# Image Watermarking between Conventional and Learning-Based Techniques: A Literature Review
## 4 Conventional Image Watermarking Schemes
### Hybrid Domain  
The spatial domain achieves better performance in terms of invisibility, while the transform domain provides high robustness. However, the watermarking schemes proposed in one domain are limited in several scenarios. Watermarking algorithms based on multi-domains are known as hybrids, and these algorithms ensure better robustness and improved information embedding properties. The next part introduces recent approaches in the hybrid domain. [PERSON] et al. [104] proposed dual color-image watermarking using DWT, WHT, and SVD to ensure image security for copyright protection. The cover image is converted to the YCbCr color space and the WHT (Hadamard transform) is employed to encrypt the watermarks. Then, the salient features are extracted based on DWT followed by SVD to calculate the singular matrix of the selected sub-band of the DWT image. In order to satisfy the perceptibility when dual watermarks are embedded, the genetic algorithm is adopted to determine the optimal embedding positions and scaling factor. Although the method shows better robustness against common image manipulation attacks, the model is partially blind, requiring more memory for the recovery process, and the time complexity is too high. The approach in [105] is proposed in the DWT-DCT-SVD domain, wherein the host image is transformed using the DWT to produce LH2 and HL2 sub-bands, and the DCT \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Reference Approaches** & **Objectives** & **Cover/ Watermark Size** & **Evaluation Metric** & **Advantages and Weaknesses** \\ \hline
[83] & DCT KELM & DE & Achieve good balance between robustness and invisibility & \(512\times 512\) \(32\times 32\) & PSNR, NC & + Robust against numerous attacks - Fragile to rotational attacks \\ \hline
[85] & DCT JND Arnold transform & Achieve good balance between robustness and invisibility & \(512\times 512\) \(64\times 64\) & PSNR, SSIM, NC, BER & + Resist different attacks, such as median filter and image rotation - Security and complexity analysis are inadequate \\ \hline
[94] & QDFT EPA MSPSAM MM, PSO & Provide better robustness against JPEG attacks and contrast enhancement & \(512\times 512\) \(64\times 64\) & Objective Value PSNR \(64\times 64\) & + Resist several image-processing attacks, including JPEG attacks - High computational time \\ \hline
[95] & DFT, PSO & Improve performance, in particular the security & COCO dataset 32-bit binary watermark & VIF PSNR Average BCR BER & + Withstand image-processing attacks and geometrical ones - Payload needs to be improved \\ \hline
[101] & DWT Entropy Alpha blending & Reduce computational cost considering robustness and imperceptibility & \(512\times 512\) \(64\times 64\) & PSNR SSIM NCC & + Robust against various sorts of attacks - Non-blind - Testing with one type of watermark data is inadequate - Security and complexity analysis need to be discussed in detail \\ \hline
[102] & DWT Capillary cryptosystems-tem Turb code Step space-filling curve & Provide robustness and security of the EPR data & \(512\times 512\) mark and different sizes of watermark image & \(512\times 512\) mark and different UACI BER NC & + Robust and secure for medical-image watermarking - Lack of computational complexity test \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of transform domain-based image watermarking techniques. is applied to these sub-bands. Finally, the chaotic logistic map is adopted to shuffle the watermark image to be inserted in the SVD domain. Additionally, an optimized version of PSO is proposed to select the most appropriate DCT sub-bands via a multi-dimensional optimization and to estimate the watermark-embedding strength considering the imperceptibility and the robustness of the watermarking scheme, which performs better in terms of watermarking requirements, especially in terms of security. However, some geometrical attacks, such as rotation and scaling, have not been treated.