Visual Cryptography for Color Images - Visual Cryptography and Secret Image Sharing - page 34

subtractive technique that starts from a white light and subtracts wavelengths
corresponding to the three colors cyan, magenta, and yellow. Figure 2.4 shows
the CMY model.
FIGURE 2.4
(See color insert.) Subtractive color model with primaries cyan, magenta, and
yellow.
Most modern printers use this model, often exploiting an additional black
ink; an additional K in the name CMYK indicates the use of the extra black
ink. Notice that we can obtain black by using all three inks (cyan, magenta,
yellow) together; however it is more ecient to cover a pixel with just black
ink, rather covering it with the three inks cyan, magenta, and yellow.
In the additive model we start from the absence of light, which gives the
black color, and we add light to obtain other colors with the extreme case
being the white color obtained when we add all possible wavelengths. In the
subtractive model we start from a white light and we subtract wavelengths
to obtain other colors with the extreme case being the color black obtained
when we take out all possible wavelengths.
If we are not very picky, and a discussion about this aspect goes beyond
the scope of this chapter, we can say that the RGB and the CMY models are
equivalent and complementary. Indeed an ink with the color cyan absorbs the
light corresponding to the red color, an ink with the color magenta absorbs
the light corresponding to the green color, and an ink with the color yellow
absorbs the light corresponding to the blue color. Because of this, for both
models, we will formally represent a color as a triple (x;y;z), where x;y, and
z denote the amount of red, green, and blue, respectively, that consists of.
The amount of each type of light (red, green, blue) is described by an integer
in the range [0, L]. With this setting, we can produce (L+ 1) 3 different colors,
which, for L suciently large, are enough to approximate all colors that the
human eyes are able to distinguish. Typically, for computers, we have L = 255.
To make things easier, throughout this chapter, we use L = 100.
Each of the components x;y, and z can be seen as a lter that lets pass