Always generate a BVH in flex.

The BVH is used also for computing normalized vertex coordinates in the trilinear interpolation, so we need to compute it also when the flex cannot collide.

PiperOrigin-RevId: 756349244
Change-Id: Ib397c472e31251df5a7ea85850d83a62f3b2c949

src/user/user_mesh.cc

@@ -3624,8 +3624,9 @@ void mjCFlex::CreateBVH() {
     elemaabb_[6*e+5] = 0.5*(xmax[2]-xmin[2]) + radius;
 
     // add bounding volume for this element
+    // contype and conaffinity are set to nonzero to force bvh generation
     const double* aabb = elemaabb_.data() + 6*e;
-    tree.AddBoundingVolume(e, contype, conaffinity, aabb, nullptr, aabb);
+    tree.AddBoundingVolume(e, 1, 1, aabb, nullptr, aabb);
     nbvh++;
   }
 

test/user/user_flex_test.cc

@@ -243,6 +243,27 @@ TEST_F(UserFlexTest, RigidFlex) {
   mj_deleteModel(m);
   mj_deleteData(d);
 }
+
+TEST_F(UserFlexTest, FlexNotCollide) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <flexcomp name="test" pos="1 0 -1" type="grid"
+                count="5 5 5" spacing="1 1 1" dim="3">
+        <contact contype="0" conaffinity="0"/>
+      </flexcomp>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  mjData* d = mj_makeData(m);
+  mj_step(m, d);
+  mj_deleteModel(m);
+  mj_deleteData(d);
+}
+
 TEST_F(UserFlexTest, BoundingBoxCoordinates) {
   static constexpr char xml[] = R"(
   <mujoco>