## 4 Experimental Results
### Comparison with Related Works  
Since the proposed two-layer RDH scheme for VQ index table is a hybrid method of de-clustering and SOC-based schemes, our experimental results are compared with the two series of schemes. The first series are the de-clustering approaches. Table 5 compares the embedding capacity (EC) and file size of the proposed scheme with the schemes proposed in [28; 30]. Although the series of schemes are all based on the VQ index table, the file size of marked index table are different. Usually, a larger file size is required to embed more secret data. To make a fair comparison, the EC and file size are converted to the total bitrate (total BR) and data bitrate (data BR) as listed in Table 6.  
Since the secret data is usually assumed to be completely random, which is supposed to be uncompressible. Therefore, image bitrate (image BR), that represents compression efficiency of a scheme, can be estimated by subtracting the data bitrate from the total bitrate of a marked VQ index table. These performance evaluations are plotted in Figure 19. As shown in the figure, the proposed scheme can embed a large amount of data with a comparable file size. More specifically, in addition to the first layer of de-clustering embedding, SOC compression in the second embedding phase efficiently vacates a large room for hiding data. Figure 19b shows that the proposed scheme successfully camouflages an uncompressed VQ index table. The second series to be compared are the SOC-based approaches, including [20; 21; 23]. EC and file size are listed in Table 7. Again, the total BR, data BR, and image BR are compared and listed in Table 8. Table values are plotted in Figure 20 for convenience of observation. As shown in the figure, the proposed scheme provides a large amount of EC, since the vacated room by compression is exploited to embed secret data. However, Figure 20d shows that the compression efficiency is comparable with related works. In addition, the IFSOC applied in our scheme is free from indicator. The series of SOC-based schemes directly hide secret data into indicators, which are exposed to eavesdroppers. So, the proposed indicator free RDH scheme is more secure than the related works. \begin{table}
\begin{tabular}{c c c c} \hline
**Images** & **Layer 1** & **Layer 2** & **Total** \\ \hline Tank & 16,414 & 55,462 & 71,876 \\ Bridge & 16,398 & 59,814 & 76,212 \\ Elaine & 16,720 & 42,814 & 59,534 \\ Lena & 16,816 & 46,483 & 63,299 \\ Peppers & 16,863 & 51,549 & 68,412 \\ Wine & 16,886 & 50,606 & 67,492 \\ Goldhill & 16,615 & 41,497 & 58,112 \\ Bird & 16,883 & 35,768 & 52,651 \\ Baboon & 17,266 & 29,803 & 47,069 \\ \hline \end{tabular}
\end{table} 
Table 4: Embedding capacity for different layers of the proposed scheme (bits). \begin{table}
\begin{tabular}{c c c c c} \hline
**Images** & **Compressible** & **Uncompressible** & **File Size** & **Bit Rate** \\ \hline Tank & 13,016 & 3368 & 18,113 & 0.55 \\ Bridge & 14,472 & 1912 & 17,361 & 0.52 \\ Elaine & 8638 & 7746 & 20,761 & 0.63 \\ Lena & 9889 & 6495 & 20,279 & 0.61 \\ Peppers & 9562 & 6822 & 22,013 & 0.67 \\ Wine & 11,240 & 5144 & 19,709 & 0.60 \\ Goldhill & 8294 & 8090 & 20,775 & 0.63 \\ Bird & 6295 & 10,089 & 22,177 & 0.46 \\ Baboon & 4179 & 12,205 & 23,809 & 0.67 \\ \hline \end{tabular}
\end{table} 
Table 3: Number of SOC compressible and uncompressible blocks.