A method of decomposing a target pattern having features to be imaged on a substrate so as to allow said features to be imaged in a multi-exposure process. The method includes the steps of: segmenting a plurality of the features into a plurality of polygons; determining the image log slope (ILS) value for each of the plurality of polygons; determining the polygon having the minimum ILS value, and defining a mask containing the polygon; convolving the defined mask with an eigen function of a transmission cross coefficient so as to generate an interference map, where the transmission cross coefficient defines the illumination system to be utilized to image the target pattern; and, assigning a phase to the polygon based on the value of the interference map at a location corresponding to the polygon, where the phase defines which exposure in said multi-exposure process the polygon is assigned.