Convert and RGBA color on a specified background to an opaque RGB color.

rgba_to_rgb.rb

#! /usr/bin/env ruby

# Pseudo Code
# - http://stackoverflow.com/questions/2049230/convert-rgba-color-to-rgb
#
# Source => Target = (BGColor + Source) =
# Target.R = ((1 - Source.A) * Source.R) + (Source.A * BGColor.R)
# Target.G = ((1 - Source.A) * Source.G) + (Source.A * BGColor.G)
# Target.B = ((1 - Source.A) * Source.B) + (Source.A * BGColor.B)

# rgba(150,150,150,0.4)
# rgb(63,45,108)

USAGE = <<-USAGE
  rgba_to_rgb "Source.R,Source.G,Source.B" ["BG.R,BG.G,BG.B" || 255,255,255]
USAGE

class Color
  attr_accessor :r, :g, :b, :a

  def initialize(r,g,b,a = nil)
    @r = r.to_f
    @g = g.to_f
    @b = b.to_f
    @a = (a || 1).to_f
  end

  def flatten_alpha(bg_color = nil)
    bg_color ||= Color.new(255,255,255,1)

    target = Color.new(0,0,0)
    target.r = ((1 - a) * r) + (a * bg_color.r)
    target.g = ((1 - a) * g) + (a * bg_color.g)
    target.b = ((1 - a) * b) + (a * bg_color.b)

    target
  end

  def to_s
    if a == 1.0
      "rgb(#{r}, #{g}, #{b})"
    else
      "rgba(#{r}, #{g}, #{b}, #{a})"
    end
  end
end

if ARGV[0].nil?
  puts USAGE; exit(1)
end

if ARGV[1].nil?
  ARGV[1] = "255,255,255"
end

source = Color.new(*ARGV[0].split(','))
bg_color = Color.new(*(ARGV[1].split(',')))

target = source.flatten_alpha(bg_color)

puts " Source: #{source}"
puts "BGColor: #{bg_color}"
puts " Target: #{target}"