1. A method for forming a mask layout, comprising: forming a plurality of phase shapes on either side of a critical feature of a design layout of an integrated circuit chip having a plurality of critical features, wherein each phase shape has an edge; identifying a plurality of transition edges from the edges, wherein each transition edge is parallel to a critical feature, and wherein the identifying of the plurality of transition edges includes identifying a substantially uniform distance from at least one of the critical feature or the edge of another phase shape; identifying a transition space defined by one of a group comprising: two transition edges, wherein the space is external to all phase shapes, and one transition edge, wherein the space is external to all phase shapes; forming a transition polygon by closing each transition space with at least one closing edge, wherein each closing edge is perpendicular to the plurality of transition edges; transforming each transition polygon into a printing assist feature; and forming a first mask layout or a second mask layout from the printing assist features and the critical features.