Visual Cryptography for Color Images - Visual Cryptography and Secret Image Sharing - page 33

but we will often use them to mean a light whose wavelength is perceived
as red, yellow, etc. Since each wavelength corresponds to a color, there are
infinite colors, one for each possible wavelength.
An object appears to be of a particular color because when light hits the
object some light is absorbed, that is some wavelengths are absorbed; the
remaining wavelengths are perceived by our eyes. When light hits an object
it can also be reflected (or pass through the object). An object of a particular
color , has strong absorption properties for the wavelengths that do not
correspond to while it reects the light with color . For example, an object
appears yellow (when hit by a white light) because it reflects the yellow light
and absorbs most strongly in the other parts of the spectrum. In the case of a
transparency, the light that is not absorbed instead of getting reflected passes
through the transparency.
A color model is a formal model that allows us to represent all (or some
of) the possible colors. One of the most used color model is the one called the
"additive color model." With this method three primary colors (usually red,
green, and blue) are mixed to obtain other colors. Figure 2.3 shows this model
with the primaries red, green, and blue; the colors yellow, cyan, and magenta
are produced when two of these primaries overlap. Varying the "intensity" of
each primary in the mixing we can obtain many other colors.
FIGURE 2.3
(See color insert.) Additive color model with primaries red, green, and blue.
The set of all possible colors that we can obtain depends on the three
primary colors that we use. Any three colors can be used as primaries; the
range of colors that we obtain is the gamut of those primaries. Unfortunately,
no three primary colors exist so that their gamut corresponds to the set of
all possible colors; however by choosing red, green, and blue as primaries we
obtain a very large number of colors in their gamut. This is why these three
colors have been chosen for the additive color model and the model is often
called Red, Green, and Blue (RGB). Most displays use this model.
Another color model is the "subtractive color model," also called the Cyan,
Magenta, and Yellow (CMY) model. In this case the colors are obtained with a