Fix: USD export: missing animation frames.

This fixes a bug where only the time sample for the first frame
is written to a USD animation.

Replaced the hard-coded USDExporterContext::time_code value with a
USDExporterContext::get_time_code function wrapper which is called
from USDAbstractWriter::get_export_time_code() to query the current
frame when writing an animation.

Pull Request: https://projects.blender.org/blender/blender/pulls/111248

source/blender/io/usd/hydra/material.cc

@@ -49,6 +49,7 @@ void MaterialData::init()
   /* Create temporary in memory stage. */
   pxr::UsdStageRefPtr stage = pxr::UsdStage::CreateInMemory();
   pxr::UsdTimeCode time = pxr::UsdTimeCode::Default();
+  auto get_time_code = [time]() { return time; };
   pxr::SdfPath material_library_path("/_materials");
   pxr::SdfPath material_path = material_library_path.AppendChild(
       pxr::TfToken(prim_id.GetElementString()));
@@ -63,7 +64,7 @@ void MaterialData::init()
                                          scene_delegate_->depsgraph,
                                          stage,
                                          material_library_path,
-                                         time,
+                                         get_time_code,
                                          export_params,
                                          image_cache_file_path()};
 

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

@@ -8,6 +8,8 @@
 #include <pxr/usd/sdf/path.h>
 #include <pxr/usd/usd/common.h>
 
+#include <functional>
+
 struct Depsgraph;
 struct Main;
 
@@ -20,7 +22,13 @@ struct USDExporterContext {
   Depsgraph *depsgraph;
   const pxr::UsdStageRefPtr stage;
   const pxr::SdfPath usd_path;
-  pxr::UsdTimeCode time_code;
+  /**
+   * Wrap a function which returns the current time code
+   * for export.  This is necessary since the context
+   * may be used for exporting an animation over a sequece
+   * of frames.
+   */
+  std::function<pxr::UsdTimeCode()> get_time_code;
   const USDExportParams &export_params;
   std::string export_file_path;
 };

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

@@ -78,9 +78,10 @@ USDExporterContext USDHierarchyIterator::create_usd_export_context(const Hierarc
    * `pxr::UsdStage::CreateNew` function). */
   const pxr::SdfLayerHandle root_layer = stage_->GetRootLayer();
   const std::string export_file_path = root_layer->GetRealPath();
+  auto get_time_code = [this]() { return this->export_time_; };
 
   return USDExporterContext{
-      bmain_, depsgraph_, stage_, path, export_time_, params_, export_file_path};
+      bmain_, depsgraph_, stage_, path, get_time_code, params_, export_file_path};
 }
 
 AbstractHierarchyWriter *USDHierarchyIterator::create_transform_writer(

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

@@ -59,7 +59,8 @@ std::string USDAbstractWriter::get_export_file_path() const
 pxr::UsdTimeCode USDAbstractWriter::get_export_time_code() const
 {
   if (is_animated_) {
-    return usd_export_context_.time_code;
+    BLI_assert(usd_export_context_.get_time_code);
+    return usd_export_context_.get_time_code();
   }
   /* By using the default timecode USD won't even write a single `timeSample` for non-animated
    * data. Instead, it writes it as non-timesampled. */