1. A method of computing the image intensity of an evaluation point for a mask shape of a mask layout comprising the steps of: a. sub-dividing said mask layout into a plurality of regions, wherein the outermost boundaries of said plurality of regions are at a predetermined distances from said evaluation point; b. considering at least one mask shape on said mask layout in at least one region of said plurality of regions; c. computing the contribution of said mask shape in said region to the image intensity at said evaluation point; d. combining the contributions from all mask shapes in said region to the image intensity at said evaluation point; and e. combining the contributions from all said regions to the image intensity at said evaluation point.