“…In practice, this kind of calculation is prohibitively time-consuming, unless to use fast Fourier transforms [13], and thus various means of approaching the correlation function are proposed in the literature [10± 11]. In particular, in the work of Boitier et al [11], the comparison of different parts of an image is approached by calculating the quadratic error between the porosity values, u(x) and u(x+h), measured in two scanned regions of interest (ROI) which are distant of h:…”