Fix Mesh PhysicsCollisionAPI by accounting for how it's referenced/instanced.

Because we introduce an additional parent scope when referencing a `Mesh` prim (required for instancing), we need to create an `over` prim that allows to actually operate on the to-be-referenced Mesh.

PiperOrigin-RevId: 753941355
Change-Id: I829be9e82fe650802afcbc46f4b2f4c81eb53bd1

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

@@ -66,7 +66,6 @@ using TfStaticData = pxr::TfStaticData<T>;
 
 // clang-format off
 TF_DEFINE_PRIVATE_TOKENS(kTokens,
-                         // Xform ops
                          ((body, "Body"))
                          ((body_name, "mujoco:body_name"))
                          ((geom, "Geom"))
@@ -89,7 +88,8 @@ TF_DEFINE_PRIVATE_TOKENS(kTokens,
                          ((outputsRgb, "outputs:rgb"))
                          ((inputsMetallic, "inputs:metallic"))
                          (repeat)
-                         );
+                         ((sourceMesh, pxr::UsdGeomTokens->Mesh))
+                        );
 
 // Using to satisfy TF_REGISTRY_FUNCTION macro below and avoid operating in PXR_NS.
 using pxr::TfEnum;
@@ -330,7 +330,7 @@ class ModelWriter {
     pxr::SdfPath subcomponent_path =
         CreatePrimSpec(data_, parent_path, name, pxr::UsdGeomTokens->Xform);
     pxr::SdfPath mesh_path =
-        CreatePrimSpec(data_, subcomponent_path, pxr::UsdGeomTokens->Mesh,
+        CreatePrimSpec(data_, subcomponent_path, kTokens->sourceMesh,
                        pxr::UsdGeomTokens->Mesh);
     mesh_paths_[*mesh->name] = subcomponent_path;
 
@@ -600,7 +600,15 @@ class ModelWriter {
     // Reference the mesh asset written in WriteMeshes.
     AddPrimReference(data_, subcomponent_path, mesh_paths_[*geom->meshname]);
 
-    return subcomponent_path;
+    // We want to use instancing with meshes, and it requires creating a parent
+    // scope to be referenced, with the Mesh prim as a child.
+    // To be able to actually manipulate the Mesh prim, we need to create and
+    // return the corresponding `over` prim as a child of the referencing prim.
+    pxr::SdfPath over_mesh_path =
+        CreatePrimSpec(data_, subcomponent_path, kTokens->sourceMesh,
+                       pxr::UsdGeomTokens->Mesh, pxr::SdfSpecifierOver);
+
+    return over_mesh_path;
   }
 
   pxr::SdfPath WriteSiteGeom(const mjsSite *site,

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

@@ -612,6 +612,9 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsRigidBody) {
 TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
   static constexpr char kXml[] = R"(
     <mujoco model="test">
+      <asset>
+        <mesh name="tetrahedron" vertex="0 0 0  1 0 0  0 1 0  0 0 1"/>
+      </asset>
       <worldbody>
         <geom name="ground" type="plane" size="5 5 0.1"
               contype="1" conaffinity="1"/>
@@ -636,6 +639,11 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
           <geom name="body_2_nocol" type="sphere" size="1"
                 contype="0" conaffinity="0"/>
         </body>
+        <body name="body_3" pos="0 3 3">
+          <joint type="free"/>
+          <geom name="body_3_col" type="mesh" mesh="tetrahedron"
+                contype="1" conaffinity="1"/>
+        </body>
       </worldbody>
     </mujoco>
   )";
@@ -655,7 +663,7 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
   // body_0/body_0 [rigidbody]
   //   body_0/body_0/body_0_col [collider]
   //
-  // body_0/body_0_0 [rigidbody]  <-- Note: USD reparents nested rigid bodies
+  // body_0/body_0_0 [rigidbody]  <-- USD reparents nested rigid bodies
   //     body_0/body_0/body_0_0/body_0_0_col [collider]
   //
   // body_1/body_1 [rigidbody]
@@ -664,6 +672,10 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
   //
   // body_2/body_2 [rigidbody]
   //   body_2/body_2/body_2_nocol []
+  //
+  // body_3/body_3 [rigidbody]
+  //   body_3/body_3/body_3_col []  <-- Intermediate prim for mesh instancing
+  //     body_3/body_3/body_3_col/Mesh [collider]
 
   // ground [collider] (Static collider)
   EXPECT_PRIM_VALID(stage, "/test/ground");
@@ -728,6 +740,26 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
                               pxr::UsdPhysicsRigidBodyAPI);
   EXPECT_PRIM_API_NOT_APPLIED(stage, "/test/body_2/body_2/body_2_nocol",
                               pxr::UsdPhysicsCollisionAPI);
+
+  // body_3/body_3 [rigidbody]
+  EXPECT_PRIM_VALID(stage, "/test/body_3/body_3");
+  EXPECT_PRIM_API_APPLIED(stage, "/test/body_3/body_3",
+                          pxr::UsdPhysicsRigidBodyAPI);
+  EXPECT_PRIM_API_NOT_APPLIED(stage, "/test/body_3/body_3",
+                              pxr::UsdPhysicsCollisionAPI);
+  //   body_3/body_3/body_3_col [] (Intermediate prim for mesh instancing)
+  EXPECT_PRIM_VALID(stage, "/test/body_3/body_3/body_3_col");
+  EXPECT_PRIM_API_NOT_APPLIED(stage, "/test/body_3/body_3/body_3_col",
+                              pxr::UsdPhysicsRigidBodyAPI);
+  EXPECT_PRIM_API_NOT_APPLIED(stage, "/test/body_3/body_3/body_3_col",
+                              pxr::UsdPhysicsCollisionAPI);
+  //     body_3/body_3/body_3_col/Mesh [collider]
+  EXPECT_PRIM_VALID(stage, "/test/body_3/body_3/body_3_col/Mesh");
+  EXPECT_PRIM_API_NOT_APPLIED(
+      stage, "/test/body_3/body_3/body_3_col/Mesh",
+      pxr::UsdPhysicsRigidBodyAPI);
+  EXPECT_PRIM_API_APPLIED(stage, "/test/body_3/body_3/body_3_col/Mesh",
+                          pxr::UsdPhysicsCollisionAPI);
 }
 
 }  // namespace