Visual Cryptography from Halftone Error Diffusion - Visual Cryptography and Secret Image Sharing - page 19

1.6 Discussion
1.6.1 Improvement of Image Quality
As stated previously, the quality index s heavily affects the image quality of the
halftone shares. A large s leads to visually pleasing halftone shares, but it also
introduces higher contrast loss in the reconstructed images. The error filter
employed in the error diffusion also affects the image quality of the shares. For
example, an error filter with longer weights leads to a sharper contrast in the
halftone image [20, 14]. Another factor that affects the image quality is the
position-dependent threshold in the thresholding block. To achieve a visually
more pleasing halftone image, output-dependent threshold modulation can be
used in the error diffusion to spread the minority pixels as homogeneously as
possible and suppress some unwanted textures [6]. For various methods that
can improve the halftone image by error diffusion, refer to [20, 6, 17, 21], etc.
Error diffusion is employed as the halftoning algorithm to generate halftone
shares since error diffusion is able to generate a visually pleasing halftone
image with simple computation. However, other halftoning algorithms can
also be applied to the second method to generate halftone shares. The DBS
algorithm can be used to generate a high quality halftone image but with
significant computation [1]. Note that the DBS algorithm for multitoning can
also be used to generate the distributions of SIPs and ABPs.
1.6.2 Comparison with Other Methods
To evaluate the performance of the introduced methods, it is illustrative to
compare our methods with VC and extended visual cryptography (EVC) [3].
VSS can be treated as a special case in our methods for s = 0, which means
that no visual information is carried by the share. In EVC, shares carry visual
information and there is a tradeoff between the contrast of the reconstructed
image and the contrast of the share image. This tradeoff is similar to the
tradeoff between the contrast of the reconstructed image and the image quality
of the halftone shares in our methods. However, the shares generated by EVC
is basically based on pixels expansion. Thus, EVC is unable to generate shares
that can show fine details.
1.6.3 Image Decoding
In the methods introduced in this chapter, a large HVC expansion is desirable
to make s large. But the contrast loss of the decoded image is severe when the
HVC expansion is large. However, a low contrast does not hinder the decoding
of the secret image if the decoding can be performed digitally. Initially shares
are supposed to be xeroxed on transparencies and decoding of the secret image
involves stacking the shares physically. However, both the distribution of the