1. A method of simulating how a target mask pattern will be printed on a wafer that is coated with a resist material, comprising: receiving a number of image intensity values that are calculated on a grid of sample points on a wafer when the wafer is exposed with the mask pattern; supplying the image intensity values to a resist simulator that determines a resist surface function; selecting a single threshold contour value for the resist surface function; and defining the boundaries of how the mask pattern will be printed on the wafer from the contours of the resist surface function at the selected threshold.