Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/IHS">IHS</a><hr>
<H3>ALGORITHM</H3>
The equations used to convert Red, Green, and Blue colour values to
Intensity, Hue, and Saturation colour values for both the Cylinder
and the Hexcone IHS models are given below.
<P>
<H4>CYLINDER</H4>
The following algorithm is used to convert RGB to IHS values using
the cylinder colour model.
<P>
<PRE>
 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 &lt;= G) H = 90
     If (G &lt; R)  H = 270

 If (B1 &lt;&gt; 0) then
     H = RadToDeg * atan(X1 / B1) 
     If (G &gt; R and H &lt; 0) H = H + 180
     If (G &lt; R and H &gt; 0) H = H + 180
     If (G = R and R &gt; B) H = 180

 If (H &lt; 0) H = H + 360
 If (H &gt;= 360) H = H - 360
</PRE>
If output channels are all 8-bit, then Intensity, Hue, and
Saturation values are scaled between 0 and 255:
<P>
<PRE>
 I = I * (255 / 442)
 H = H * (255 / 360)
 S = S * (255 / 208.2066)
</PRE>
The above algorithm was derived from the following references:
<P>
<PRE>
 Kruse, F.A. and G.L. Raines, 1984. &quot;A Technique For Enhancing Digital
 Colour Images by Contrast Stretching in Munsell Colour Space&quot;,
 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.
</PRE>
<H4>HEXCONE</H4>
The following algorithm is used to convert RGB to IHS values using
the single-hexcone colour model.
<P>
<PRE>
 R = Red, G = Green, B = Blue
 I = Intensity, H = Hue, S = Saturation

 Max = Maximum(R,G,B)
 Min = Minimum(R,G,B)
 Delta = Max - Min

 I = Max

 If (Max &lt;&gt; 0) S = Delta / Max
 If (Max = 0)  S = 0

 If (S = 0) H = 0                       (Hue is undefined)

 If (R = Max) H = (G - B) / Delta       (between yellow and magenta)
 If (G = Max) H = 2 + (B - R) / Delta   (between cyan and yellow)
 If (B = Max) H = 4 + (R - G) / Delta   (between magenta and cyan)
 H = H * 60                             (convert Hue to degrees)
 If (H &lt; 0) H = H + 360                 (Hue must be positive)
 If (H &gt;= 360) H = H - 360              (Hue must be less than 360)
</PRE>
If output channels are all 8-bit, then Hue and Saturation values are
scaled between 0 and 255:
<P>
<PRE>
 H = H * (255 / 360)
 S = S * 255
</PRE>
The above algorithm was obtained from page 592 of the following text
book:
<P>
<PRE>
 Foley, J.D., A. van Dam, S.K. Feiner and J.F. Hughes, 1990.
 Computer Graphics: Principles and Practice (second edition).
 Addison-Wesley Publishing Company.
</PRE>
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