Hue Shift

Ok. The HSV formula for transferring RGB triplet into the HSV form is widely published. But I've failed to find any hints to how one arrives at it, so I could try to optimize it for my game's blitter routine. Therefore I tried to infer h,s,v formula myself, using basic vector math.

v = (r+g+b)/3 //value
hs = v-r,v-g,v-b //hue*saturation vector
s = hs.length //saturation
h = angle(hs/s) //hue is the angle of the vector

Guess when one needs to just change the saturation, there is no reason compute the angle or even vector length in full, but the common requirement to change the hue requires nasty computation. Can anyone hint at any efficient way of shifting hue?

>>36
Do you still have the presquared values?

function sqraprx(a, b) // ~= sqrt a^2 + b^2

c = max(a,b) + 0.41* min(a,b) / max(a,b)

return c;