Image Sharing with Steganography and Authentication - Visual Cryptography and Secret Image Sharing - page 445

to retrieve the gray-scale image GI 0 , i.e., the secret image.

16.5 Experimental Results
The feasibility of the proposed scheme was confirmed by the experimental
results discussed in this section. We conducted three experiments, one to esti-
mate the visual quality of the stego-images; the second to estimate the visual
quality of the reconstructed secret images; and the last to evaluate the ca-
pability of identifying the tampered region. The test images shown in Figure
16.27 are four grayscale images "Jet-F16", "Baboon", "Lena", and "Pepper".
For our experiments, we chose the peak-signal-to-noise ratio (PSNR) as
the criterion to measure the visual quality of the grayscale images, which is
defined in Equation 16.19 and Equation 16.20.

255 2
MSN

PSNR = 10log 10 (16.19)

X X

1
H W

(P hw P 0 hw ) 2

MSN = (16.20)

h=1 w=1

H and W is the height and width of the images P and P 0 respectively. P hw
and P 0 hw are the corresponding pixel values, and MSE is the mean-square-error
between the two images.
In the first experiment, to compare the visual quality (i.e., PSNRs) between
the cover images and the corresponding stego-images in Lin-Tsai's scheme,
Yang et al.'s scheme, Chang et al.'s scheme, and ours, we took the image "Jet-
F16" as the secret image and the images "Lena", "Pepper", and "Baboon" as
the cover images, as shown in Figure 16.27. Figure 16.28 demonstrates the
experimental results. The PSNRs of different schemes reveal that the visual
quality of the stego-images generated by our scheme is the best of the schemes
compared in our experiments.
To evaluate the visual quality of the recovered gray-scale images, we com-
pared the PSNRs of the reconstructed grayscale images and the original secret
image in Figure 16.29. From Figure 16.29, we can see the PSNR of Jet-F16,
which is extracted from "Lena", "Baboon" and "Pepper", is larger than 30 dB.
It means that not only does the revealing function of our scheme work well
but also that the visual quality of the secret grayscale image is acceptable.
In addition, storage space and stego-image transmission time are important
issues in secret sharing. Consequently, the pixel expansion problem should be
eliminated or minimized with any useful secret image sharing scheme. In other