A method performed by a computing system includes receiving a layout pattern, receiving a target pattern associated with the layout pattern, receiving a set of constraints related to the target pattern, simulating a first contour associated with the layout pattern, determining a first difference between the first contour and the target pattern, simulating a second contour associated with a modified layout pattern, and determining a second difference between the second contour and a modified target pattern. The modified target pattern is different than the target pattern and within the constraints. The method further includes fabricating a mask having the final layout pattern.