1. A method comprising: by a computer: reading at least a portion of a layout file defining one or more features to be created via a photolithographic process, wherein the features defined in the layout file are divided into a number of edge fragments, determining initial simulation sites for one or more of the edge fragments, estimating an image intensity at one or more of the initial simulation sites, adding one or more additional simulation sites to an edge fragment based on the estimated image intensity, estimating an image intensity at the one or more additional simulation sites, determining an edge placement error from the estimated image intensities, and using the edge placement error determined at the one or more simulation sites to compute an OPC correction for creating at least one of the features defined in the layout file.