Illustration of the process of reference point adaptation. (a) Based on the DPD indicator, the four candidate solutions all meet the conditions of the contributing solution. (b) The four contributing solutions and two noncontributing solutions are copied to the new archive A′. (c) The four reference points satisfying the conditions of the valid reference point are copied into the valid reference point set Rvalid. (d) The four valid reference points from Rvalid and the projection points of the two candidate solutions in the new file A′ on the hyperplane are copied to the set of reference points R′.