6-3 Color Conversion between RGB and HIS

While color output by computer is based on three primary colors of red (R), green (G) and blue (B) and their mixture depending on bits or gray scale, human visual sense of color will rely on hue (H), intensity (I) and color purity or saturation (S), the relationship that has been already established with the Munsell color system, one of the most popular color appearance systems in the world.

The Munsell color system as shown in Figure 6.5 consists of a hue ring with forty colors 11 intensity levels, from 0 (black) to 10 (white) and saturation ranging differently from low (mixed; 0~2) to high (pure; 10~20) depending on the hue and intensity. Any color in the Munsell color system is identified with a combination of HIS, for example 2.5 R 6/4, that is 2.5R (H), 6(I) and 4(S).

The Munsell color samples are available in the commercial market or publications.

When users want to change the brightness or intensity of a certain color, it would be really difficult to change RGB combination directly, but it is very easy to change only I of HIS if RGB is converted to HIS.

Color conversation between RGB and HIS is physically established, though there are several HIS color space systems such as cube, cone, hexagon and double hexagon color space, which are a little different from the Munsell color system.

Figure 6.6 show the relationship between RGB and HIS color space.

Table 6.2 shows the algorithm of the conversion from RGM to HIS and from HIS to RGB with the range of H (0, 60), I (0, 1) and S (0, 1); R (0, 1), G (0, 1), B (0, 1) for the hexagon color space, where H = 0 or 60 (Red), H = 10 (Yellow), H = 20 (Green), H = 30 (Cyan), H = 40 (Blue), H= 50 (Magenta).