To provide fast mask defect scoring, approximated wafer simulations (e.g. using one convolution) are performed on the defect inspection image and its corresponding reference inspection image. Using the approximated defect wafer image and the approximated reference wafer image generated by these approximated wafer simulations, a defect maximum intensity difference (MID) is computed by subtracting one approximated wafer image from the other approximated wafer image to generate a difference image. After a defect region of the difference image is clearly defined, a simulation at the centroid (i.e. a single point) of the defect region is performed. After the defect MID is computed (represented by an intensity) it can be compared to a prototype MID, which can represent a generic nuisance defect.