applied sciences Article Screen-Cam Robust Image Watermarking with Feature-Based Synchronization Weitong Chen 1,2,3 , Na Ren 1,2,3, *, Changqing Zhu 1,2,3 , Qifei Zhou 1,2,3 , Tapio Seppänen 4 and Anja Keskinarkaus 4 1 Key Laboratory of Virtual Geographic Environment (Nanjing Normal University), Ministry of Education, Nanjing 210023, China; 171301018@njnu.edu.cn (W.C.); 09322@njnu.edu.cn (C.Z.); 181301014@njnu.edu.cn (Q.Z.) 2 State Key Laboratory Cultivation Base of Geographical Environment Evolution, Nanjing 210023, China 3 Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210023, China 4 Physiological Signal Analysis Team, Center for Machine Vision and Signal Analysis, University of Oulu, 90014 Oulu, Finland; Tapio.Seppanen@oulu.fi (T.S.); Anja.Keskinarkaus@oulu.fi (A.K.) * Correspondence: 09359@njnu.edu.cn; Tel.: +86-136-1157-8959 Received: 23 September 2020; Accepted: 22 October 2020; Published: 25 October 2020   Abstract: The screen-cam process, which is taking pictures of the content displayed on a screen with mobile phones or cameras, is one of the main ways that image information is leaked. However, traditional image watermarking methods are not resilient to screen-cam processes with severe distortion. In this paper, a screen-cam robust watermarking scheme with a feature-based synchronization method is proposed. First, the distortions caused by the screen-cam process are investigated. These distortions can be summarized into the five categories of linear distortion, gamma tweaking, geometric distortion, noise attack, and low-pass filtering attack. Then, a local square feature region (LSFR) construction method based on a Gaussian function, modified Harris–Laplace detector, and speeded-up robust feature (SURF) orientation descriptor is developed for watermark synchronization. Next, the message is repeatedly embedded in each selected LSFR by an improved embedding algorithm, which employs a non-rotating embedding method and a preprocessing method, to modulate the discrete Fourier transform (DFT) coefficients. In the process of watermark detection, we fully utilize the captured information and extract the message based on a local statistical feature. Finally, the experimental results are presented to illustrate the effectiveness of the method against common attacks and screen-cam attacks. Compared to the previous schemes, our scheme has not only good robustness against screen-cam attack, but is also effective against screen-cam with additional common desynchronization attacks. Keywords: screen-cam process; local square feature region; synchronization; DFT; robust watermarking 1. Introduction Currently, the ubiquity of the computer office and the development of communication technology render digital image storage, copying, and transmission convenient and fast. Security problems involving digital images, such as leakage, malicious theft, and illegal dissemination, still frequently occur. In order to protect the data on the computer side, data encryption [1–6] and software watermarking [7,8] schemes are proposed. Similarly, some scholars have also investigated access control technology [9–11] to prevent illegal copying and transmission of data by restricting internal operations. Although these methods can effectively prevent the illegal acquisition of digital data directly from a computer, they cannot prohibit using a camera to take a photo of sensitive information Appl. Sci. 2020, 10, 7494; doi:10.3390/app10217494 www.mdpi.com/journal/applsci