Preserve physical properties in USD IO

CMakeLists.txt

@@ -475,6 +475,18 @@ mt_library(
     io_file_save_options_public_headers
 )
 
+mt_library(
+  NAME io_json_utils
+  HEADERS_VARS io_json_utils_public_headers
+  SOURCES_VARS io_json_utils_sources
+  PUBLIC_LINK_LIBRARIES
+    character
+    nlohmann_json::nlohmann_json
+  PRIVATE_LINK_LIBRARIES
+    common
+    math
+)
+
 mt_library(
   NAME io_fbx
   HEADERS_VARS
@@ -488,6 +500,7 @@ mt_library(
     io_skeleton
   PRIVATE_LINK_LIBRARIES
     io_common
+    io_json_utils
     ${io_fbx_private_link_libraries}
     OpenFBX
 )
@@ -496,18 +509,6 @@ if(MOMENTUM_ENABLE_FBX_SAVING)
   target_compile_definitions(io_fbx PRIVATE MOMENTUM_WITH_FBX_SDK)
 endif()
 
-mt_library(
-  NAME io_json_utils
-  HEADERS_VARS io_json_utils_public_headers
-  SOURCES_VARS io_json_utils_sources
-  PUBLIC_LINK_LIBRARIES
-    character
-    nlohmann_json::nlohmann_json
-  PRIVATE_LINK_LIBRARIES
-    common
-    math
-)
-
 mt_library(
   NAME io_gltf
   HEADERS_VARS

momentum/io/fbx/fbx_builder.cpp

@@ -78,6 +78,7 @@ void FbxBuilder::addCharacter(const Character& character, const FileSaveOptions&
   // Create skeleton hierarchy
   auto skeletonResult = createSkeletonNodes(character, impl_->scene);
   addMetaData(skeletonResult.rootNode, character);
+  addPhysicalProperties(character, skeletonResult.jointToNodeMap);
 
   if (options.locators) {
     createLocatorNodes(character, impl_->scene, skeletonResult.nodes);
@@ -119,6 +120,7 @@ void FbxBuilder::addRigidBody(
   // Create skeleton hierarchy
   auto skeletonResult = createSkeletonNodes(character, impl_->scene);
   addMetaData(skeletonResult.rootNode, character);
+  addPhysicalProperties(character, skeletonResult.jointToNodeMap);
 
   // Prefix skeleton joint names with charName to avoid collisions when
   // multiple rigid bodies share the scene (e.g. "root" -> "controller_l:root").

momentum/io/fbx/fbx_io.cpp

@@ -20,6 +20,8 @@
 #include "momentum/io/skeleton/parameter_transform_io.h"
 #include "momentum/io/skeleton/parameters_io.h"
 
+#include <gsl/gsl>
+
 #include <variant>
 #endif // MOMENTUM_WITH_FBX_SDK
 
@@ -47,6 +49,11 @@ void saveFbxCommon(
   // initialize FBX SDK and prepare for export
   // ---------------------------------------------
   auto* manager = ::fbxsdk::FbxManager::Create();
+  // FBX SDK uses manual memory management. Use a scope guard so the manager — and the exporter,
+  // scene, and nodes it owns — are always destroyed, including when an exception is thrown mid-
+  // export (e.g. by addPhysicalProperties). Without it, a throw leaks the manager (flagged by
+  // ASAN).
+  const auto managerGuard = gsl::finally([manager]() { manager->Destroy(); });
   auto* ios = ::fbxsdk::FbxIOSettings::Create(manager, IOSROOT);
   manager->SetIOSettings(ios);
 
@@ -89,6 +96,7 @@ void saveFbxCommon(
   // Create skeleton hierarchy
   auto skeletonResult = createSkeletonNodes(character, scene);
   addMetaData(skeletonResult.rootNode, character);
+  addPhysicalProperties(character, skeletonResult.jointToNodeMap);
   createLocatorNodes(character, scene, skeletonResult.nodes);
   createCollisionGeometryNodes(character, scene, skeletonResult.nodes);
 
@@ -158,15 +166,9 @@ void saveFbxCommon(
   // close the fbx exporter
   // ---------------------------------------------
 
-  // finally export the scene
+  // finally export the scene; managerGuard destroys the exporter, scene, and manager (and
+  // everything they own) when this function returns or throws.
   lExporter->Export(scene);
-  lExporter->Destroy();
-
-  // destroy the scene and the manager
-  if (scene != nullptr) {
-    scene->Destroy();
-  }
-  manager->Destroy();
 }
 
 } // namespace

momentum/io/fbx/fbx_io_internal.h

@@ -12,11 +12,13 @@
 
 #include "momentum/character/blend_shape.h"
 #include "momentum/character/character.h"
+#include "momentum/character/character_utility.h"
 #include "momentum/character/collision_geometry_state.h"
 #include "momentum/character/marker.h"
 #include "momentum/character/skin_weights.h"
 #include "momentum/common/exception.h"
 #include "momentum/common/log.h"
+#include "momentum/io/common/json_utils.h"
 #include "momentum/io/file_save_options.h"
 #include "momentum/math/constants.h"
 #include "momentum/math/mesh.h"
@@ -28,13 +30,16 @@
 // They do the most awful things to isnan in here
 #include <fbxsdk.h>
 #include <fbxsdk/fileio/fbxiosettings.h>
+#include <nlohmann/json.hpp>
 
 #ifdef isnan
 #undef isnan
 #endif
 
 #include <span>
+#include <string>
 #include <unordered_map>
+#include <unordered_set>
 
 namespace momentum::fbx_internal {
 
@@ -177,6 +182,37 @@ inline SkeletonNodeResult createSkeletonNodes(
   return result;
 }
 
+inline void addPhysicalProperties(
+    const Character& character,
+    const std::unordered_map<size_t, fbxsdk::FbxNode*>& jointToNodeMap) {
+  std::unordered_set<size_t> seenJoints;
+  for (const auto& jointProperties : character.physicalProperties) {
+    const size_t jointIndex =
+        resolvePhysicalPropertiesJointIndex(jointProperties, character.skeleton);
+
+    MT_THROW_IF(
+        jointIndex == kInvalidIndex || jointIndex >= character.skeleton.joints.size(),
+        "Physical properties entry references unknown joint '{}'",
+        jointProperties.jointName);
+    MT_THROW_IF(
+        !seenJoints.insert(jointIndex).second,
+        "Multiple physical properties entries resolve to joint '{}' (index {})",
+        character.skeleton.joints.at(jointIndex).name,
+        jointIndex);
+
+    const auto nodeIt = jointToNodeMap.find(jointIndex);
+    MT_THROW_IF(
+        nodeIt == jointToNodeMap.end() || nodeIt->second == nullptr,
+        "Unable to find FBX node for physical properties joint '{}'",
+        character.skeleton.joints.at(jointIndex).name);
+
+    nlohmann::json physicalPropertiesJson;
+    jointPhysicalPropertiesToJson(jointProperties, physicalPropertiesJson);
+    ::fbxsdk::FbxProperty::Create(nodeIt->second, ::fbxsdk::FbxStringDT, "physicalProperties")
+        .Set(FbxString(physicalPropertiesJson.dump().c_str()));
+  }
+}
+
 // ============================================================================
 // Metadata, locators, collision geometry
 // ============================================================================

momentum/io/fbx/openfbx_loader.cpp

@@ -16,6 +16,7 @@
 #include "momentum/common/filesystem.h"
 #include "momentum/common/log.h"
 #include "momentum/io/common/gsl_utils.h"
+#include "momentum/io/common/json_utils.h"
 #include "momentum/io/fbx/polygon_data.h"
 #include "momentum/io/skeleton/locator_io.h"
 #include "momentum/io/skeleton/parameter_limits_io.h"
@@ -30,6 +31,7 @@
 #include <gsl/span_ext>
 
 #include <cmath>
+#include <exception>
 #include <fstream>
 #include <functional>
 #include <numeric>
@@ -113,6 +115,44 @@ const char* propertyTypeStr(ofbx::IElementProperty::Type type) {
   }
 }
 
+ofbx::IElement* resolveProperty(const ofbx::Object& object, const char* name);
+std::string resolveStringProperty(const ofbx::Object& object, const char* name);
+
+PhysicalProperties loadPhysicalPropertiesFromNodes(
+    const Skeleton& skeleton,
+    const std::vector<const ofbx::Object*>& jointFbxNodes) {
+  PhysicalProperties result;
+
+  for (size_t jointIndex = 0; jointIndex < jointFbxNodes.size(); ++jointIndex) {
+    const ofbx::Object* jointNode = jointFbxNodes.at(jointIndex);
+    if (jointNode == nullptr) {
+      continue;
+    }
+
+    // Reading the FBX property itself can throw: resolveProperty/resolveStringProperty use MT_THROW
+    // (std::runtime_error) when a node carries a malformed "physicalProperties" property, and the
+    // embedded JSON may be corrupt. Catch broadly so a single bad joint is skipped rather than
+    // aborting the whole character load.
+    try {
+      if (resolveProperty(*jointNode, "physicalProperties") == nullptr) {
+        continue;
+      }
+
+      const nlohmann::json physicalPropertiesJson =
+          nlohmann::json::parse(resolveStringProperty(*jointNode, "physicalProperties"));
+      result.push_back(jointPhysicalPropertiesFromJson(
+          physicalPropertiesJson, skeleton.joints.at(jointIndex).name, jointIndex));
+    } catch (const std::exception& e) {
+      MT_LOGW(
+          "Skipping physical properties for FBX joint '{}': {}",
+          skeleton.joints.at(jointIndex).name,
+          e.what());
+    }
+  }
+
+  return result;
+}
+
 template <typename T>
 Eigen::Quaternion<T> computeEulerRotation(
     const Eigen::Vector3<T>& angles,
@@ -1471,6 +1511,7 @@ std::tuple<Character, std::vector<MatrixXf>, float> loadOpenFbx(
       metadata);
   result.resetJointMap();
   result.inverseBindPose = inverseBindPoseTransforms;
+  result.physicalProperties = loadPhysicalPropertiesFromNodes(result.skeleton, jointFbxNodes);
 
   return {result, jointParamMotions, scene->getSceneFrameRate()};
 }