# 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 semi-fragile watermarking algorithms in spatial domain perform some kinds of reversible mathematical transforms for the image, and then achieve the watermark-embedding process by modifying the coefficients of the transform domain. The semi-fragile watermarking algorithms in transform domain can take human perception into account, they can balance invisibility and robustness, and they can adapt the compression standard, so they have become a research hotspot in recent years. Among them, the discrete cosine transform (DCT) and discrete wavelet transform (DWT) are the two most commonly used transforms. * DCT Domain Since the block DCT is the basis of JPEG compression, it is possible to embed semi-fragile watermarks into DCT domains to enhance the ability to resist JPEG compression. [PERSON] et al. [3] put forward a semi-fragile watermarking algorithm using spread spectrum technology. The correlation about pixel differences in spatial domains is performed by the detector, which is modified. The watermarking information is embedded into the low- and medium-frequency areas in the DCT domain. Under moderate compression, the tamper detection accuracy of the algorithm in this paper can reach to 75%, while under light compression the accuracy can reach to 90%. In smooth areas, the algorithm can perform well, but in edge or texture areas, its performance is no longer good. To enhance the functions of the algorithm in [3], [PERSON] proposed a method that can not only adjust to the watermarking strength but the detector type based on the image content characteristics [14]. The algorithm can improve the correlation calculation in edge or texture areas. The watermark embedded in the textured blocks is stronger than in other kinds of blocks. Compared with the algorithm in [3], this algorithm has a much smaller number of false positives, especially in edge and texture areas. However, the switching in the algorithm may lead to lower PSNR. Many watermarking algorithms in DCT domains implement the processes of watermark generation and embedding based on important properties of JPEG compression [15; 16]. In [15], [PERSON] and [PERSON] put forward a watermarking algorithm that embedded watermarks in the DCT domain. The corresponding sign and size of coefficients in the DCT domain are changeless before and after JPEG compression. Based on this point, the feature codes are extracted as watermarking information. To enhance the ability to resist malicious tampering, the algorithm uses the nine-neighborhood system to ensure that the nondeterministic block dependency is implemented. If the host image is quantized to the predetermined QF firstly, it can accept any further compression with the shorter quantization step size. The reason is that the initial quantization coefficients can be accurately re-quantified by using the original step size. According to the invariance principle of DCT coefficients in JPEG image compression process, [PERSON] and [PERSON] carried out the processes of watermark generation and embeddingmodulation [16]. Under the condition that the embedding intensity of watermarking information is not increased, the watermarked image can be protected more effectively when faced with transform domain attacks. In this algorithm, the judgment about the false alarm area is added, and the five-neighborhood strategy is combined to realize the correlation between the image blocks, which increases the security of the algorithm and the accuracy of the localization for tampered areas. In addition, this method has the ability to tolerate a certain degree of JPEG compression, but no other non-malignant tampering is involved in the paper. To avoid cut-and-paste attacks, [PERSON] and [PERSON] [17] proposed a semi-fragile watermarking algorithm based on JPEG compression in the DCT domain.