# Review on Semi-Fragile Watermarking Algorithms for Content Authentication of Digital Images
## 5 Overview on Semi-fragile Watermarking Algorithms
### Unrecoverable Semi-Fragile Watermarking Algorithms
#### 5.1.2 Transform Domain
The watermarking information is generated by a pseudo-random sequence that combines the secret key with block-dependent properties, and then it is embedded into the low-frequency DCT coefficients selected by the key based on the modified quantization index modulation method. To resist the JPEG compression with high quality factor, the algorithm quantizes the selected DCT coefficients by using the JPEG quantization matrix before embedding the watermark. The algorithm has good invisibility and high accuracy of tampering localization. In terms of \(R_{\text{FP}}\) and \(R_{\text{FN}}\), it has achieved good performance as well. In addition, the algorithm can be extended into color images easily. * DWT Domain DCT is a global transform from spatial domain to frequency domain for an image, while DWT is a local transform with the capability of multi-scale analysis. Because the DWT has the advantages of both spatial domain and DCT domain, it has become the focus and hotspot in the current research. [PERSON] et al. [18] combined the spatial domain method with the DWT domain method to propose a semi-fragile watermarking algorithm. The signature is extracted using the spatial domain approach and the watermarking information is inserted by the frequency domain method. The extracted signatures are encoded by the ECC method to eliminate the errors created by normal image processing operations that may bring about changes in the image signatures. To strengthen the security of the watermarking algorithm, the private keys are used to encrypt and decrypt the watermark. This algorithm controls the fragility of the system by adjusting the size of quantization step, and it can resist the traditional image compression. However, since the algorithm extracts the mean value of the pixels in every block as the watermarking information, when the maximum or minimum block is tampered with, the size of quantization step may be altered and then the accurate location of the tampered areas cannot be achieved. The algorithms in DWT domains can also be compatible with JPEG2000 compression [19] and JPEG compression [20]. In [19], [PERSON] and [PERSON] proposed a semi-fragile digital watermarking method for JPEG2000 image content authentication. This algorithm combined with JPEG2000 encoder and decoder carries out the processes of the watermark generation and embedding modulation based on the invariant parameters in JPEG2000 image compression process. The tampered regions are located by the wavelet transform characteristic so that the reliability of the image authentication system can be improved. This algorithm selects the important coefficients in DWT domain to achieve the embedding modulation of the watermarking information. The size of the macroblock can be adjusted adaptively according to texture features of the image, so the algorithm can be carried out effectively in both the texture areas and the flat areas of the image. The algorithm can tolerate the normal natural images with the compression code larger than 0.5 bits per pixel (bpp). When the code rate is less than 0.5 bpp, there is a significant distortion in a considerable part of image. In [20], according to the fact that the size relationship of the adjacent wavelet coefficients in high frequency is not changed after JPEG compression, [PERSON] and [PERSON] proposed a novel semi-fragile watermarking algorithm against the JPEG compression. The algorithm has high watermarking capacity and good anti-JPEG compression performance. At the same time, it can also accurately locate the tampered areas. To obtain a higher PSNR value, the watermarking information is embedded in sub-bands with smaller variance of adjacent coefficients. The algorithm uses parametric integer wavelets to decompose the image and scramble the watermark before embedding. In this way, it becomes difficult for the attacker to get any information about the watermarking information, so the security of the authentication system can be ensured.