Ruby version of Darel Rex Finley's HSP (hue, saturation, perceived brightness) color model. Go to http://alienryderflex.com/hsp.html for his original work.

perceived_brightness.rb

class WebColor
  attr_accessor :r, :g, :b

  @@pr, @@pg, @@pb = 0.299, 0.587, 0.114
  @@err = 0.0000001

  def initialize r = 0.0, g = 0.0, b = 0.0
    if r.is_a? Numeric
      @r, @g, @b = r, g, b
    else
      hex = stripHex r
      @r = ('0x'+hex[0..1]).to_i(16)/255.0
      @g = ('0x'+hex[2..3]).to_i(16)/255.0
      @b = ('0x'+hex[4..5]).to_i(16)/255.0
    end
  end

  def stripHex(hex)
    /^#?([0-9a-f]{6});?$/.match(hex.downcase)[1] || "000000"
  end

  def hex
    "#%02x%02x%02x" %
        [((@r+@@err)*255).to_i, ((@g+@@err)*255).to_i, ((@b+@@err)*255).to_i]
  end

  def hue
    if (@r == @g && @r == @b)
      0.0
    elsif (@r >= @g && @r >= @b)
      if (@b >= @g)
        1.0-1.0*(@b-@g)/(@r-@g)/6
      else
        0.0+1.0*(@g-@b)/(@r-@b)/6
      end
    elsif (@g >= @r && @g >= @b)
      if (@r >= @b)
        2.0/6-1.0*(@r-@b)/(@g-@b)/6
      else
        2.0/6+1.0*(@b-@r)/(@g-@r)/6
      end
    else  # B is largest
      if (@g >= @r)
        4.0/6-1.0*(@g-@r)/(@b-@r)/6
      else
        4.0/6+1.0*(@r-@g)/(@g-@g)/6
      end
    end
  end

  def saturation
    if (@r == @g && @r == @b)
      0.0
    elsif (@r >= @g && @r >= @b)
      if (@b >= @g)
        1.0-@g/@r
      else
        1.0-@b/@r
      end
    elsif (@g >= @r && @g >= @b)
      if (@r >= @b)
        1.0-@b/@g
      else
        1.0-@r/@g
      end
    else  # B is largest
      if (@g >= @r)
        1.0-@r/@b
      else
        1.0-@g/@b
      end
    end
  end

  def perceived_brightness
    Math.sqrt(@@pr*@r*@r + @@pg*@g*@g + @@pb*@b*@b)
  end

  def WebColor.fromHSP h, s, p
    sa = 1.0-s
    if sa > 0.0
      if h < 1.0/6
        h = (h-0.0/6)/6
        pa = (1.0-h*(1.0-1.0/sa))
        b = p/Math.sqrt(@@pr/(sa*sa)+@@pg*(pa*pa)+@@pb)
        r = b/sa
        g = b+h*(r-b)
      elsif h < 2.0/6
        h = (2.0/6-h)*6
        pa = (1.0-h*(1.0-1.0/sa))
        b = p/Math.sqrt(@@pg/(sa*sa)+@@pr*(pa*pa)+@@pb)
        g = b/sa
        r = b+h*(g-b)
      elsif h < 3.0/6
        h = (h-2.0/6)*6
        pa = (1.0-h*(1.0-1.0/sa))
        r = p/Math.sqrt(@@pg/(sa*sa)+@@pb*(pa*pa)+@@pr)
        g = r/sa
        b = r+h*(g-r)
      elsif h < 4.0/6
        h = (4.0/6-h)*6
        pa = (1.0-h*(1.0-1.0/sa))
        r = p/Math.sqrt(@@pb/(sa*sa)+@@pg*(pa*pa)+@@pr)
        b = r/sa
        g = r+h*(b-r)
      elsif h < 5.0/6
        h = (h-4.0/6)*6
        pa = (1.0-h*(1.0-1.0/sa))
        g = p/Math.sqrt(@@pb/(sa*sa)+@@pr*(pa*pa)+@@pg)
        b = g/sa
        r = g+h*(b-g)
      else
        h = (6.0/6-h)*6
        pa = (1.0-h*(1.0-1.0/sa))
        g = p/Math.sqrt(@@pr/(sa*sa)+@@pb*(pa*pa)+@@pg)
        r = g/sa
        b = g+h*(r-g)
      end
    else
      if h < 1.0/6
        h = (h-0.0/6)*6
        r = Math.sqrt(p*p/(@@pr+@@pg*(h*h)))
        g = r*h
        b = 0.0
      elsif h < 2.0/6
        h = (2.0/6-h)*6
        g = Math.sqrt(p*p/(@@pg+@@pr*(h*h)))
        r = g*h
        b = 0.0
      elsif h < 3.0/6
        h = (h-2.0/6)*6
        g = Math.sqrt(p*p/(@@pg+@@pb*(h*h)))
        b = g*h
        r = 0.0
      elsif h < 4.0/6
        h = (4.0/6-h)*6
        b = Math.sqrt(p*p/(@@pb+@@pg*(h*h)))
        g = b*h
        r = 0.0
      elsif h < 5.0/6
        h = (h-4.0/6)*6
        b = Math.sqrt(p*p/(@@pb+@@pr*(h*h)))
        r = b*h
        g = 0.0
      else
        h = (6.0/6-h)*6
        r = Math.sqrt(p*p/(@@pr+@@pb*(h*h)))
        b = r*h
        g = 0.0
      end
    end

    r = 1.0 if r > 1.0
    g = 1.0 if g > 1.0
    b = 1.0 if b > 1.0

    WebColor.new r, g, b
  end
end


def test_conversions hex
  color = WebColor.new hex
  puts "------------"
  puts "  incoming hex  = #{hex}"
  puts "  converted rgb = (#{color.r}, #{color.g}, #{color.b})"
  h = color.hue
  s = color.saturation
  p = color.perceived_brightness
  puts "  computed hsp  = (#{h}, #{s}, #{p})"
  color2 = WebColor.fromHSP h, s, p
  puts "  reverse rgb   = (#{color2.r}, #{color2.g}, #{color2.b})"
  hex2 = color2.hex
  puts "  reverse hex   = #{color2.hex}  - #{ hex == hex2 ? "passed" : "failed" }"
end

test_conversions "#000000"
test_conversions "#ff0000"
test_conversions "#00ff00"
test_conversions "#0000ff"
test_conversions "#ffff00"
test_conversions "#00ffff"
test_conversions "#ff00ff"
test_conversions "#ffffff"

Line 46 has a bug. @g-@g should be @b-@g