# Image Watermarking between Conventional and Learning-Based Techniques: A Literature Review
## 1 Introduction
As a result, robust video watermarking algorithms for copyright protection have developed to meet the actual needs [14]. Cyber-physical systems, the Internet of Things (IoT), the on-demand availability of computer system resources, and cognitive computing are four key elements of Industry 4.0. The effectiveness of this industrial revolution depends on how successfully these parts can cooperate and communicate with one another. This communication is accomplished by sharing a wide range of data acquired from a network of sensors, attracting the unwanted attention of hackers. As a result, one of the key concerns is data protection, and improved watermarking techniques are needed [9]. The ever-increasing volume of medical digital images, as well as the necessity to distribute them among specialists and hospitals for better and more accurate diagnoses, necessitate the protection of patients' privacy [10]. Important information is concealed within a cover medical image, and it should not be detected, retrieved, or modified by an unauthorized user. The challenge is high, and medical-image watermarking must be performed with extreme caution. The watermarking technique must not degrade image quality, and personal patient information included within the image must be retrievable without error following image decompression [15]. To provide services to residents, smart cities make use of information and communication technologies (5G, AI, cloud, and edge computing). A huge amount of information is collected, transmitted, and analyzed from each point within a smart city. This increases the vulnerabilities and risks of the acquired data [16]. Data validation is frequently requiredin order to identify the location and technical details of how and when the data are generated. Improving their security is critical because malicious exploitation and piracy have become recurrent phenomena, imposing the need for digital content protection. Smart City Security is a collaborative project with many partners who are critical components of a complete, vital, and interconnected IoT security ecosystem. The present challenge is to identify smarter protection approaches and define a uniform framework for data-use policies. Digital watermarking techniques are highly advocated [16; 17]. Watermarking techniques have been implemented by various algorithms employing spatial and frequency domains, as well as deep learning [18], with each having distinct benefits and limitations. Regardless of the field or the technology used, there are three common and mutually restricted properties in a watermarking scheme: invisibility, robustness, and watermark capacity or information quantity to insert. These three constraints are contradictory. For example, increasing watermarking strength to make the watermark more robust will have the opposite effect of making the watermark more visible. In the same way, increasing the capacity will result in more noticeable watermark that is also less robust. Therefore, we have to respect a compromise between these three criteria to build an acceptable watermarking method [19]. Yet, there are other significant properties related to watermarking algorithms, such as security and computational complexity. Overall, achieving a good relationship along with other watermarking properties is difficult. Deep learning (DL) has attracted considerable attention in the field of digital image processing, and it has achieved great success in improving prediction performance by analyzing and learning from massive data and computational resources [20]. Furthermore, problems considered to be unsolvable are now solved with extraordinary precision. Likewise, digital image watermarking is a prime example of deep learning's significant contributions [21], giving rise to a large number of deep learning-based watermarking schemes with substantially better performance compared with traditional methods. However, deep learning will not solve all digital watermarking problems.