1. A method 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, the method comprising: simulating a first partial image within the resist layer using interaction of the incident radiation and the first feature without using interaction of the incident radiation and the second feature; simulating a second partial image within the resist layer using the interaction of the incident radiation and of the second feature without using the interaction of the incident radiation and the first feature; 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 and the first feature is different from the interaction of the incident radiation and the second feature.