The Maggi-Rubinowicz method (MRM) is a useful tool to compute diffraction patterns from opaque planar objects. We adapted the MRM to planar rectangles. In the first part of this study, differences between diffraction patterns, both the intensity and the phase distributions, from a tilted rectangle and from the square having the same orthogonal projection on the observation plane, have been analyzed. In the second part, we compared results obtained with the MRM to those obtained with angular spectrum theory (AST) coupled to fast Fourier transform (FFT). The main novelty of this work is the fact that MRM is particularly well suited for evaluating anti-aliasing procedures applied to AST-FFT calculations.