For a typical PSF h, only two or three passes of iterative refinement are needed for convergence. Note that the best-fitting stencils are selected once and are fixed for each evaluation of R. To approximate
the interpolation and convolution h ∗ Rv i , the interpolation is performed on a grid with twice the resolution of v, and the convolution is approximated by discrete convolution.
The final interpolation is obtained by interpolating the prefiltered image u = Rv i at the desired points.