Information Technology Reference
In-Depth Information
5.2
Watermark Removal
Once the presence of hidden information was found, attempts for erasing the
embedded data were made using the dither technique described in Section 4.
Experimental results for Baboon and Lena, respectively, with hidden data emedded
using the single-layered method are shown in Figures 8 and 9. Data were embedded in
the 20th and 30th DCT coefficients with ∆=20 and ∆=25 respectively. The quantity
S
on the abscissa of the plots represents the assumed quantization step used in the at-
tempts. As observed from Figures 8(a) and 9(a), the relative extraction bit error-rate,
BER, remained to be 0 when
S
was small, and started going up as
S
increased. When
S
approached and then exceeded the ∆ value actually used in the embedding (20 for
Baboon, and 25 for Lena), the relative bit error rate grew sharply to nearly 0.5 indicat-
ing that the watermarks were no longer extractable.
0.6
54
52
0.4
50
48
0.2
46
44
0
10
20
30
10
20
30
S
S
(a) Relative bit error rate
vs.
S
(b) PSNR vs.
S
Fig. 8.
Removal of QIM watermark in Baboon
0.6
55
0.4
50
0.2
45
0
10
20
30
10
20
30
S
S
(a) Relative bit error rate vs.
S
(b) PSNR vs.
S
Fig. 9.
Removal of QIM watermark in Lena
Influence of the dithering attack on the image quality is shown in Figures 8(b) and
9(b) in which the ordinates represent peak-signal-to-noise ratio (PSNR) of the images
with respect to the original unwatermarked cover images. The horizontal dashed lines
give the initial PSNR due to QIM embedding. It can be seen that, after the embedded
information was removed at
S
> ∆, PSNR remained at a high level. Note that PSNR
