# Screen-Cam Robust Image Watermarking with Feature-Based Synchronization
###### Abstract
The Fourier domain-based algorithms can be subdivided into two categories. The first category is transform-invariant domain-based methods [24; 25], which employed log-polar mapping (LPM) and discrete Fourier transform (DFT) to achieve robustness to rotation, scaling, and translation (RST) distortions. The second category uses the selected magnitude coefficients of the DFT domain as the message carrier [26; 27; 28; 29]. Researchers have improved the method by choosing the optimal radius of the implementation to minimize quality degradation [28] or by color correction to enhance the extraction accuracy [29]. For the wavelet domain-based algorithms, messages are embedded by modulating the coefficients of the wavelet transform. These algorithms commonly combine with another domain-based watermark to achieve robustness to affine distortion attacks [30], to rotation and scale attacks [31], or implement fragile watermarking [32]. Similarly, because the frequency domain-based methods are global watermarks, they are also weak to cropping attacks. Furthermore, the experiments in [33] show that these print-scan and print-cam robust watermarking methods have a relatively higher bit error rate that cannot be directly applied to the screen-cam process. Considering the particularity of the screen-cam process, [PERSON] et al. [33] proposed an effective screen-cam robust image watermark algorithm with an intensity-based scale-invariant feature transform (I-SIFT)-based synchronization. The message is embedded in the discrete cosine transform (DCT) domain of each feature region. However, this method needs to record the four vertices of the image in advance, which means it cannot cope with the situations where the image's original size is unknown. Besides, this method cannot address orientation or scale desynchronization attacks. However, normal user operations may cause watermark desynchronization inevitably. For example, the image displayed on the screen is blocked by other application windows or the image has been zoomed, rotated, and cropped for usage. Because the spy may just capture the content displayed on the screen, if we do not know the original information of the captured image, we may not be able to restore the image to its original orientation and scale. Therefore, to address possible desynchronization attacks besides screen-cam attack, a screen-cam, RST, and cropping invariant watermark synchronization method need to be further investigated. To solve these issues, a feature and Fourier-based screen-cam robust watermarking scheme is proposed in this paper. The main contributions are as follows:  
* We analyze the performance of commonly used feature operators and the variation rules of DFT magnitude coefficients during the screen-cam process. * We designed an orientation and scale invariant local square feature region (LSFR) construction method, which can achieve watermark synchronization against screen-cam attack and also common desynchronization attacks. * We employ a non-rotating embedding algorithm based on the properties of the DFT coefficients, which can avoid further distortions that may be caused by orientation normalization. * We present a preprocessing method for message embedding. By working in combination with the proposed local statistical feature-based message extraction method, it can improve the extraction accuracy. The remainder of the paper is organized as follows: Section 2 summarizes different distortions in the screen-cam process. Section 3 describes the implementation details of the proposed method. The selection of parameters and experiment results are presented in Sections 4 and 5. Finally, Section 6 concludes the paper.