“…In this case, we need to compute the Green's function samples only once, and a single matrix product per line is required for yielding the transformed image. The complexity of the original filtering process, as described in (Ferreira Júnior et al, 2008;2009) , is of order O(M.N 2 ) per each image line, and since the filtering along the image columns is also required, we have a total complexity of order O(M.N 2 )+O(N.M 2 ). With the matrix approach, this will be reduced to O(M)+O(N).…”