A fast method simulates photolithography using conventional image processing techniques. Convolution simulates for blurring; erosion and dilation correct for edge diffraction. In one technique, the source image of the photomask is deconvolved to sharpen it and then dilated to remove edge diffraction. The image is eroded, and then convolved according to the resolution of the stepper at the photomask plane. This aerial image can be further eroded to match the effects of resist and developing. Optional thresholding is done to produce a simulated processed wafer image. In a fast technique, the deconvolution step is eliminated. Dilation and erosion are combined into a single erosion. Where a phase shift mask is involved, a complex convolution is used. Source data can come from the photomask electronic design or from a visual image of the actual photomask. Optimizations include: special microprocessor instructions, floating point pixel values, separable convolution and annular illumination simulation.