1. A method implemented by a computer for placing sub-resolution assist features (“SRAF”) with respect to one or more target patterns in an original mask layout, comprising: generating an initial SRAF guidance map for the original mask layout; generating a first set of one or more SRAFs according to the initial SRAF guidance map; generating a first altered mask layout corresponding to the original mask layout by placing the first set of one or more SRAFs in the original mask layout; determining, by the computer, a lithographic response for the first altered mask layout as a function of one or more process window parameters; and determining whether the lithographic response for the first altered mask layout accommodates a predefined process window size.