1. A method of generating a fast mask defect score, the method comprising: performing a first approximated wafer simulation of a defect inspection image to generate an approximated defect wafer image; performing a second approximated wafer simulation of a corresponding reference inspection image to generate an approximated reference wafer image, wherein performing the first and second approximate wafer simulations includes performing a single convolution; and computing a defect maximum intensity difference (MID) using the approximated defect and reference wafer images.