1. A method for decomposing a particular pattern on a layer of a design layout into a set of images for generating multiple sub-patterns for multiple photolithographic operations, the method comprising: a) by a computer, imposing a grid on the particular pattern to define a set of nodes, each node associated with a cost; b) based on said cost, searching through a space of lithographically feasible images to identify a set of at least two images for decomposing the particular pattern; c) determining whether a union of the identified set of images adequately approximates the particular pattern; and d) repeating operations (b) and (c) until a particular set of images is identified that has a union that adequately approximates the particular pattern.