Image Sharing with Steganography and Authentication - Visual Cryptography and Secret Image Sharing - page 420

secret image. In addition, generating authentication bits with CRT increased
computational complexity.
This chapter proposes an enhanced scheme with the aim of maintaining
the effective authentication but eliminating the pixel expansion problem while
retaining an acceptable computational cost. Our scheme uses two techniques in
combination to reduce cover image size. The first is an error diffusion technique
[13,22] that helps to transform a grayscale image into a binary image. The
second, called ELUT, is an edge lookup inverse halftoning technique that relies
on edge detection and a lookup table to generate a reconstructed grayscale
image from a halftone image [11]. Inverse halftoning is a kind of commonly
used technique to reconstruct grayscale images from halftone images. In 2005,
a new edge-based lookup table scheme for inverse halftoning was proposed
by Chung and Wu [11], by which, the quality of the reconstructed grayscale
images was improved. In this chapter we call Chung and Wu's scheme ELUT
for short.
Experimental results confirm that no pixel expansion occurs in the pro-
posed scheme and the visual quality of the stego-image carrying the shadows
is much better. Moreover, the proposed scheme successfully reduces compu-
tational complexity while maintaining an acceptable authentication ability,
which is much better than the performance of either Lin-Tsai's scheme [15] or
Yang et al.'s scheme [28].
The rest of this chapter is organized as follows. In Section 16.2, we briefly
introduce the Lin-Tsai scheme [15], the Yang et al. scheme [28], and the Chang
et al. scheme [9]. In Section 16.3, we briefly review the techniques adopted
in the proposed scheme. Full details of our proposed scheme are explained
in Section 16.4. Section 16.5 gives some experimental results. Finally, our
conclusions are presented in Section 16.6

16.2 Related Work
In this section three existing schemes, the Lin-Tsai scheme [15], Yang et al.'s
scheme [28] and Chang et al.'s scheme [9] are introduced, in turn.

16.2.1 Lin and Tsai's Scheme
Lin-Tsai's scheme is a (k,n)-threshold polynomial-based scheme that combines
steganography and authentication to enable the sharing of secret images [15].
Their scheme uses parity check to achieve authentication, and each pixel in a
secret image is used to produce n

shared pixels for n participants by using

the polynomial function in Equation 16.1.

F(x) = (c 0 + c 1 x + ::: + c k1 x k1 ) mod p: (16.1)