1. A method for placing an assist feature in a layout, comprising: receiving a layout of an integrated circuit; selecting an evaluation point in the layout; identifying locations of constructive and destructive interference nodes using the magnitude of an image-intensity gradient of an aerial-image at the evaluation point, wherein the aerial-image is calculated using an image intensity model, the layout, and by placing one or more representative assist features in the layout; and placing an assist feature in the layout based on the locations of the constructive and destructive interference nodes.