  In many fields of robotics, knowing the relative position and orientation
between two sensors is a mandatory precondition to operate with multiple
sensing modalities. In this context, the pair LiDAR-RGB cameras offer
complementary features: LiDARs yield sparse high quality range measurements,
while RGB cameras provide a dense color measurement of the environment.
Existing techniques often rely either on complex calibration targets that are
expensive to obtain, or extracted virtual correspondences that can hinder the
estimate's accuracy. In this paper we address the problem of LiDAR-RGB
calibration using typical calibration patterns (i.e. A3 chessboard) with
minimal human intervention. Our approach exploits the planarity of the target
to find correspondences between the sensors measurements, leading to features
that are robust to LiDAR noise.
  Moreover, we estimate a solution by solving a joint non-linear optimization
problem. We validated our approach by carrying on quantitative and comparative
experiments with other state-of-the-art approaches. Our results show that our
simple schema performs on par or better than other approches using complex
calibration targets. Finally, we release an open-source C++ implementation at
\url{https://github.com/srrg-sapienza/ca2lib}
