1. A method of designing a lithographic mask, including the use of a lithographic process model for simulating an image formed by illumination of the lithographic mask in a lithographic system, the method comprising: determining a first region of influence (ROI 1 ) around a point of interest on the mask design, such that mask features within said first ROI 1 will contribute a relatively large amount of flare energy at said point of interest; determining a second region of influence (ROI 2 ) around said point of interest, such that mask features outside of said ROI 2 will contributed a relatively small amount of flare energy at said point of interest in accordance with a predetermined criterion, such that the region between ROI 1 and ROI 2 comprises an intermediate region of influence (intermediate ROI); identifying an initial mask polygon shape having a first plurality of vertices located within the intermediate ROI; and smoothing said initial mask polygon shape to form a smoothed mask polygon shape that has fewer vertices within the intermediate ROI than said first plurality of vertices; determining a smoothed flare contribution at said point of interest from said vertices of said smoothed mask polygon within the intermediate ROI; and determining an image at the point of interest comprising using said smoothed flare contribution in the lithographic process model rather than a flare contribution from said initial mask polygon shape.