Parent Topic: ALGORITHM

CYLINDER

The following algorithm is used to convert RGB to IHS values using the cylinder colour model.

 R = Red, G = Green, B = Blue
 I = Intensity, H = Hue, S = Saturation

 K2 = 1 / sqrt(2)
 K3 = 1 / sqrt(3)
 K6 = 1 / sqrt(6)
 PI is the constant PI (3.14159)
 RadToDeg = 180 / PI (factor to convert radians to degrees)

 B1 = K6 * (2*B - R - G)
 X1 = K2 * (G - R)

 I = K3 * (R + G + B)

 If (B1 = 0) then
     If (R <= G) H = 90
     If (G < R)  H = 270

 If (B1 <> 0) then
     H = RadToDeg * atan(X1 / B1) 
     If (G > R and H < 0) H = H + 180
     If (G < R and H > 0) H = H + 180
     If (G = R and R > B) H = 180

 If (H < 0) H = H + 360
 If (H >= 360) H = H - 360

If output channels are all 8-bit, then Intensity, Hue, and Saturation values are scaled between 0 and 255:

 I = I * (255 / 442)
 H = H * (255 / 360)
 S = S * (255 / 208.2066)

The above algorithm was derived from the following references:

 Kruse, F.A. and G.L. Raines, 1984. "A Technique For Enhancing Digital
 Colour Images by Contrast Stretching in Munsell Colour Space",
 Proceedings of the International Symposium on Remote Sensing of
 Environment, 3rd Thematic Conference, Environmental Research Institute
 of Michigan, Colorado Springs, Colorado, pp. 755-773.

 Bonham-Carter, Graeme F., 1994.  Geographic Informations Systems for
 Geoscientists: Modelling with GIS. Computer Methods in the
 Geosciences, Volume 13, published by Pergamon (Elsevier Science Ltd),
 pp. 120-125.

Parent Topic: ALGORITHM
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