feature extraction sources. Fortunately, these redundancies
do not exist in the filtered images generated by basis filters,
which can also represent the texture and edge information in
any arbitrary orientation. Therefor e, it should be possible to
obtain low-complexity sensitive features using basis filters.
Considering that Gauss partial derivative filter bank can
be used as basis filters to construct steerable filter, a stega-
nalysis feature extraction method based on Gauss partial
derivative filter bank is proposed in this paper. In the pro-
posed method, the filter bank is first constructed with eight
scales and the first five-order Gauss partial derivatives. Then,
the filtered images comprising rich texture and edge infor-
mation are generated. Finally, 5000-dimensional histograms
are extracted for detection. Experimental results for UED,
JUNIWARD, and SIUNIWARD indicate that the proposed
feature set results in competitive performance with fewer fea-
ture dimension compared with DCTR, PHARM, and GFR.
The rest of this paper is organized as follows. Section 2
describes steerable filter using Gauss partial derivative filter
bank as basis filters. The property of filtered image coeffi-
cients is first analyzed in Sec. 3, and then the feature extrac-
tion steps of the proposed method are outlined. In Sec. 4, the
recommended parameters of feature extraction are obtained
according to the experimental results, and the validity of the
proposed feature set is verified by comparing it with other
three steganalysis feature sets. The paper is concluded
in Sec. 5.
2 Steerable Filter
Steerable filter
15
has the advantages of translational invari-
ance and directional operability. Its most remarkable charac-
teristic is that it can be constructed in any orientation by
taking a suitable linear combination of a small number of
basis filters,
16
resulting in the capture of texture and edge
information in multiple orientations conveniently. It has
been successfully used for edge detection,
17
texture analy-
sis,
18
and image denoising.
19
Researchers have also applied
it to the steganalysis of stego images in color domain.
20
In
this paper, Gauss partial derivative filter bank is used as basis
filters to construct steerable filter.
The expression for Gaussian function with scale factor σ
is as follows:
EQ-TARGET;temp:intralink-;e001;63;289Gðx; yÞ¼
1
2πσ
2
e
−
x
2
þy
2
2σ
2
; (1)
where x; y ∈ R and its m-order partial derivative set is G
m
¼
fG
m;i
g;i ¼ 0;:::;m.ForG
m;i
, the orders of partial deriva-
tive with respect to x and y are m − i and i respectively:
EQ-TARGET;temp:intralink-;e002;63;215G
m;i
¼
∂
m−i
∂x
m−i
∂
i
∂y
i
Gðx; yÞ; (2)
G
m
exactly signify m -order Gauss partial derivative filter
bank, and steerable filter f
m;θ
constructed on G
m
can be
expressed as
EQ-TARGET;temp:intralink-;e003;63;137f
m;θ
¼
X
m
i¼0
k
m;i
ðθÞG
m;i
; (3)
where θ is the orientation of steerable filter and k
m;i
ðθÞ is the
weighting coefficient for G
m;i
:
EQ-TARGET;temp:intralink-;e004;326;752k
m;i
ðθÞ¼ð−1Þ
i
m
i
cos
m−i
ðθÞ sin
i
ðθÞ;i¼ 0;1;:::;m:
(4)
From the above, it can be seen that f
m;θ
is obtained by accu-
mulating the product of each G
m;i
and k
m;i
ðθÞ. Figure 1 gives
m-order Gauss partial derivative filter bank and correspond-
ing steerable filter f
m;π∕3
with scale parameter σ ¼ 0.8. It can
be seen that f
m;π∕3
is G
m;0
rotated by π∕3 along the positive
x-axis countercloc kwise. From Eqs. (3) and (4 ), it is clear
that if θ ¼ 0 deg, then f
m;0 deg
¼ G
m;0
. In fact, f
m;θ
can
be obtained by rotating f
m;0 deg
(or G
m;0
)byθ along the pos-
itive x-axis counte rclockwise.
The process of generating filtered image Z
m;θ
from spatial
image X and steerable filter f
m;θ
can be summarized as fol-
lows. First, construct basis filter G
m;i
and calculate weighting
coefficient k
m;i
ðθÞ according to parameters m, σ, and θ. Next,
generate basis filtered images Y
m;i
by convolving X with
G
m;i
. Last, filtered image Z
m;θ
can be generated by accumu-
lating the product of each Y
m;i
and k
m;i
ðθÞ. The process can
be expressed as follows:
EQ-TARGET;temp:intralink-;e005;326;516
Z
m;θ
¼ X f
m;θ
¼ X
X
m
i¼0
k
m;i
ðθÞG
m;i
¼
X
m
i¼0
k
m;i
ðθÞ
X G
m;i
|fflfflfflffl{zfflfflfflffl}
Y
m;i
;
(5)
where “*” denotes the convolution without padding. Figure 2
shows the schematic of filtered image Z
3;π∕3
generated using
steerable filter. The input image is “70.pgm” in the Boss-base
1.01 image databa se. From Fig. 2, it can be seen that filtered
image Z
3;π∕3
is a linear combination of basis filtered
images Y
3;0
;:::;Y
3;3
.
Textures and edges appear in arbitrary orientations in
natural images. One approach to capture texture and edge
information in many orientations is to apply many versions
of the same filter (such as Gabor filter), each different from
the others by some rotation in orientation. The number of
convolution operations needed is equal to the number of ori-
entations. Another more efficient approach is to apply steer-
able filters as described above, in which case the number of
convolution operations needed is independent of the number
of orientations but is equal to the number of basis filters. The
number of orientations is usually greater than that of basis
filters. Although the second approach is more efficient
than the first, both approac hes have shortcomings in stega-
nalysis. For example, there may be redundancies in filtered
images generated by filters with different orientations, which
would result in increased feature complexity and reduced
sensitivity. Such as, the dimension and extraction time of fea-
tures are directly propor tional to the number of Gabor filter
orientations in GFR. When the number is less than 32, the
detection performance of features improves with increases in
the orientation number. Conversely, when the number is
greater than 32, the detection performance of features
reduces with increases in the orientation number. For steer-
able filter, the linear relationship with the basis filters results
in the filtered images generated in different orientations hav-
ing a certain association, e.g., Z
2;θ
having a linear relation-
ship with Z
2;0
and Z
2;π∕2
, which is not conducive to the
Journal of Electronic Imaging 013011-2 Jan∕Feb 2017
•
Vol. 26(1)
Zhang et al.: Steganalysis of content-adaptive JPEG steganography based on Gauss partial derivative filter bank
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