Image Forensics Based on Planar Contact Constraints of 3D Objects

Standing objects on planar surfaces are common to see in images, e.g., people on the ground. For most objects to stay stable on the plane, planar contact is a necessary requirement. However, 2D image splicing usually disregards this physical constraint of 3D world, leading to a potential artifact of object not attached to the plane. This paper is the first attempt to use the contact constraint of standing objects as a new clue for image forensics. Accordingly, we propose a novel approach to first reconstruct the 3D poses of standing objects and their supporting plane and then measure the contact conditions for splicing detection. To tackle the problem of unknown object shape for pose estimation, we effectively employ the prior knowledge of 3D morphable model to simultaneously estimate both shape and pose parameters by fitting to image observations. The 3D normal orientation of the supporting plane is estimated given its vanishing line. Dealing with uncertainty factors in estimations, we approximate a distribution of estimates using sampling strategies and then make the final decision. Particularly, we focused our method on the important scenario of human figure splicing detection, and comprehensive experiments on multiple data sets and typical images proved the encouraging effectiveness of the new forensic clue and the proposed approach.