Apply articulation API to the correct body when converting to USD.

PiperOrigin-RevId: 776206773
Change-Id: I84b5687a9900b42fd208124414ac98feaaa9f8c4

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

@@ -1821,6 +1821,17 @@ class ModelWriter {
                                          : pxr::KindTokens->subcomponent;
     SetPrimKind(data_, body_path, kind);
 
+    // If the parent is not the world body, but is child of the world body
+    // then we need to apply the articulation root API.
+    if (parent_id != kWorldIndex) {
+      int parent_parent_id =
+          mjs_getId(mjs_getParent(parent->element)->element);
+      if (parent_parent_id == kWorldIndex) {
+        ApplyApiSchema(data_, parent_path,
+                       pxr::UsdPhysicsTokens->PhysicsArticulationRootAPI);
+      }
+    }
+
     // Apply the PhysicsRigidBodyAPI schema if we are writing physics.
     if (write_physics_) {
       // If the body had a mass specified then it must have either inertia or
@@ -1926,13 +1937,6 @@ class ModelWriter {
                        pxr::UsdGeomTokens->Xform);
     SetPrimKind(data_, world_group_path, pxr::KindTokens->group);
 
-    if (write_physics_) {
-      // Apply the PhysicsArticulationRootAPI to the world body so that
-      // everything under it is automatically considered an articulation.
-      ApplyApiSchema(data_, world_group_path,
-                     pxr::UsdPhysicsTokens->PhysicsArticulationRootAPI);
-    }
-
     return world_group_path;
   }
 };

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

@@ -1139,6 +1139,9 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsRigidBody) {
             <geom name="test_geom_2" type="sphere" size="1"/>
           </body>
         </body>
+        <body name="test_body_3" pos="0 0 0">
+          <geom name="test_geom_3" type="sphere" size="1"/>
+        </body>
       </worldbody>
     </mujoco>
   )";
@@ -1153,18 +1156,26 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsRigidBody) {
   EXPECT_PRIM_VALID(stage, "/physics_test/test_body");
   EXPECT_PRIM_VALID(stage, "/physics_test/test_body/test_body_2");
 
-  // Articulation root is applied to the root of the physics scene (worldbody).
-  EXPECT_PRIM_API_APPLIED(stage, "/physics_test",
-                          pxr::UsdPhysicsArticulationRootAPI);
-
   EXPECT_PRIM_API_APPLIED(stage, "/physics_test/test_body",
                           pxr::UsdPhysicsRigidBodyAPI);
-  EXPECT_PRIM_API_NOT_APPLIED(stage, "/physics_test/test_body",
-                              pxr::UsdPhysicsArticulationRootAPI);
   EXPECT_PRIM_API_APPLIED(stage, "/physics_test/test_body/test_body_2",
                           pxr::UsdPhysicsRigidBodyAPI);
+  EXPECT_PRIM_API_APPLIED(stage, "/physics_test/test_body_3",
+                          pxr::UsdPhysicsRigidBodyAPI);
+
+  // Articulation root is applied to the children of the world body.
+  EXPECT_PRIM_API_APPLIED(stage, "/physics_test/test_body",
+                          pxr::UsdPhysicsArticulationRootAPI);
+  // test_body_3 is a child of the world but has no children so should not be
+  // an articulation root.
+  EXPECT_PRIM_API_NOT_APPLIED(stage, "/physics_test/test_body_3",
+                          pxr::UsdPhysicsArticulationRootAPI);
+
+  // Articulation root is not applied to other bodies or world body.
+  EXPECT_PRIM_API_NOT_APPLIED(stage, "/physics_test",
+                          pxr::UsdPhysicsArticulationRootAPI);
   EXPECT_PRIM_API_NOT_APPLIED(stage, "/physics_test/test_body/test_body_2",
-                              pxr::UsdPhysicsArticulationRootAPI);
+                          pxr::UsdPhysicsArticulationRootAPI);
 
   // Geoms should not have RigidBodyAPI applied either.
   EXPECT_PRIM_API_NOT_APPLIED(stage, "/physics_test/test_body/test_geom",