A fast method simulates photolithography using conventional image processing techniques. Convolution simulates blurring due to optics; erosion and dilation correct for edge diffraction. To produce the convolution kernel, an effective projection lens image for the image source is produced by convolving the lens image with an image of the illuminator aperture shape. An effective projection lens image for the stepper is produced similarly. The stepper effective lens image is divided by the image source effective lens image to produce a corrected effective lens image. A corrected convolution kernel is produced by taking a Fourier transform of the corrected effective lens image. The kernel is used to convolve the image, once using energy and once using voltage, and then squaring the result. The aerial image is produced by blending the energy and voltage convolutions according to the computed partial coherence of the optics. Complex convolution is used to represent relative phases other than 180 degrees.