-For whole experiments, a set of 500 images is randomly extracted
-from the database taken from the BOSS contest~\cite{Boss10}.
+For whole experiments, the whole set of 10000 images
+of the BOSS contest~\cite{Boss10} database is taken.
In this set, each cover is a $512\times 512$
-grayscale digital image.
+grayscale digital image in a RAW format.
+We restrict experiments to
+this set of cover images since this paper is more focussed on
+the methodology than benchmarking.
\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 is the message length that can be inserted.
+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 the half of this set cardinality.
+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 as \emph{adaptive+STC}.
-The second one randomly choose the subset of pixels to modify by
+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.
-On our experiments and with the adaptive scheme,
-the average size of the message that can be embedded is 16445.
+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 the modified pixels and the length of the bit message.
+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
-a set of edge pixels with threshold that is decreasing until its cardinality
+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.
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 PSNR-HVS-M family~\cite{psnrhvsm11},
+the BIQI~\cite{MB10}, 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 last ones have been designed to tackle this problem.
+The other ones have been designed to tackle this problem.
-\begin{table}
+\begin{table*}
\begin{center}
-\begin{tabular}{|c|c|c||c|c|}
+\begin{tabular}{|c|c|c||c|c|c|}
\hline
-Schemes & \multicolumn{3}{|c|}{STABYLO} & HUGO\\
+Schemes & \multicolumn{3}{|c|}{STABYLO} & \multicolumn{2}{|c|}{HUGO}\\
\hline
-Embedding & \multicolumn{2}{|c||}{Adaptive} & Fixed & Fixed \\
+Embedding & \multicolumn{2}{|c||}{Adaptive} & Fixed & \multicolumn{2}{|c|}{Fixed} \\
\hline
-Rate & + STC & + sample & 10\% & 10\%\\
+Rate & + STC & + sample & 10\% & 10\%&6.35\%\\
\hline
-PSNR & 66.55 & 63.48 & 61.86 & 64.65 \\
+PSNR & 66.55 & 63.48 & 61.86 & 64.65 & 67.08 \\
\hline
-PSNR-HVS-M & 78.6 & 75.39 & 72.9 & 76.67\\
+PSNR-HVS-M & 78.6 & 75.39 & 72.9 & 76.67 & 79.23 \\
\hline
-BIQI & 28.3 & 28.28 & 28.4 & 28.28\\
+BIQI & 28.3 & 28.28 & 28.4 & 28.28 & 28.28 \\
\hline
-wPSNR & 86.43& 80.59 & 77.47& 83.03\\
+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}
+\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 relevant.
+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 a higher threshold
+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 better results with the strategy
-\emph{adaptive+STC} in a lightweight manner, as motivated in the introduction.
+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
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 the approaches~\cite{DBLP:journals/eswa/ChenCL10,Chang20101286}
+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 rate from a large base of images.
+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.
(AUMP) test is designed (theoretically and practically), to check whether
an image has stego content or not.
In the latter, the authors show that the
-machine learning step, (which is often
-implemented as support vector machine)
-can be favorably executed thanks to an Ensemble Classifiers.
+machine learning step, which is often
+implemented as support vector machine,
+can be favorably executed thanks to an ensemble classifier.
-\begin{table}
+\begin{table*}
\begin{center}
-\begin{tabular}{|c|c|c|c|c|}
+%\begin{small}
+\begin{tabular}{|c|c|c|c|c|c|}
\hline
-Schemes & \multicolumn{3}{|c|}{STABYLO} & HUGO\\
+Schemes & \multicolumn{3}{|c|}{STABYLO} & \multicolumn{2}{|c|}{HUGO}\\
\hline
-Embedding & \multicolumn{2}{|c|}{Adaptive} & Fixed & Fixed \\
+Embedding & \multicolumn{2}{|c|}{Adaptive} & Fixed & \multicolumn{2}{|c|}{Fixed} \\
\hline
-Rate & + STC & + sample & 10\% & 10\%\\
+Rate & + STC & + sample & 10\% & 10\%& 6.35\%\\
\hline
-AUMP & 0.39 & 0.33 & 0.22 & 0.50 \\
+AUMP & 0.39 & 0.33 & 0.22 & 0.50 & 0.50 \\
\hline
-Ensemble Classifier & 0.47 & 0.44 & 0.35 & 0.48 \\
+Ensemble Classifier & 0.47 & 0.44 & 0.35 & 0.48 & 0.49 \\
\hline
\end{tabular}
+%\end{small}
\end{center}
\caption{Steganalysing STABYLO\label{table:steganalyse}}
-\end{table}
+\end{table*}
Results show that our approach is more easily detectable than HUGO, which
is the most secure steganographic tool, as far as we know. However due to its
-huge number of features integration, it is not lightweight.
+huge number of features integration, it is not lightweight, which justifies
+in the authors' opinion the consideration of the proposed method.
