Resolving the Alignment Problem in Visual Cryptography - Visual Cryptography and Secret Image Sharing - page 312

TABLE 11.5
The advantages and disadvantages of large and small subpixels.
size of the
subpixels

advantages disadvantages

easier to align and
better misalignment
tolerance

larger transparency size
and lower resolution

larger

harder to align and
worse misalignment
tolerance

smaller transparency size
and higher resolution

smaller

lowing problems: (1) What is the appropriate percentage of the large subpixel
in a share? (2) What is the appropriate size ratio of the large subpixel to the
small subpixel? (3) How do we arrange the large and small subpixels in one
share?

11.5 A Misalignment Tolerant VCS
11.5.1 The Algorithm
Concerning the first two problems, both items (the percentage and the size
ratio) can be used to trade the quality of the recovered secret image for the
misalignment tolerance. Different percentages and size ratios will be experi-
mented and analyzed in Section 11.5.2. Consider the third problem, how to
arrange the large and small subpixels in one share. In this chapter, we ar-
range them in a regular mask or a random mask. Subsequently, we describe
the encrypting/decrypting algorithm of a (k;n)-threshold misalignment toler-
ant VCS, when given a percentage (the item in Problem (1)), a size ratio (the
item in Problem 2) and an arrangement mask (the item in problem (3)).
Notation Used:

I A gray secret image of size xy.
2 The size ratio of a large to a small subpixel, i.e., 2 = (s 2 =s 1 ) 2 .
I S A small-scaled secret image of size xy. This image is obtained by
halftoning I into a binary image. Note that, this image is used for
embedding small subpixels.
I B A large-scaled secret image of size (x=) (y=). This image is
obtained by reducing the secret image I to size (x=) (y=) and
then halftoning to a binary image. Note that, this image is used for
embedding large subpixels.
p B (p S ) p B and p S are percentages of large and small subpixels, respectively.