1. A method of generating complementary masks for use in a dark field double dipole imaging process, said method comprising the steps of: identifying a target pattern having a plurality of features, said plurality of features comprising horizontal and vertical features; generating a horizontal dark field mask based on said target pattern, said horizontal dark field mask including low contrast features, said generation of said horizontal dark field mask comprising the steps of: optimizing the bias of said low contrast features contained in said horizontal dark field mask; and applying assist features to said horizontal dark field mask; generating a vertical dark field mask based on said target pattern, said vertical dark field mask containing low contrast features, said generation of said vertical dark field mask comprising the steps of: optimizing the bias of low contrast features contained in said vertical dark field mask; and applying assist features to said vertical dark field mask; and decomposing the target pattern into the horizontal dark field mask and the vertical dark field mask, the decomposing comprising: performing model-based optimization of both masks so as to optimize imaging performance criteria after double exposure of a feature in the target pattern during the dark field double dipole imaging process using both masks.