# Image Watermarking between Conventional and Learning-Based Techniques: A Literature Review
## 4 Conventional Image Watermarking Schemes
### Transform Domain
#### 4.2.3 Discrete Wavelet Transform (DWT)
In view of the experimental tests, the obtained results reveal a good resistance of the proposed method against a wide range of attacks, with high watermarking imperceptibility. Despite this, it suffers from high computational complexity. For color image authenticity and copyright protection, [PERSON] and [PERSON] [101] presented a DWT-based watermarking method using alpha blending and entropy concepts. The DWT is applied on the YCbCr host image to obtain the non-overlapping sub-bands of the \"Y\" channel. Next, the block with the highest entropy value is adaptively chosen for watermark embedding. Finally, alpha blending is performed to conceal the watermark in the selected block. Results have revealed that alpha blending successfully strikes a balance between the watermarking system's resilience and imperceptibility. The main benefits of the proposed approach include low computational cost, enhanced PSNR value, and high robustness against various sorts of attacks. However, the watermarking scheme requires the host image in the extraction stage and it has only been tested with one kind of watermark data, and with inadequate time and security analysis. On the other hand, [PERSON] and [PERSON] [102] proposed a Paillier cryptosystem and turbo code-based DWT image watermarking technique for grayscale medical images in telehealth applications. In order to provide authentication, the watermark is first scrambled using a step space-filling curve, then encoded using turbo code. Furthermore, the retrieved watermark is concealed in the host image using the homomorphic properties of the Paillier Cryptosystem. The experimental results show that the discussed technique is very robust and meets all watermarking requirements for telemedicine applications in terms of imperceptibility, robustness, capacity, and security. Since the encryption operation provides a high level of computational complexity, selective encryption techniques can be used to improve performances in this aspect. Lastly, [PERSON] et al. [103] designed a secure and blind watermarking method based on discrete wavelet transform and computational ghost imaging (CGI). The watermark is encrypted by the CGI system with a chaotic key. Then, the quantization algorithm is applied to the ciphertext, which compresses the amount of information to be concealed in the wavelet coefficients of the original image. In this approach, the chaotic key is applied in both the embedding and encryption stages, which improves the security of the watermarking scheme. Moreover, it can withstand both salt and pepper noise and Gaussian noise at the same time. However, It should be noted that with increasing noise and cropping, the quality of the reconstructed image gradually declines, and the compression of the ciphertext can induce serious distortion in the retrieved watermark. The wavelet transform-based image watermarking schemes provide good performance in terms of invisibility and robustness to various types of attacks. Moreover, The main advantage of DWT over DFT is the temporal resolution, which makes DWT a more attractive research area. Yet, it is still sensitive to geometric transformations and it requires a high complexity value. Table 2 summarizes an overview of the transform domain-based state-of-the-art schemes.