# Convolutional Neural Network Architecture for Recovering Watermark Synchronization
## 2 Related Work  
To date, numerous watermarking techniques have been proposed to protect the copyright of content. Methods of inserting watermarks into various transform domains such as discrete cosine transform (DCT), discrete Fourier transform (DFT), radon transform, curvelet transform, Dual-tree complex wavelet transform (DT-CWT), and contourlet transform have been proposed [1; 2; 3; 4; 5; 6]. Various insertion methods such as spread-spectrum, quantization index modulation (QIM), angle QIM, and absolute angle QIM have also been proposed [7; 8; 9; 10]. However, these watermarking  
Figure 1: Illegal distribution scenario for real-time web-comics and watermark extraction process suitable for this scenario. Illegal distribution is frequently caused by screen capturing on Internet browsers and smartphone applications. Scaling and translation occur in this process and these distortions must be corrected before watermark decoding. techniques often show weaknesses after geometric distortion. Because geometric distortion causes synchronization errors, it is not easy to ensure robustness of these watermarks to geometric attacks. In addition, the print-scan process, which is regarded as a watermark removal attack, is commonly used for image reproduction and distribution. Print-scan resilient data hiding provides an authentication method of an important document, which is becoming more significant issue because of the security problems [11; 12; 13]. To reduce the vulnerability to geometric attacks, many studies have been carried out to identify the transform domain and insertion methods with invariant characteristics against geometric distortions [14; 15; 16; 17]. However, these algorithms have drawbacks such as low invisibility, lack of bit capacity such as zero-bit watermarking, vulnerability to specific geometric distortion such as translation, or necessity of additional information. Many template-based watermarking methods for decoding watermarks using template matching techniques have been proposed [18; 19; 20; 21], but these methods also have a drawback in that it is difficult to insert sufficient copyright information because the bit capacity is low. In addition, since these templates are additionally inserted over the watermarked image in which the copyright information is embedded, the invisibility is further reduced, and the template and the watermark signal may interfere with each other. These watermarking techniques robust to geometric attacks have many disadvantages compared to watermarking techniques that do not consider geometric attacks. In addition, if the synchronization problem is solved from the geometrically distorted image, the watermark can be normally decoded. For this reason, watermarking techniques that do not consider geometric attacks are used in many cases. If a geometric attack occurs, watermark decoding is performed after the image is recovered into the geometric characteristics of the original form. However, this approach requires a process for finding the original image information, comparing the original image with the geometrically distorted image, and then recovering the geometrical characteristics. This process is often inefficient because it is done manually or using heuristic search. Conventional template-based watermarking techniques usually use whole image-unit transform rather than block-unit transform. This is because it is advantageous to use the image-unit transform when searching for invariant domains and insertion methods against rotation, scaling and translation (RST) attacks. When using the block-unit transform, it is necessary to solve the problem of correcting the block synchronization after geometrical distortion, which is difficult. However, the image-unit template has a problem in that the template cannot be inserted independently with the watermark containing copyright information unless they use the same transform domain. Therefore, there is a limitation that the template and watermark signal can interfere with each other.