDelegate::~USDSceneDelegate()
+{
+  BLI_delete(temp_dir_.c_str(), true, true);
+}
+
+void USDSceneDelegate::populate(Depsgraph *depsgraph, bContext * /*context*/)
+{
+  /* TODO: implement MaterialX support in USD exporter. */
+  const bool use_materialx = !settings_.mx_filename_key.IsEmpty();
+  (void)use_materialx;
+
+  USDExportParams params = {};
+  params.export_hair = true;
+  params.export_uvmaps = true;
+  params.export_normals = true;
+  params.export_materials = true;
+  params.selected_objects_only = false;
+  params.visible_objects_only = true;
+  params.use_instancing = true;
+  params.evaluation_mode = DEG_get_mode(depsgraph);
+  params.generate_preview_surface = true;
+  params.export_textures = true;
+  params.overwrite_textures = true;
+  params.relative_paths = true;
+
+  /* Create clean directory for export. */
+  BLI_delete(temp_dir_.c_str(), true, true);
+  BLI_dir_create_recursive(temp_dir_.c_str());
+
+  /* Free previous delegate and stage first to save memory. */
+  delegate_.reset();
+  stage_.Reset();
+
+  /* Convert depsgraph to stage + aditional file in temp directory. */
+  stage_ = io::usd::export_to_stage(params, depsgraph, temp_file_.c_str());
+  delegate_ = std::make_unique<pxr::UsdImagingDelegate>(render_index_, delegate_id_);
+  delegate_->Populate(stage_->GetPseudoRoot());
+}
+
+}  // namespace blender::render::hydra

source/blender/render/hydra/scene_delegate/usd_scene_delegate.hh

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: Apache-2.0
+ * Copyright 2023 Blender Foundation */
+
+#pragma once
+
+#include <pxr/imaging/hd/renderIndex.h>
+#include <pxr/usdImaging/usdImaging/delegate.h>
+
+#include <string>
+
+#include "settings.h"
+
+struct Depsgraph;
+struct bContext;
+
+namespace blender::render::hydra {
+
+/* Populate Hydra render index using USD file export, for testing. */
+class USDSceneDelegate {
+  pxr::HdRenderIndex *render_index_;
+  pxr::SdfPath const delegate_id_;
+  pxr::UsdStageRefPtr stage_;
+  std::unique_ptr<pxr::UsdImagingDelegate> delegate_;
+  const SceneDelegateSettings &settings_;
+
+  std::string temp_dir_;
+  std::string temp_file_;
+
+ public:
+  USDSceneDelegate(pxr::HdRenderIndex *render_index,
+                   pxr::SdfPath const &delegate_id,
+                   const SceneDelegateSettings &settings_);
+  ~USDSceneDelegate();
+
+  void populate(Depsgraph *depsgraph, bContext *context);
+};
+
+}  // namespace blender::render::hydra