A method for determining a mask pattern to be used on a photo-mask in a photolithographic process is described. During the method, a target pattern is partitioning into subsets, which are distributed to processors. Then, a set of second mask patterns, each of which corresponds to one of the subsets, is determined. Moreover, at least one of the second set of mask patterns may be determined by: providing a first mask pattern that includes distinct types of regions corresponding to distinct types of regions of the photo-mask, calculating a gradient of a function, and determining a second mask pattern based, at least in part, on the gradient. Note that the function may depend on the first mask pattern and an estimate of a wafer pattern that results from the photolithographic process, and that the gradient may be calculated in accordance with a formula obtained by taking a derivative of the function.