A method for forming a mask layout is described. A plurality of phase shapes are formed on either side of a critical feature of a design layout of an intergrated circuit chip having a plurality of critical features. A plurality of transition edges are identified from the edges of each phase shape. Each transition edge is parallel to critical feature. A transition space is identified as defined by one of the group including two transition edges and one transition edge. A transition polygon is formed by closing each transition space with at least one closing edge. Each transition polygon is transformed into a printing assist feature. A mask layout is formed from the printing assist features and critical features.