Graphics Reference
In-Depth Information
Figure 8.6.
The HSV hexcone.
See [Blin93]. We get a new space of primaries called the
transmission
primaries, which
were recommended by the National Television System Committee (NTSC) in 1953 as
the basis for generating color television broadcast signals in the United States. The
value Y is called the
luminance
and measures brightness. It is the only signal received
by a black and white TV. This is a good basis with respect to properties of RGB phos-
phors and the “standard” observer. The triangular region in the chromaticity diagram
shown in Figure 8.4 is the possible gamut of the 1953 NTSC recommended RGB
monitor, although actual monitors have a smaller triangle.
Although the RGB model is very simple mathematically, getting a desired color
by varying the R, G, B guns of a monitor is not so simple. Artists especially would
find the RGB model very unintuitive. For that reason other models were introduced.
The HSV (Hue-Saturation-Value) “Hexcone” Model.
We get this model
by looking down at the color cube along its main diagonal (see Figure 8.6) and re-
coordinatizing (see Figure 8.7). A color is now specified by three numbers h (hue), s
(saturation), and v (value). The hue corresponds to an angle from 0 to 360 degrees,
where, for example, red is at 0 degrees and green at 120 degrees. The saturation s, s
Π[0,1], measures the departure of a hue from white or gray. The value v, v Π[0,1],
measures the departure of a hue from black, the color of zero energy. See Figure 8.8.
The hexcone model tries to mimic how artists work. Their way of working with
color has been described as follows in [AgoG87]:
“Artists choose a pure hue, or pigment, and lighten it to a
tint
of that hue by adding white
or darken it to a
shade
of that hue by adding black, or in general obtain a
tone
of that hue
by adding a mixture of white and black, that is, gray.”
The HSL (Hue-Saturation-Brightness) Triangle Model.
See Figure 8.9. The
reason for not using the letter “B” is so as not to cause confusion with “blue.” The
hue is again specified as an angle between 0 and 360 degrees about the vertical axis
and the saturation and brightness are values between 0 and 1.
The HSL model is good for color gradations found in nature.
