# Screen-Cam Robust Image Watermarking with Feature-Based Synchronization
## 2 Screen-Cam Process Analysis  
The screen-cam process contains various distortions [34]. The subprocesses of the screen-cam process produce different types of distortions, as shown in Figure 1, and cause severe image quality degradation. This section aims to provide a basis for the design of screen-cam robust watermarking schemes by analyzing the different types of distortions generated in each step of the screen-cam process. The screen-cam process can be divided into three subprocesses: screen display, while shooting, and camera imaging. In the screen display process, the main factors that affect image signal are the quality of different monitors and their settings. Regular user operations will also cause distortions. With regard to the while shooting process, the main factors are the shooting environment, the relative position of screen and camera, and the moire phenomenon. If shooting at a large angle, the focusing problem cannot be disregarded [23]. Besides, camera shake may occur when pressing the shutter. The camera imaging process of a mobile phone is the process of converting optical signals to digital image signals and processing them. The main types of equipment in the process are the optical lens, the CMOS sensor [35], and the digital signal processor (DSP). With regard to CMOS sensors,  
Figure 1: Various processes and corresponding distortions in screen-cam process. the most important components are the photoelectric sensor and analog-to-digital sensor in receiving and processing signals. Therefore, the factors can be divided into hardware parts and software parts. The hardware part is related to the quality of the cameras' hardware devices. The software part is related to the image processing algorithm performed by the image signal processor (ISP). This part contains signal correction and format conversion before storage. Assuming an original image is displayed on a screen, and it is well captured by a camera, the distortions created during this screen-cam process can be divided into five categories. Linear distortion: The luminance, contrast, and color distortions caused by the quality and settings of the monitor can be approximated as a linear change. Linear distortion can also occur through linear correction of the image by the ISP. Gamma tweaking: To fit the human vision, the monitor performs gamma correction on the digital image, which is a nonlinear distortion. The ISP of the mobile phone performs a gamma compensation according to the algorithms on the digital image. The effect of gamma compensation may be amplified by other previous attacks. Geometric distortion: (1) Different degrees of perspective distortion are caused by the distance and angle of capturing, which will generate an uneven scaling attack on the digital image. (2) Another perspective distortion, such as pincushion distortion and barrel distortion, is caused by lens distortion of the optical lens. By controlling the photographer, the effects of these attacks can be reduced to a certain extent. Noise attack: Noise attacks are important factors that cause a sharp reduction in image quality during the screen-cam process. Noise attacks can be divided into four categories: moire noise, noise caused by the external environment, hardware, and software. (1) In physics, moire noise is a phenomenon of beat noise, which is the wave interference that occurs between two different objects [36]. During the screen-cam process, the light-sensitive elements in a camera emit high-frequency electromagnetic waves and the object being photographed, such as a liquid crystal display (LCD), emits some electromagnetic interference. When the two electromagnetic waves intersect, the waveforms are superimposed and cause electronic fluctuations of the acquiring device. The original waveforms are changed to form a new waveform that is moire. Generally, the moire phenomenon is most severe when the two wave frequencies are similar or in a multiple relationship.