RGB Model

i. In the RGB model each color appears in its primary spectral components of red, green and blue. This model is based on the Cartesian co-ordinate system. The RGB model is shown below (FIG).

In the above figure, the three primary colours, that is Red, Green and Blue are shown at the three corners of the cube. The secondary colors cyan, magenta and yellow are shown at the other three corners of the cube.

ii. Black is at the origin and white is at the corner furthest from the origin. In the RGB model, the gray scale (points of equal RGB values) extends from black to white along the line joining these two points.

iii. The different colors in this model are points on or inside the cube, and are defined by vectors extending from the origin. For convenience all color values have been normalized so the cube shown in the figure above is a unit cube.

iv. All the values of R,G and B are assumed to be in the range [0,1]

HSI Model

i. HSI stands for Hue, Saturation and Intensity. Now when humans view a color object it is described by its hue, saturation and brightness.

ii. Hue is a color attribute that describes a pure color (pure yellow, orange or red)

Saturation gives a measure of the degree to which a pure color is diluted by white light.

Brightness is practically impossible to measure, however it depends upon color intensity, which is key factor in describing color sensation. The intensity is easily measurable and the results are also easily interpretable

iii. Thus the model that is used to describe a color object is the HSI model.

iv. The HSI model decouples the intensity from color-carrying information (hue and saturation) in a color image

v. Shown above is the HSI model. The angle from the red axis gives the Hue, the length of the vector is the saturation and the intensity is given by the position of the plane on the vertical intensity axis.