Image Sharing with Steganography and Authentication - Visual Cryptography and Secret Image Sharing - page 434

image, respectively.

Step 1: Generate the lookup table (LUT) by using the five substeps that

follow. An array LUT[ ] is used to map the given halftone image to
the corresponding grayscale image.

(a) For initialization, i = 1, LUT[k] = 0, where 0 k 2 16 1.

(b) Divide images double HIi i and GI i into overlapping 4 4
blocks and denote them as BH ij and BG ij . In other words,
BH ij and BG ij are the j-th halftone block of halftone im-
age HI i and the j-th grayscale block of grayscale image
GI i , respectively.

(c) Calculate index k for each halftone block BH ij and update
the value of the intermediate LUT[k] by using Equation
16.10. Here, BG ij (3; 3) is a representative pixel for each
grayscale block.
k = P u=1 P v=1 2 (v1)+4(u1) BH ij (u;v)
LUT[K] = LUT[K] + BG ij (3; 3)

;

(16.11)

(d) i = i + 1. If i n, go to Step b. Otherwise, compute and
archive the final LUT[ ]. N[k] is another array used to store
the number of halftone blocks that obtain the same index
value k.

(e) For 0 k 2 16 1, LUT[k] = LUT[k]
N[k]

.

Step 2: Replace the element BH ij (3; 3) in every block BH ij by LUT[k],

where k is the index value of block BH ij calculated in Step c. When
the replacement procedure is finished, retrieve the grayscale image
with pixel values corresponding to LUT[k].

Step 3: After applying Step 2 to a set of n training images in succession,

a set of reconstructed grayscale images called GI 0 i , i = 1, 2,..., n,
can be retrieved. Adopt Canny's edge detector to each reconstructed
grayscale image GI 0 i to generate an edge map EM i , for i = 1, 2,..., n.
Each edge map EM i consists of a set of 44 blocks. Therefore, the
j-th block of the edge map EMi i is denoted as BE ij . By combining
the lookup table LUT generation procedure described in Step 1 with