1. A method for simulating contours produced by a lithographic process for a design, wherein the method is implemented by a computer, the method comprising: maintaining a differential model that defines differences in imaging results obtained when using a second lithographic process from imaging results obtained when using a different first lithographic process, wherein the differences in imaging results are attributable to differences in lithographic process parameters between the second lithographic process and the different first lithographic process; and generating, using the computer and the differential model, a predicted difference in simulated wafer contours for the second lithographic process, wherein the predicted difference is a difference between simulated wafer contours respectively produced by the first and second lithographic processes for the design.