1. A computer implemented method for generating simulated spectral information in optical critical dimension (OCD) metrology, the method comprising: receiving an intensity distribution function over a numerical aperture space for a target diffracting structure; generating a set of points and weights for a non-linear transformation of the intensity distribution function over a plurality of subregions within the numerical aperture space; and rigorously computing the simulated spectral information based on the set of generated points and weights.