## 4 Discussion  
Recently, several researchers proposed VC-based watermarking schemes for image copyright protection [43; 44; 45]. [PERSON] and [PERSON] [43] utilized a VC algorithm, DWT, error correction codes and noise-removing techniques to develop a watermarking approach to protect the copyright of multiple images and multiple users. Their approach is robust against several attacks. However, it cannot resist geometrical attacks including scaling and rotation, and the size of watermark is restrained by the size of the cover image. In the same direction, [PERSON] and [PERSON] [44] proposed a novel watermarking scheme to create feature shares from cover images by using VC, SVD, DWT and scale invariant feature transform (SIFT). Then, these feature shares were used with watermarks and private keys to compose the secret share. In case of any conflict, the watermark is recovered by stacking the feature shares, secret share and private key. This approach was proven to be robust against attacks, especially rotation and scaling. However, this approach provides copyright protection for single-user-only (not multiple users) and multiple images. Furthermore, it needs to send extra information (selected SIFT features) in an insecure channel to the receiver in order to create the master share. Hence, this approach is regarded as being too insecure to protect the copyright of the images. [PERSON] et al. [45] proposed a novel watermarking scheme to protect the copyright of multiple users and multiple images by using VC, LBP, k-means clustering and speeded-up robust features (SURF). In this approach, the master share is constructed by LBP, SURF and a clustering algorithm. Then, this share is used with a watermark to create ownership shares that are used later to create the key share. In case of any dispute, the key share, master share and ownership share are used together to recreate the watermark to confirm the possession of the images. The key difference between this approach and the earlier approaches is that it needs only a subset of qualified owners to recover the watermark, and not all of them, like the other approaches, to assure the copyright of multiple images and multiple users. To sum up, all of these approaches have their own limitations and drawbacks in the protection of the copyright of a single image (or multiple images) or a single user (or multiple users), which can be summarized as follows:  
* The size of the watermark is restrained by the size of the image [43].
* They cannot resist rotation and scale attacks [43].
* The approach is limited only for a single user and multiple images [44].
* Side information should be sent to the other side [44].
* The main aim of these approaches is to protect the copyright of multiple users and multiple images only [43; 44; 45].  
Based on the above discussion, the proposed VC-based watermarking scheme can remedy all of the above limitations, and can be used solely to detect and localize forgeries in the digital images. In addition, we can use this approach to provide protection for the copyright of single or multiple digital images. To the best of researchers' knowledge, this first approach uses VC-based watermarking to detect and localize image forgeries. Finally, we have to mention that the described experimental results are based on standard images collected by the research community under real scenarios, and that they mimic the actual operational environments. Therefore, we can say that there are no internal or external threats that may change the validity of these results, but as a future work we are planning to implement these algorithms in the real environments.