A non-transitory, computer readable storage medium is encoded with computer program instructions, such that, when the computer program instructions are executed by a computer, the computer performs a method. The method generates mask assignment information for forming a plurality of patterns on a layer of an integrated circuit (IC) by multipatterning. The mask assignment information includes, for each of the plurality of patterns, a mask assignment identifying which of a plurality of masks is to be used to form that pattern, and a mask assignment lock state for that pattern. User inputs setting the mask assignment of at least one of the plurality of patterns, and its mask assignment lock state are received. A new mask assignment is generated for each of the plurality of patterns having an “unlocked” mask assignment lock state.