1. A method for modifying a photomask layout comprising: providing a photomask layout having at least an edge with at least a first evaluation point and a second evaluation point on the edge; interpreting the photomask layout to generate an interpreted photomask layout, wherein the interpreted photomask layout comprises an interpreted edge pattern, and the interpreted edge pattern has a third evaluation point corresponding to the first evaluation point and a fourth evaluation point corresponding to the second evaluation point; measuring a distance between the first evaluation point and the third evaluation point to obtain a first shift; measuring a distance between the second evaluation point and the fourth evaluation point to obtain a second shift; calculating a shift gradient between the first shift and the second shift; and estimating the number of segment(s) between the first evaluation point and the second evaluation point.