Zero‐mean normalized cross‐correlation (ZNCC)1 11, 17 was used to assess the similarity between DR and DRR. ZNCC is shown in eq. (1):(1)ZNCC(di)=∑w(IDR−IDR¯)(IDRR(di)−IDRR¯)∑w(IDR−IDR¯)2∗∑w(IDRR(di)−IDRR¯)2(i=0,1,2)[Correction added on 8th February 2018, after first online publication: Equation was corrected.]