+For whole experiments, a set of 500 images is randomly extracted
+from the database taken from the BOSS contest~\cite{Boss10}.
+In this set, each cover is a $512\times 512$
+grayscale digital image.
+
+
+\subsection{Adaptive Embedding Rate}
+
+Two strategies have been developed in our scheme, depending on the embedding rate that is either \emph{adaptive} or \emph{fixed}.
+
+In the former the embedding rate depends on the number of edge pixels.
+The higher it is, the larger the message length that can be inserted is.
+Practically, a set of edge pixels is computed according to the
+Canny algorithm with an high threshold.
+The message length is thus defined to be half of this set cardinality.
+In this strategy, two methods are thus applied to extract bits that
+are modified. The first one is a direct application of the STC algorithm.
+This method is further referred to as \emph{adaptive+STC}.
+The second one randomly chooses the subset of pixels to modify by
+applying the BBS PRNG again. This method is denoted \emph{adaptive+sample}.
+Notice that the rate between
+available bits and bit message length is always equal to 2.
+This constraint is indeed induced by the fact that the efficiency
+of the STC algorithm is unsatisfactory under that threshold.
+In our experiments and with the adaptive scheme,
+the average size of the message that can be embedded is 16,445 bits.
+Its corresponds to an average payload of 6.35\%.
+
+
+
+
+In the latter, the embedding rate is defined as a percentage between the
+number of modified pixels and the length of the bit message.
+This is the classical approach adopted in steganography.
+Practically, the Canny algorithm generates
+a set of edge pixels related to a threshold that is decreasing until its cardinality
+is sufficient. If the set cardinality is more than twice larger than the
+bit message length, a STC step is again applied.
+Otherwise, pixels are again randomly chosen with BBS.
+
+
+
+\subsection{Image Quality}
+The visual quality of the STABYLO scheme is evaluated in this section.
+For the sake of completeness, four metrics are computed in these experiments:
+the Peak Signal to Noise Ratio (PSNR),
+the PSNR-HVS-M family~\cite{PSECAL07,psnrhvsm11},
+the BIQI~\cite{MB10,biqi11}, and
+the weighted PSNR (wPSNR)~\cite{DBLP:conf/ih/PereiraVMMP01}.
+The first one is widely used but does not take into
+account the Human Visual System (HVS).
+The other ones have been designed to tackle this problem.
+
+\begin{table*}
+\begin{center}
+\begin{tabular}{|c|c|c||c|c|c|}
+\hline
+Schemes & \multicolumn{3}{|c|}{STABYLO} & \multicolumn{2}{|c|}{HUGO}\\
+\hline
+Embedding & \multicolumn{2}{|c||}{Adaptive} & Fixed & \multicolumn{2}{|c|}{Fixed} \\
+\hline
+Rate & + STC & + sample & 10\% & 10\%&6.35\%\\
+\hline
+PSNR & 66.55 & 63.48 & 61.86 & 64.65 & 67.08 \\
+\hline
+PSNR-HVS-M & 78.6 & 75.39 & 72.9 & 76.67 & 79.23 \\
+\hline
+BIQI & 28.3 & 28.28 & 28.4 & 28.28 & 28.28 \\
+\hline
+wPSNR & 86.43& 80.59 & 77.47& 83.03 & 87.8\\
+\hline
+\end{tabular}
+\end{center}
+\caption{Quality Measures of Steganography Approaches\label{table:quality}}
+\end{table*}
+
+Let us give an interpretation of these experiments.
+First of all, the adaptive strategy produces images with lower distortion
+than the one of images resulting from the 10\% fixed strategy.
+Numerical results are indeed always greater for the former strategy than
+for the latter, except for the BIQI metrics where differences are not really relevant.
+These results are not surprising since the adaptive strategy aims at
+embedding messages whose length is decided according to an higher threshold
+into the edge detection.
+Let us focus on the quality of HUGO images: with a given fixed
+embedding rate (10\%),
+HUGO always produces images whose quality is higher than the STABYLO's one.
+However, our approach nevertheless provides equivalent
+results with the strategy
+\emph{adaptive+STC} than HUGO with an average embedding rate set to
+6.35\%.
+This occurs with a lightweight manner, as motivated in the introduction.
+
+
+Let us now compare the STABYLO approach with other edge based steganography
+schemes with respect to the image quality.
+First of all, the Edge Adaptive
+scheme detailed in~\cite{Luo:2010:EAI:1824719.1824720}
+executed with a 10\% embedding rate
+has the same PSNR but a lower wPSNR than ours:
+these two metrics are respectively equal to 61.9 and 68.9.
+Next, both approaches~\cite{DBLP:journals/eswa/ChenCL10,Chang20101286}
+focus on increasing the payload while the PSNR is acceptable, but do not
+give quality metrics for fixed embedding rates from a large base of images.
+Our approach outperforms the former thanks to the introduction of the STC
+algorithm.
+
+
+
+
