A received layout identifies a plurality of circuit components to be included in an integrated circuit (IC) layer for double patterning the layer using two photomasks, the layout including a plurality of first patterns to be included in the first photomask and at least one second pattern to be included in the second photomask. A selected one of the first patterns has first and second endpoints, to be replaced by a replacement pattern connecting the first endpoint to a third endpoint. At least one respective keep-out region is provided adjacent to each respective remaining first pattern except for the selected first pattern. Data are generated representing the replacement pattern, such that no part of the replacement pattern is formed in any of the keep-out regions. Data representing the remaining first patterns and the replacement pattern are output.