+For the whole experiment, 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 with respect to the rate of
+embedding which 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 considered.
+Practically, a set of edge pixels is computed according to the
+Canny algorithm with high threshold.
+The message length is thus defined to be the 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
+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 two.
+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.
+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.
+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
+is sufficient. If the set cardinality is more than twice larger than the
+bit message length an STC step is again applied.
+Otherwise, pixels are again randomly chosen with BBS.
+
+
