1. A computer-implemented method for optimizing a lithographic process having an illumination source and a mask, the method comprising: a free-form optimization process; placing sub-resolution assist feature (SRAF) seeds in a description of the mask based on a result of the free-form optimization process; and a constrained optimization process, including growing the SRAF seeds while taking into account manufacturability constraints for both the illumination source and the mask, wherein one or more steps are performed by the computer.