Local JPEG Grid Detector via Blocking Artifacts
Tina Nikoukhah, Rafael Grompone von Gioi
⚠ This is a preprint. It may change before it is accepted for publication.


Image JPEG compression leaves blocking artifact traces. This paper describes an algorithm that exploits those traces to locally recover the grid embedded in the image by the JPEG compression. The algorithm returns a list of grids associated with different parts of the image. The method uses Chen and Hsu’s cross-difference to reveal the artifacts. Then, an a contrario validation step according to Desolneux, Moisan and Morel’s theory is used to control the number of false alarms, producing on average no more than one false detection per image. The only parameter is the size of the smallest window, which represents the exhaustiveness of the method. The application to image forgery detection is twofold: first, the presence of multiple JPEG grids is a sign of forgery; second, knowledge of the grid is required for further JPEG forensic analysis.