### HSI: Hue Saturation Intensity.

RGB to HSI (Gonzalez and Woods)

I = 1/3(R+G+B)
S = 1 - ( 3/(R+G+B))*a where a is the minimum of R, G and B
H = cos^(-1) ( (0.5*((R-G)+(R-B))) / ((R-G)^2 + (R-B)*(G-B))^(0.5) )
If S = 0, H is meaningless.
If (B/I) > (G/I) then H = 360 - H since H is an angle in degrees we
then normalise to 0,1 with H=H/360

HSI to RGB (Gonzalez and Woods)

First we restore H to degrees with H = 360*H
If 0 < H <= 120 then
B = 1/3(1-S)
R = 1/3(1+ [(S cos H) / (cos(60 - H))] )
G = 1 - (B+R)
If 120 < H <= 240 then
H = H - 120
R = 1/3(1-S)
G = 1/3(1+ [(S cos H) / (cos(60 - H))] )
B = 1 - (R+G)
If 240 < H <= 360 then
H = H - 240
G = 1/3(1-S)
B = 1/3(1+ [(S cos H) / (cos(60 - H))] )
R = 1 - (G+B)