A method is described for performing model-based optical proximity corrections on a mask layout used in an optical lithography process having a plurality of mask shapes. Model-based optical proximity correction is performed by computing the image intensity on selected evaluation points on the mask layout. The image intensity to be computed includes optical flare and stray light effects due to the interactions between the shapes on the mask layout. The computation of the image intensity involves sub-dividing the mask layout into a plurality of regions, each region at an increasing distance from the evaluation point. The contributions of the optical flare and stray light effects due to mask shapes in each of the regions are then determined. Finally, all the contributions thus obtained are combined to obtain the final computation of the image intensity at the selected point.