1. A method of modifying a plurality of sub-resolution assist features, comprising: receiving a mask pattern for a photolithographic mask, the mask pattern comprising one or more main features, the photolithographic mask operable to pattern a wafer pattern for a semiconductor wafer; estimating placement of one or more sub-resolution assist features for the one or more main features; and repeating the following for one or more iterations: correcting the one or more main features using a wafer pattern model, the wafer pattern model operable to estimate the wafer pattern; evaluating the one or more sub-resolution assist features according to the wafer pattern model; and modifying at least one sub-resolution assist feature of the one or more sub-resolution assist features in accordance with the evaluation.