# A Self-Recovery Fragile Image Watermarking with Variable Watermark Capacity
## 4 Experimental Results and Comparison Analysis
### Performance of Tamper Detection and Self-Recovery
#### 4.2.5 Content-Only Attack  
Content-only attack [37] is another special attack that is commonly used in fragile watermarking. In some watermarking schemes with recovery ability, the authentication bits of a block are completely irrelevant to the image content. The content-only attack exploits this flaw and alters the image content while keeping their watermarking bits untouched. In this way, the image content has been tampered, but the watermarking bits used for tamper detection remain unchanged, which would lead to the authentication failure in the detection side. The proposed scheme has avoided this drawback. The feature of the singular values in each block is adopted as the authentication watermark. Besides, the use of three-level detection strategy can also protect the watermarked image from the content-only attack effectively.  
Figure 11 shows the algorithm performance comparisons for content-only attack. The airplane is inserted into the watermarked Clock image without changing its watermarking bits. From the detection results presented in Figure 11d-f, we can observe that reference [13] has failed in detecting this attack. This is because its generation processes of authentication watermark and recovery watermark are mutually independent, which gives a chance for content-only attack. The proposed scheme and the method in [25] avoid this shortcoming and achieve similar detection results subjectively. Due to the authentication failure, the attacked image in [13] cannot be recovered. To make a better comparison for the restored images, Figure 12 exhibits the partial enlarged details of the restored images shown in Figure 11h,i. As shown in the enlarged drawing, there are some image blocks (as circled in red) that are unnaturally distributed in the restored image of reference [25]. Besides, compared to reference [25], the restored image by the proposed scheme is much closer to the original image.  
Table 7 lists the corresponding experimental data. From the table, we can learn that the FNR of the proposed method is little bit lower than [PERSON] et al.'s method [25]. The PSNR value of the recovered image obtained by the proposed scheme is improved more than 3 dB, which is benefited from the effective block classification method and recovery strategy.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Algorithm** & **FPR** & **FNR** & **PSNR (dB) of Recovered Image** \\ \hline [PERSON] et al. [13] & 0.22\% & 0 & 35.19 \\ [PERSON] et al. [25] & 0.25\% & 0 & 35.21 \\ The proposed method & 0.30\% & 0 & 37.27 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Performance comparisons for image deletion attack using Venice image.  
Figure 10: Partial enlarged details of the restored images: (**a**) Original ship image; (**b**) restored ship image using the method in [13]; (**c**) restored ship image using the method in [25]; (**d**) restored ship image of the proposed method. To highlight the differences among these restored images, some regions in restored images are selected and circled in red.  
Figure 11: Tamper detection and recovery performance for content-only attack: (**a**) Original Clock image; (**b**) watermarked Clock image; (**c**) attacked Clock image; (**d**) tamper detection result using the method in [13]; (**e**) tamper detection result using the method in [25]; (**f**) tamper detection result of the proposed method; (**g**) image recovery result using the method in [13]; (**h**) image recovery result using the method in [25]; (**i**) image recovery result of the proposed method.  
Figure 12: Partial enlarged details of the restored images: (**a**) Original image; (**b**) restored image using the method in [25]; (**c**) restored image of the proposed method. To highlight the differences between the restored images, some regions in restored images are selected and circled in red.