A method of adding optical proximity correction, OPC, to a base pattern is described. A base pattern is represented by a digital base pattern data file. A first OPC data file representing a first OPC pattern which adds pattern width at exterior corners of the pattern and a second OPC data file representing a second OPC pattern which reduces pattern width at interior corners of the pattern are formed. The final data file results from the logical subtraction of the second OPC data file from an interim data file formed by the logical OR of the base pattern data file and the first OPC data file. The final data file can be used to form masks or to inspect masks.