1. A method of automatically distinguishing a reference image from a defect image, the method comprising: using a mask defect printability system, accessing multiple images, the multiple images being undesignated as reference or defect images; aligning the multiple images; defining a common area of the multiple images; computing a complexity of each of the images, as defined by the common area, by performing at least a portion of a spatial frequency transform, the spatial frequency transform including generating a Fourier transform (FFT); and designating the reference and defect images by comparing the complexity of each of the multiple images, wherein higher frequencies or larger values of high frequencies appear on an FFT representation of the defect image compared to an FFT representation of the reference image, wherein subtracting FFT representations of the multiple images from each other results in difference images, and wherein a difference image having more positive values in a high frequency domain identifies a defect image.