Write mesh inertia to defaults when exporting to xml. Fixes #2478

PiperOrigin-RevId: 735338748
Change-Id: Id0b71d06c77baef417f7968ae8a14154243a1f83

src/xml/xml_native_writer.cc

@@ -220,9 +220,6 @@ void mjXWriter::OneMesh(XMLElement* elem, const mjCMesh* mesh, mjCDef* def) {
     }
     WriteAttrTxt(elem, "content_type", mesh->ContentType());
     WriteAttrTxt(elem, "file", mesh->File());
-    if (mesh->Inertia() != def->Mesh().Inertia()) {
-      WriteAttrTxt(elem, "inertia", FindValue(meshinertia_map, 4, mesh->Inertia()));
-    }
 
     // write vertex data
     if (!mesh->UserVert().empty()) {
@@ -250,6 +247,9 @@ void mjXWriter::OneMesh(XMLElement* elem, const mjCMesh* mesh, mjCDef* def) {
   }
 
   // defaults and regular
+  if (mesh->Inertia() != def->Mesh().Inertia()) {
+    WriteAttrTxt(elem, "inertia", FindValue(meshinertia_map, 4, mesh->Inertia()));
+  }
   WriteAttr(elem, "refpos", 3, mesh->Refpos(), def->Mesh().Refpos());
   WriteAttr(elem, "refquat", 4, mesh->Refquat(), def->Mesh().Refquat());
   WriteAttr(elem, "scale", 3, mesh->Scale(), def->Mesh().Scale());

test/xml/xml_api_test.cc

@@ -34,6 +34,7 @@ namespace {
 using ::testing::IsNull;
 using ::testing::NotNull;
 using ::testing::StartsWith;
+using ::testing::HasSubstr;
 
 static constexpr char xml[] = R"(
   <mujoco>
@@ -151,11 +152,46 @@ TEST_F(MujocoTest, SaveXml) {
   mjModel* saved_model = mj_compile(saved_spec, 0);
   EXPECT_THAT(saved_model, NotNull()) << "Invalid model: " << error.data();
 
-  mjtNum tol = 0;
+  mj_deleteSpec(spec);
-  std::string field = "";
+  mj_deleteSpec(saved_spec);
-  EXPECT_LE(CompareModel(model, saved_model, field), tol)
+  mj_deleteModel(model);
-            << "Expected and attached models are different!\n"
+  mj_deleteModel(saved_model);
-            << "Different field: " << field << '\n';
+}
+
+TEST_F(MujocoTest, SaveXmlWithDefaultMesh) {
+  static constexpr char xml[] = R"(
+    <mujoco>
+      <default>
+        <mesh inertia="shell"/>
+      </default>
+      <asset>
+        <mesh name="test_mesh" vertex="0 0 0 1 0 0 0 1 0 0 0 1"/>
+      </asset>
+      <worldbody>
+        <body>
+          <geom mesh="test_mesh" type="mesh"/>
+        </body>
+      </worldbody>
+    </mujoco>
+    )";
+
+  std::array<char, 1024> error;
+  mjSpec* spec = mj_parseXMLString(xml, 0, error.data(), error.size());
+  EXPECT_THAT(spec, NotNull()) << "Failed to parse spec: " << error.data();
+  mjModel* model = mj_compile(spec, 0);
+  EXPECT_THAT(model, NotNull()) << "Failed to compile model: " << error.data();
+
+  std::array<char, 1024> out;
+  EXPECT_THAT(mj_saveXMLString(spec, out.data(), out.size(), error.data(),
+                               error.size()), 0) << error.data();
+
+  mjSpec* saved_spec = mj_parseXMLString(xml, 0, error.data(), error.size());
+  EXPECT_THAT(saved_spec, NotNull()) << "Invalid saved spec: " << error.data();
+  mjModel* saved_model = mj_compile(saved_spec, 0);
+  EXPECT_THAT(saved_model, NotNull()) << "Invalid model: " << error.data();
+
+  // check that the mesh has inertia="shell"
+  EXPECT_THAT(out.data(), HasSubstr(R"(<mesh inertia="shell"/>)"));
 
   mj_deleteSpec(spec);
   mj_deleteSpec(saved_spec);