1. A method for merging a plurality of sub-resolution assist features, comprising: receiving a mask pattern for a photolithographic mask, the mask pattern comprising a plurality of sub-resolution assist features; selecting a first sub-resolution assist feature to merge with a second sub-resolution assist feature; establishing a merge bar width of a merge bar for merging the first sub-resolution assist feature and the second sub-resolution assist feature; determining a distance in a predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature; identifying a merging technique in accordance with the distance and the merge bar width; and merging the first sub-resolution assist feature and the second sub-resolution assist feature according to the identified merging technique.