A method for determining a mask pattern is described. During the method, a first mask pattern that includes a plurality of second regions corresponding to the first regions of the photo-mask is provided. Then, a second mask pattern is determined based on the first mask pattern and differences between a target pattern and an estimate of a wafer pattern that results from the photolithographic process that uses at least a portion of the first mask pattern. Note that the determining includes different treatment for different types of regions in the target pattern, and the second mask pattern and the target pattern include pixilated images.