Calculate opacity for hexadecimal colors in Ruby.

hexcolor.rb

#!/usr/bin/env ruby
#
# Calculate opacity for hexadecimal colors.
# By Henrik Nyh <http://henrik.nyh.se> 2009-02-10 under the MIT License.

module Hexcolor
  extend self

  def opacity(fg, opa=1.0, bg='FFFFFF')
    raise "Opacity must be between 0 and 1.0, but was #{opa}" unless (0..1.0).include?(opa)
    fgs = fg.scan(/../)
    bgs = bg.scan(/../)
    fgs.zip(bgs).map do |fg,bg|
      fg = fg.to_i(16)
      bg = bg.to_i(16)
      fg = (1 - opa) * bg + opa * fg
      format('%02X', fg.round)
    end.join
  end
  
end

if __FILE__ == $0
  require 'test/unit'
  class OpacityTest < Test::Unit::TestCase
    def assert_opacity(expected_color, opa, input_color)
      assert_equal(expected_color, Hexcolor.opacity(input_color, opa))
    end
    
    def test_black
      assert_opacity('FFFFFF', 0.0, '000000')
      assert_opacity('808080', 0.5, '000000')
      assert_opacity('000000', 1.0, '000000')
    end
    
    def test_white
      assert_opacity('FFFFFF', 0.0, 'FFFFFF')
      assert_opacity('FFFFFF', 0.5, 'FFFFFF')
      assert_opacity('FFFFFF', 1.0, 'FFFFFF')
    end
    
    def test_red
      assert_opacity('FFFFFF', 0.0, 'FF0000')
      assert_opacity('FF8080', 0.5, 'FF0000')
      assert_opacity('FF0000', 1.0, 'FF0000')
    end
  end
end