1. A method implemented by a computer for simulating an image formed within a resist layer on a substrate resulting from an incident radiation, the substrate having a first feature and a second feature underlying the resist layer that are simultaneously subjected to the incident radiation, the method comprising: simulating, using the computer, a first partial image within the resist layer that represents interaction of the incident radiation with the first feature exclusive of any interaction of the incident radiation with the second feature; simulating, using the computer, a second partial image within the resist layer that represents interaction of the incident radiation with the second feature exclusive of any interaction of the incident radiation with the first feature; and computing the image formed within the resist layer from the first partial image, and the second partial image; wherein the interaction of the incident radiation with the first feature is different from the interaction of the incident radiation with the second feature.