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, wherein the generating and the placing are performed by using a computer, and wherein generating an SRAF guidance map comprises: computing an image gradient map of the mask layout; and for each field point in the mask layout, computing a total vote sum for a unit source at the field point using the image gradient map.