The HSI Model

In the HSI model, color is specified by the three quantities namely,

• hue,
• saturation and
• intensity.

Hue
In the visible spectrum, hue directly corresponds to the dominant wavelength of color.

Saturation
Saturation refers to the degree to which a color deviates from a neutral grey of equal intensity. It is also identified as pastel and vividness. Saturation may also be defined as color's purity or the amount of white contained in a specific color (Camci, 1992). When highly desaturated, any color of the spectrum should approach the standard white color. The analogy here is to white noise when the signal strength is progressively reduced.

Intensity
Intensity of a color refers to its relative brightness in the color mixture. It represents spectral energy at the specific wavelength arriving at the sensor from all directions.

The HSI model can be represented in terms of the color-space by defining a three dimensional cylindrical coordinate system and a subspace as shown in Figure 6.1. The hue distribution (H) is represented as an angle varying from 0^o to 360^o . Saturation (S) corresponds to the radius, varying from 0 to 1. Intensity (I) varies along the Z-axis with 0 being black and 1 being white. Adjusting hue will vary the color from red at 0^o through green at 120^o , blue at 240^o , and back to red at 360^o .

Figure 6.1 Double cone model of HSI color space; Color triangle is suitable for color matching.