Spectral methods to determine the exact scaling factor of resampled digital images

This paper combines analytical models of periodic interpolation artifacts with recent empirical findings on the spectral energy distribution of rescaled images to infer exact transformation parameters in a passive-blind forensic setting. We present a measure to solve a long-known ambiguity between upscaling and downscaling in the forensic analysis of resampled signals and thus substantially limit the range of candidate scaling factors. The effectiveness of our method is backed with empirical evidence on a large set of images and scaling factors.