# Screen-Cam Robust Image Watermarking with Feature-Based Synchronization
## 5 Experimental Results and Analysis
### Robustness against Screen-Cam  
In this section, we verify the robustness against a screen-cam attack. First, we compare the proposed method with two existing algorithms [21,33]. Since the size of the host images used in their articles is different from this one, we use the same host images here. In order to improve the independence of the experimental results to the host images, we use additional twelve images from the database [52] to verify the performance. The PSNR values of the images generated by the proposed method are controlled to be not lower than by other methods, which are at around 42 dB. An example of Lena embedded with different methods is shown in Table 3. All the watermarked images are displayed on the screen at the original resolution. The comparison of BER for different shooting conditions is shown in Figure 15. The result shows method [21] designed for print-cam is not applicable for screen-cam process, and the proposed method and method [33] both have good robustness against screen-cam attack.  
In theory, without considering external interference, the distortion caused by shooting from the horizontal left and horizontal right is similar. Shooting at different vertical angles is also similar to shooting at different horizontal angles with a 90-degree rotation of the image. Therefore, as shown in Figure 16, the shooting angle is set from being perpendicular to the screen up to 60 degrees of horizontal left at intervals of fifteen degrees. The shooting distance is set from 40 to 110 cm at intervals of 10 cm. When the shooting angle is 45 or 60, the shooting distance of 40 cm is too small to capture the entire image. Therefore, the distance is selected to be over 50 cm.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Host Image** & **Lena** & **Baboon** & **Airplane** & **Peppers** & **Building** & **Pentagon** & **White House** & **Naval Base** \\ \hline PSNR (dB) & 42.3166 & 44.0559 & 43.7674 & 43.0367 & 42.4984 & 43.4840 & 44.1091 & 42.7127 \\ MSSIM & 0.9544 & 0.9831 & 0.9659 & 0.9564 & 0.9733 & 0.9890 & 0.9839 & 0.9894 \\ JPEG 40\% & 0 & 0.0667 & 0 & 0.0833 & 0.0667 & 0.0333 & 0.0333 & 0.1000 \\ JPEG 30\% & 0 & 0.0833 & 0 & 0.1167 & 0.0833 & 0.0500 & 0.0500 & 0.0833 \\ JPEG 20\% & 0 & 0.0667 & 0 & 0.2833 & 0.0500 & 0.0333 & 0.1167 & 0.1167 \\ JPEG 10\% & 0.0667 & 0.2333 & 0.1500 & 0.4167 & 0.1333 & 0.1000 & 0.2500 & 0.2167 \\ Scaling 0.6 & 0 & 0.0500 & 0 & 0.0333 & 0.0833 & 0.0500 & 0.0333 & 0.0333 \\ Scaling 0.5 & 0 & 0.0500 & 0 & 0.0500 & 0.0500 & 0.0667 & 0.0333 & 0.0500 \\ Scaling 0.4 & 0 & 0.0333 & 0 & 0.2167 & 0.1167 & 0.0500 & 0.0333 & 0.0833 \\ Scaling 0.3 & 0.1667 & 0.4000 & 0.3500 & 0.4500 & 0.4500 & 0.4667 & 0.4667 & 0.3500 \\ \multirow{2}{*}{BER} & Cropping-off 20\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Cropping-off 30\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Cropping-off 40\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Cropping-off 50\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Rotation 15\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Rotation 30\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Rotation 45\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Rotation 90\% & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ Median filter 3 \(\times\) 3 & 0 & 0.0667 & 0 & 0.0333 & 0.0333 & 0.0333 & 0.0667 \\ Median filter 5 \(\times\) 5 & 0.0667 & 0.1500 & 0.0500 & 0.4500 & 0.2667 & 0.3000 & 0.4000 & 0.2833 \\ \hline \hline \end{tabular} Note: The underlined coefficient represents failed detection.  
\end{table}
Table 2: Performance of image quality and robustness against common attacks.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Methods** & **[PERSON] et al. [21]** & **[PERSON] et al. [33]** & **Proposed** \\ \hline Watermarked images & & & \\ PSNR (dB) & 40.0784 & 42.0148 & 42.3166 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Images generated by different methods.  
We also tested the performance at other tilt shooting angles with a handheld shooting, as shown in Table 5; it also has excellent performance. Therefore, the proposed algorithm is robust to a screen-cam attack.