1. A method of designing a mask layout comprising: providing a layout of mask shapes; identifying a region of maximal influence on at least one of said mask shapes; determining an expected image within said region of maximal influence by simulating said expected image using a CAD tool configured to perform a model-based process; determining the location of an extremum in said expected image; and defining a primary simulation site on an edge of said at least one of said mask shapes based upon a projection between said extremum and said edge, wherein said at least one mask shape comprises a polygon including a vertex, and said region of maximal influence comprises a search region around said vertex, and wherein said polygon comprises a plurality of neighboring vertices, and wherein said search region is formed by determining for each of said plurality of neighboring vertices a neighboring vertex region of influence around each of said plurality of neighboring vertices, and combining said neighboring vertex regions of influence where said neighboring vertex regions of influence overlap to form said search region.