The non-normal incidence of semi-guided plane waves on step-like or tapered transitions between thin film regions with different thicknesses, an early problem of integrated optics, is being reconsidered. As a step beyond the common effective index picture, we compare two approaches on how this problem can be tackled-at least approximately-by nowadays readily available simulation tools for integrated optics design. Accepting the scalar approximation, using an ansatz of harmonic field dependence on the position along the interface, the 3-D problem reduces to a 2-D Helmholtz problem, for guided wave input and transparent-influx boundary conditions, with an effective permittivity that depends on the incidence angle. Alternatively, one complements the structure with a second mirrored interface, such that the 2-D cross section of a wide multimode rib waveguide emerges. Constraints for transverse resonance then permit to translate the propagation constants of its polarized modes into discrete samples of the phase changes experienced by an in-plane guided wave upon total internal reflection at the sidewalls.