# E-SAWM: A Semantic Analysis-Based ODF Watermarking Algorithm for Edge Cloud Scenarios
## 4 Experiments
### Watermark Capacity  
Watermark capacity refers to the proportion of the watermark information size to the size of the document being watermarked. It can be calculated as follows:  
\[\text{WatermarkCapacity}=\frac{\text{watermark data bits}}{\text{OFD file bits}} \tag{10}\]  
By utilizing a watermark algorithm with a higher watermark capacity, the document can be embedded more effectively within the watermark information. This capability enables the handling of diverse document lengths and multilevel watermarks, facilitating the transmission of larger amounts of information in practical applications.  
In the annotation structure-based watermarking algorithm, the process of adding a watermark involves appending both annotation structure information and the watermark content itself to the corresponding Annot.xml file of the target watermark page. Conversely, E-SAWM converts and encrypts the watermark information into multiple groups (referred to as \(k\) groups). These \(k\) groups, along with their corresponding encrypted watermark characters, are then added to the Content.xml file of the designated watermark page. Compared to the annotation structure-based algorithm, our algorithm enriches the original OFD file with additional information during the watermarking process.  
To assess whether the increased information can be accommodated within the acceptable carrying range, we conducted a watermarking capacity test. For this evaluation, we randomly selected twenty OFD files of various sizes as samples. Additionally, we generated twenty watermarks with different information contents. Each watermark was individually matched with a corresponding file and embedded using the watermark algorithm. To measure the impact of watermarking on document size, we calculated the rate of change in the OFD file size by comparing its size before and after the watermark-embedding process, as shown in Figure 12.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Attack Type** & **Example of Attack Content** & \begin{tabular}{c} **Watermark Extraction** \\ **Success Rate** \\ \end{tabular} \\ \hline Highlight & \
\begin{tabular}{c} Left Column—Line 1 \\ Right Column—Lines 2-5 \\ Left Column—Line 2 \\ Right Column—Lines 2-3 \\ Left Column—Line 5 \\ Right Column—Lines 2-3 \\ Left Column—Line 6 \\ Right Column—Line 4 \\ Right Column—Lines 2 \\ Text overlay \\ \end{tabular} &
\begin{tabular}{c} 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ 100\% \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Extraction success rate of watermarks following each attack.  
Figure 11: Example of each post-attack OFD page. Please note that in the visual representation presented: Colorful lines symbolize distinct attack methods, with yellow representing “highlight”, blue indicating “Wavy line”, a green line representing “underline”, and red signifying “Strikethrough”. Green font signifies “Handwritten graffiti”, while gray font indicates “Text overlay”.  
The experiments demonstrate that when embedding watermarks of various sizes into each sample OFD document, the document size experiences minimal fluctuations, which suggests that E-SAWM effectively handles the embedding of high-capacity watermark information without significantly impacting the document size. Moreover, it showcases the algorithm's ability to accommodate a substantial amount of watermark information.