1. A computer-implemented method for placing sub-resolution assist features (SRAFs) in a mask layout, comprising: obtaining the mask layout; generating, by the computer, an SRAF guidance map from the mask layout, wherein the SRAF guidance map is an image having a plurality of pixels corresponding to the mask layout, and wherein the generating step includes computing, for each pixel in the image, whether a sub-resolution assist feature located in the mask layout corresponding to the pixel would contribute positively to edge behavior of features in the mask layout; placing sub-resolution assist features in the mask layout according to the SRAF guidance map; and applying optical proximity correction including the SRAFs to produce an updated mask layout.