“…Then, we approximate the first-order and second-order regularization terms by using the forward difference, central difference, mid-point rule, and Dirichlet boundary conditions. For more information and details, the reader is referred to Lajili et al 36 and Zhang et al 37 overcome these limitations by a multilevel technique, which assures the numerical solution serves as a perfect starting point and speeds up the registration process. Indeed, starting with the initial guess U = 0, the idea is to improve this guess by resizing the original images to a sequence of coarser ones where computations are cheap and register these smaller images (see Figure 3).…”