1. A method for placing sub-resolution assist features (“SRAF”) in a mask layout, comprising: generating an SRAF guidance map for the mask layout, wherein the SRAF guidance map is an image in which each pixel value indicates whether the pixel would contribute positively to edge behavior of features in the mask layout if the pixel is included as part of a sub-resolution assist feature; and placing sub-resolution assist features in the mask layout according to the SRAF guidance map.