Every Shred Helps: Assembling Evidence from Orphaned JPEG Fragments

In this paper, we address the problem of forensic photo carving, which serves as one of the key sources of digital evidence in modern law enforcement. We propose efficient algorithms for assembling meaningful photographs from orphaned photofragments, carved without access to file headers, meta-data, or compression settings. The addressed problem raises a novel variant of a jigsaw puzzle with an unknown number of mixed images, missing pieces, and severe brightness and colorization artifacts. We construct an efficient compatibility metric for matching puzzle pieces and a corresponding image stitching procedure which allows us to mitigate these artifacts. To facilitate photo assembly, we perform a forensic analysis of the fragments to provide clues about their location within the frame of the imaging sensor. The proposed algorithm formulates the assembly problem as finding non-overlapping sets in an interval graph spanned over the input fragments. The algorithm exhibits lower computational complexity compared with a popular puzzle-solving approach based on minimal spanning trees.