Raise error if mesh has flipped faces.

PiperOrigin-RevId: 461860424
Change-Id: Ib23c50cf7f64de1173404d2f65ad87177fab45b3

test/user/testdata/malformed_face.obj

@@ -0,0 +1,9 @@
+v 0 0 0
+v 1 0 0
+v 0 1 0
+v 0 0 1
+f 3 1 4
+f 1 2 4
+f 2 3 4
+# malformed face
+f 1 2 3

test/user/testdata/malformed_face.xml

@@ -0,0 +1,8 @@
+<mujoco>
+  <asset>
+    <mesh file="malformed_face.obj"/>
+  </asset>
+  <worldbody>
+    <geom type="mesh" mesh="malformed_face"/>
+  </worldbody>
+</mujoco>

test/user/user_mesh_test.cc

@@ -43,6 +43,8 @@ static const char* const kTexturedTorusPath =
     "user/testdata/textured_torus.xml";
 static const char* const kDuplicateOBJPath =
     "user/testdata/duplicate.xml";
+static const char* const kMalformedFaceOBJPath =
+    "user/testdata/malformed_face.xml";
 
 using ::testing::HasSubstr;
 
@@ -174,5 +176,31 @@ TEST_F(MujocoTest, TinyInertiaFails) {
           "mass and inertia of moving bodies must be larger than mjMINVAL"));
 }
 
+TEST_F(MujocoTest, MalformedFaceFails) {
+  const std::string xml_path = GetTestDataFilePath(kMalformedFaceOBJPath);
+  std::array<char, 1024> error;
+  mjModel* model = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
+  ASSERT_THAT(model, testing::IsNull());
+  EXPECT_THAT(error.data(), HasSubstr("faces have inconsistent orientation"));
+}
+
+TEST_F(MujocoTest, FlippedFaceFails) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <asset>
+      <mesh name="example_mesh"
+        vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+        face="2 0 3  0 1 3  1 2 3  0 1 2" />
+    </asset>
+    <worldbody>
+      <geom type="mesh" mesh="example_mesh"/>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(error.data(), HasSubstr("faces have inconsistent orientation"));
+}
+
 }  // namespace
 }  // namespace mujoco