1. A method implemented by a computer, the method being for simulating an image of a mask pattern expected to be produced using a photolithographic process in a resist coated on a substrate, the method comprising: determining an aerial image of the mask pattern at a level of the substrate; and convolving, using a processor of the computer, the aerial image with at least two mathematical orthogonal convolution kernels to determine a resist image that is representative of the expected image of the mask pattern in the resist, wherein the convolution kernels are orthogonal solutions of a two dimensional quantum harmonic oscillator.