Visual Cryptography for Color Images - Visual Cryptography and Secret Image Sharing - page 38

FIGURE 2.8
The darkening problem.
2.2.4 The Annihilator Color
Since for any color we have that add (;) = , in many visual cryptography
schemes for color images the black color is often used to "cover up" other
colors so that they don't show up in the reconstructed image. For this reason
we call the black color the "annihilator" color. The presence of the annihilator
color in the reconstructed image has no meaning and thus the observer has to
ignore it. The use of the annihilator color is not a problem from a formal point
of view but the visual effect is not good: in many cases the presence of the
annihilator color in the reconstructed image is overwhelming (e.g., 90% of the
image) and thus it is not reasonable to assume that the observer can recognize
the secret image. This is clearly a problem if we want to share images, but it
doesn't rule out some applications as we will see in later sections. We remark
that the annihilator color, has nothing special: it is just the black color! If
the secret image contains black pixels, then we will not be able to distinguish
amongst the black pixels in the reconstructed image which ones were originally
black and which ones were annihilated.
2.2.5 The Identity Color
Color is the "identity" color, in the sense that for any color we have that
add (;) = . In some schemes the identity color is used, together with the
annihilator color, as a special color. Recall that that in the context of visual
cryptography white is actually transparent.