Photolithographic process simulation is described in which fast computation of resultant intensity for a large number of process variations and/or target depths (var,zt) is achieved by computation of a set of partial intensity functions independent of (var,zt) using a mask transmittance function, a plurality of illumination system modes, and a plurality of preselected basis spatial functions independent of (var,zt). Subsequently, for each of many different (var,zt) combinations, expansion coefficients are computed for which the preselected basis spatial functions, when weighted by those expansion coefficients, characterize a point response of a projection-processing system determined for that (var, zt) combination. The resultant intensity for that (var,zt) combination is then computed as a sum of the partial intensity functions weighted according to corresponding products of those expansion coefficients. Prediction of a mask transmittance function as a function of illumination incidence angle for a regional cluster of source emitters is also described.