1. A computer-implemented method for placing sub-resolution assist features (“SRAF”) with respect to one or more target patterns in a mask layout, the method comprising: generating an SRAF guidance map for the mask layout, wherein signal strength in a SRAF guidance map determines spatial distribution of SRAFs; placing a first set of one or more SRAF in the mask layout according to the initial SRAF guidance map; generating an updated SRAF guidance map using improvement in signal strength in the initial SRAF guidance map due to the placement of the first set of one or more SRAF and the one or more target patterns; placing a subsequent set of one or more SRAF in the mask layout according to the updated SRAF guidance map; and repeating the steps of generating an updated SRAF guidance map and placing the subsequent set of one or more SRAF in an iterative process, until a predefined lithographic process window criterion is satisfied, wherein one or more of the above steps are implemented by the a computer.