Comparison between the E-modes power spectra of the input displacement field (black) and the displacement field after NGP assignment for several values of the oversampling factor κ. The input displacement is computed on an ECP grid with a number of pixel N_pix = 16   384² while the discretized one is the result of an NGP assignment on a grid of κ²N_pix. With progressively higher resolution the extra power due to discretization becomes negligible and the two spectra become almost indistinguishable. The discretization-induced error power spectrum is shown as a dotted line for reference; both E and B modes of the discretization error have the same power spectra.