# A Brief, In-Depth Survey of Deep Learning-Based Image Watermarking
## 2 Preliminaries  
### Traditional Image Watermarking Components  
Image watermarking entails the incorporation of watermark data within an image. This watermark, an encoded digital signal, is meticulously crafted to be inconspicuous to human vision yet readily identifiable and extractable via computational algorithms. As elaborated in Section 1, the application spectrum of image watermarking, delineated by the nature of the watermark information, spans copyright protection, authenticity verification, covert communication, and tampering detection, among others. Figure 1 succinctly encapsulates the components and steps inherent in the traditional paradigm of image watermarking.  
**Embedding and extraction.** In the embedding step, the watermark is integrated into the cover image via a watermarking algorithm. The overarching objective is to ensure the embedded watermark is robust and resistant to removal or alteration, whilst concurrently maintaining the visual integrity of the cover image. Various techniques exist for the watermark embedding, such as the modification of pixel values in the spatial domain [6], and the manipulation of coefficients within the frequency domain representation [18]. Post-embedding, the watermarked image is disseminated to the designated audience, potentially via online platforms. Authorized recipients are equipped to extract the embedded watermark utilizing a specialized extraction algorithm.  
**Key.** Numerous image watermarking methodologies incorporate a key, denoting secret values instrumental in modulating the embedding and extraction processes of the watermark. Typically, this key is conjointly generated and disseminated among the content owner and authorized users. Its application within the watermarking procedure varies, contingent on the algorithmic design, predominantly aiming to augment security and  
Figure 1: General components and steps of traditional image watermarking.  
robustness. For instance, the key can be employed to govern the generation of a pseudo-random sequence integrated into the image for watermark embedding [18], or to designate the precise location of the embedded watermark within the image [6].  
**Watermark preprocessing.** Watermark preprocessing can be instrumental in augmenting both security and robustness. One classic approach involves the encryption of the watermark using cryptographic techniques, such as AES [19] or RSA [20]. In this process, an encryption key is employed to convert the watermark into a ciphertext, enhancing the security of the embedded data. The decryption and, hence, the accessibility of the watermark, is contingent upon the application of the corresponding cryptographic key, ensuring the watermark remains impervious to unauthorized access and enhancing its applicability in high-security contexts. In addition to security, the watermark can be encoded using methodologies, such as the error correction code [21], facilitating the rectification of errors within the extracted watermark, and enhancing its robustness. The Reed-Solomon code [22] and convolutional code [23] are classic exemplary methodologies that infuse redundancy into the watermark. This inclusion of redundant data is strategically orchestrated to ameliorate errors encountered during watermark extraction, bolstering the accuracy and reliability of the extraction process, even amidst distortions.