colors - Fastest formula to get Hue from RGB

Question

If you are given red, green, and blue values that range from 0-255, what would be the fastest computation to get just the hue value? This formula will be used on every pixel of a 640x480 image at 30fps (9.2 million times a second) so every little bit of speed optimization helps.

I've seen other formulas but I'm not happy with how many steps they involve. I'm looking for an actual formula, not a built in library function.

Answer 1

1. Convert the RGB values to the range 0-1, this can be done by dividing the value by 255 for 8-bit color depth (r,g,b - are given values):

   R = r / 255 = 0.09
   G = g / 255 = 0.38
   B = b / 255 = 0.46
   

2. Find the minimum and maximum values of R, G and B.

3. Depending on what RGB color channel is the max value. The three different formulas are:

   • If Red is max, then Hue = (G-B)/(max-min)
   • If Green is max, then Hue = 2.0 + (B-R)/(max-min)
   • If Blue is max, then Hue = 4.0 + (R-G)/(max-min)

The Hue value you get needs to be multiplied by 60 to convert it to degrees on the color circle. If Hue becomes negative you need to add 360 to, because a circle has 360 degrees.

Here is the full article.