Obstacle detector, the trace is not visible

Hi, there

I set the ‘debug_linetrace’ be True, but the trace is not visible. Anyone gives me a hint? the code relevanted is here, thanks a lot!

obs_bp = world.get_blueprint_library().find(‘sensor.other.obstacle’)
obs_bp.set_attribute(“debug_linetrace”,str(True))
obs_location = carla.Location(0,0,0)
obs_rotation = carla.Rotation(0,0,0)
obs_transform = carla.Transform(obs_location,obs_rotation)
ego_obs = world.spawn_actor(obs_bp,obs_transform,attach_to=ego_vehicle, attachment_type=carla.AttachmentType.Rigid)
def obs_callback(obs):
print(“Obstacle detected:\n”+str(obs)+’\n’)
ego_obs.listen(lambda obs: obs_callback(obs))