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, the merge bar width being greater than a minimum spacing value and less than a minimum printability value; 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 by establishing from the distance where there is: a separation in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the separation less than the merge bar width; a separation in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the separation greater than the merge bar width; an overlap in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the overlap less than the merge bar width; or an overlap in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the overlap greater than 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 by performing at least one of the following: if there is a separation in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the separation less than the merge bar width, inserting the merge bar to merge the first sub-resolution assist feature and the second sub-resolution assist feature; if there is a separation in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the separation greater than the merge bar width: inserting the merge bar within the separation; and extending at least one of the first sub-resolution assist feature and the second sub-resolution assist feature to merge the first sub-resolution assist feature and the second sub-resolution assist feature; if there is an overlap in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the overlap less than the merge bar width, inserting the merge bar to merge the first sub-resolution assist feature and the second sub-resolution assist feature; and if there is an overlap in the predetermined direction between the first sub-resolution assist feature and the second sub-resolution assist feature, the overlap greater than the merge bar width: shortening at least one of the first sub-resolution assist feature and the second sub-resolution assist feature; and inserting the merge bar to merge the first sub-resolution assist feature and the second sub-resolution assist feature.