1. A system comprising: a memory to store machine readable instructions; and a processing unit to access the memory and execute the machine readable instructions, the machine readable instructions comprising: a feature set extractor comprising a Fast Fourier Transform (FFT) extractor to extract a feature set from a region of interest (ROI) of each of a plurality of digital images of print samples, wherein the feature set comprises a differential coefficient for the ROI based on an energy spectral density (ESD) of a first Fast Fourier Transform (FFT) of the ROI and an ESD of a FFT of a reference ROI, wherein the feature set is a filtered feature set that includes a feature set characterizing a printer that printed a given print sample of the print samples; a cluster component to determine clusters of the print samples based on the feature set of each of the plurality of scanned images of the print samples; and a printer identifier to identify the printer of the print samples based on the clusters of the print samples.