Color - Computer Graphics and Geometric Modeling - page 295

nate the object in a dark room. Another is to view it through a hole in a black panel
while focusing on the perimeter of the hole. In the latter case, the perceived colors
are called film or aperture colors .
Perceived colors have a number of characteristics. In the case of film colors, the
simplest case, there are only three, namely, hue, saturation, and brightness:
Perceived hue :
Looking at red light we experience a sensation of a red “hue.” It is hard to say
exactly what hue is. The problem is similar to trying to describe the sensation of
bitterness or shrillness in voice. It can also be thought of as the “color” of the color
by which the color is designated. Colors are subdivided into
chromatic colors - those perceived colors that have hue
achromatic colors - those perceived colors that do not have hue (for example,
the colors from a fluorescent lamp)
There are four basic hues (the unitary or unique hues ): unitary red, yellow, green,
and blue. All other hues are mixtures.
Perceived saturation :
This is the perception of the relative amount of a hue in a color and can be thought
of as a number between 0 and 1. Since light can be thought to have two compo-
nents, a chromatic and an achromatic one, a working definition of saturation is
as the ratio of the chromatic to the sum of the chromatic and achromatic com-
ponents of a color. For example, pink has a lower chromatic component than red.
Saturation measures how much a pure color is diluted by white.
Perceived brightness :
Brightness is an attribute of the illumination in which a nonisolated object is
viewed. It is a “perception of the general luminance level.” Brightness applies to
the color of an object only when the object is isolated and the light comes to the
eye from nowhere else. One generally talks about it as ranging from “dim” to
“dazzling.”
Perceived colors other than film colors have additional characteristics such as:
Perceived lightness :
This is an attribute of a nonisolated color produced in the presence of a second
stimulus. One uses terms such as “lighter than” or “darker than” for this.
Perceived brilliance :
This is perceived only when the object is not isolated as, for example, in the case
of an area of paint in a painting or a piece of glass among others in a stained glass
window.