On the robustness of JPEG post-compression to resampling factor estimation

The research on image resampling detection is urgently called for digital image forensics recently. In this paper, the forensics of resampling operation with JPEG compression as post-process is addressed, namely post-JPEG resampling. We first discover the coupling effect between resampling and post-JPEG compression, which is shown as features of frequency mixing in spectrum and named as octant symmetric aliasing peaks(OSAP). The frequency location of OSAP depends on the resampling factor. We verify that the feature of OSAP is prone to be more prominent than resampling when quality factor (QF) of post-JPEG is low enough. By addressing the frequency response of post-JPEG compression, an approximate model of OSAP is derived. The prior knowledge of OSAP can help to enhance the resampling feature, based on which a new method of resampling factor estimation is proposed. In the proposed method, the location of OSAP is estimated from local maximum of spectrum with strong symmetry, and a candidate set of resampling factor is derived from OSAP. The optimal estimation among candidates is chosen in a heuristic way, which weakens the feature of OSAP appropriately. Experiments implemented on natural image set show the effectiveness of proposed method.