In Mujoco USD interop, only consider dynamic friction when writing to and parsing from USD.

PiperOrigin-RevId: 783741987
Change-Id: I23c47a6e95c160c5c7d5c513e268c79e4236e2e4

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -831,9 +831,8 @@ class ModelWriter {
 
     mjsGeom *geom_default = mjs_getDefault(geom->element)->geom;
     if (geom->friction[0] != geom_default->friction[0]) {
-      WriteUniformAttribute(material_path, pxr::SdfValueTypeNames->Float,
-                            pxr::UsdPhysicsTokens->physicsStaticFriction,
-                            (float)geom->friction[0]);
+      // Since MuJoCo has no concept of static friction, only write dynamic
+      // friction to remain truthful to how MuJoCo perceives the data.
       WriteUniformAttribute(material_path, pxr::SdfValueTypeNames->Float,
                             pxr::UsdPhysicsTokens->physicsDynamicFriction,
                             (float)geom->friction[0]);

src/experimental/usd/usd_to_mjspec.cc

@@ -66,6 +66,7 @@
 #include <pxr/usd/usdPhysics/revoluteJoint.h>
 #include <pxr/usd/usdPhysics/rigidBodyAPI.h>
 #include <pxr/usd/usdPhysics/scene.h>
+#include <pxr/usd/usdShade/material.h>
 #include <pxr/usd/usdShade/materialBindingAPI.h>
 namespace {
 
@@ -1136,19 +1137,8 @@ void ParseMjcPhysicsJointAPI(mjsJoint* mj_joint,
 
 void ParseUsdPhysicsMaterialAPI(
     mjsGeom* geom, const pxr::UsdPhysicsMaterialAPI& material_api) {
-  auto static_friction_attr = material_api.GetStaticFrictionAttr();
   auto dynamic_friction_attr = material_api.GetDynamicFrictionAttr();
-  if (static_friction_attr.HasAuthoredValue()) {
-    if (dynamic_friction_attr.HasAuthoredValue()) {
-      mju_warning(
-          "Material %s has both static and dynamic friction authored, taking "
-          "the static value.",
-          material_api.GetPath().GetString().c_str());
-    }
-    float static_friction;
-    static_friction_attr.Get(&static_friction);
-    geom->friction[0] = static_friction;
-  } else if (dynamic_friction_attr.HasAuthoredValue()) {
+  if (dynamic_friction_attr.HasAuthoredValue()) {
     float dynamic_friction;
     dynamic_friction_attr.Get(&dynamic_friction);
     geom->friction[0] = dynamic_friction;
@@ -1205,10 +1195,6 @@ void ParseUsdPhysicsCollider(mjSpec* spec,
   geom->contype = 1;
   geom->conaffinity = 1;
 
-  if (prim.HasAPI<pxr::UsdPhysicsMassAPI>()) {
-    ParseUsdPhysicsMassAPIForGeom(geom, pxr::UsdPhysicsMassAPI(prim));
-  }
-
   if (prim.HasAPI<pxr::MjcPhysicsCollisionAPI>()) {
     ParseMjcPhysicsCollisionAPI(geom, pxr::MjcPhysicsCollisionAPI(prim));
   }
@@ -1225,6 +1211,13 @@ void ParseUsdPhysicsCollider(mjSpec* spec,
     }
   }
 
+  // Parse the Mass API after the physics material APIs since the density attribute
+  // from the Mass API is supposed to override the Material API density attribute.
+  // See https://openusd.org/dev/api/usd_physics_page_front.html
+  if (prim.HasAPI<pxr::UsdPhysicsMassAPI>()) {
+    ParseUsdPhysicsMassAPIForGeom(geom, pxr::UsdPhysicsMassAPI(prim));
+  }
+
   // Convert displayColor and displayOpacity to rgba.
   // We want to support primvar inheritance, hence FindPrimvarWithInheritance.
   pxr::UsdGeomPrimvarsAPI primvarsAPI(prim);

test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc

@@ -155,10 +155,6 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsMaterials) {
       stage,
       "/physics_materials_test/test_body/geom_with_friction.material:binding",
       "/physics_materials_test/PhysicsMaterials/geom_with_friction");
-  ExpectAttributeEqual(stage,
-                       "/physics_materials_test/PhysicsMaterials/"
-                       "geom_with_friction.physics:staticFriction",
-                       4.0f);
   ExpectAttributeEqual(stage,
                        "/physics_materials_test/PhysicsMaterials/"
                        "geom_with_friction.physics:dynamicFriction",