A method for generating sub-resolution assist features (SRAFs) for target features of a photomask includes generating a 2D rules-based assist feature (RBAF) rules table that estimates results obtained from a model-based SRAF method. The 2D RBAF rules table defines seed skeletons using polar coordinates. Empty space surrounding the target features is divided into corresponding owned regions. Seed skeletons are placed in the owned regions using the 2D RBAF rules table. The seed skeletons are widened, and the resulting structure is re-skeletonized. A cleaning process is performed on the re-skeletonized structure, eliminating potentially troublesome features, including stubs, forks, triple-points and quad-points. The cleaned skeleton structure is straightened, and polygonal SRAFs are placed at locations specified by the straightened skeleton structure.