TheMunsell color system is a typical color appearance system, in which color is represented with hue (H), saturation(S) and intensity (I) as a psychological response. Hue is composed of the five basic color; red (R), yellow (Y), green(G), blue (B) and purple (P) which are located along a hue ring with intervals of 72 degrees as shown in Figure 10.6.1. Intermediate colors between the above five basic colors; YR, GY, BG, PB and RP are located in between each other. Finally each hue is divided into ten but actually four. For example 1R, 5R, 10R, 1YR, 5YR, 10YR, 1Y, ...... are a series along the hue ring.
Intensity is an index of brightness with 11 ranks from 0 (dark) to 10 (light). Saturation is an index of pureness ranging from 0 to 16 depending on the hue and intensity. Color in the Munsell color system is identified as a combination of hue, intensity / saturation, for example 5R4 / 10, which means 5R (hue), 4 (intensity) and 10 (saturation).
Figure 10.6.2 shows a three dimensional color solid as called the Munsell's solid, with the 40 panels with color samples of intensity and saturation with respect to the hue. Munsell color samples are available in the commercial market. Any user can identity arbitrary colors by comparison with the Munsell's color samples. Psychologically defined HSI has been correlated with physically defined RGB or Yxy as mentioned in 10.5. Therefore conversion between RGB and HSI can be made mathematically. In the case of a color display using a digital image processing device, the RGB signal has to be input, though color control is much easier using HSI indices. Figure 10.6.3 shows the relationship between RGB space and HSI space.
The following are conversions from RGB to HSI, and from HSI to RGB. The ranges of R,G,B,S,I are [0,1] :, the range of H is [0,2p].
(1) from RGB to HIS
I = Max. (R,G,B)
1) I = 0 ; S = 0, H= indeterminate
S = (I-i)/I , where i = min. {R, G, B}
Let r = (I-R) / (I-i), g = (I-G) / (I-i), b = (I-B) / (I-i), then
if R = I H = (b-g) / 3
if G = I H = (2+r-b) / 3
if B = I H = (4+g-r) / 3
(2) from HSI to RGB
1) S = 0 ; R = G = B = I regardless of value of H
H' = 3H / h = floor(H') If H = 2
, then H = 0
(floor (x): the function of getting the truncated value of x)
P = I(1-S), Q = I{1-S (H' - h)} , T = I {1-S(1-H'+h)} , then
h = 0 R = I, G = T, B = P
h = 1 R = Q, G = I, B = P
h = 2 R = P, G = I, B = T
h = 3 R = P, G = Q, B = I
h = 4 R = I, G = P, B = Q
h = 5 R = I, G = P, B = Q
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