SHARE - How to calculate how much brighness from color

My client ask me about how to make text black or white color base on background color for that text.

example:

    - background color black, color of text is white
    - background color white, color of text is black
    - background color yellow, color of text is black
    - background color dark-purple, color of text is white

I think I need to calculate how much brighness from background color that used.

After googling I found this formula for calculating brighness.

    brightness  =  sqrt( .241 R2 + .691 G2 + .068 B2 )
   
    R = red color (decimal)
    G = green color (decimal)
    B = blue color (decimal)

example:

   
    background color : EA0516, text color : FFFFFF
    backgorund color : FFFF00, text color : 000000

on my model

    class model
   
      def self.calculate_brightness(color) #format color on hex without #, example FFFFFF
        r = color[0..1].hex
        g = color[2..3].hex
        b = color[4..5].hex
        value = Math.sqrt(r * r * 0.241 + g * g * 0.691 + b * b * 0.068)
        (value < 130 ? 'FFFFFF' : '000000')
      end
   
    end

You can see more information here
http://alienryderflex.com/hsp.html
http://www.nbdtech.com/Blog/archive/2008/04/27/Calculating-the-Perceived-Brightness-of-a-Color.aspx