gistfile1.rb

#!/usr/bin/env ruby
#coding: binary
require 'zpng'
require 'sugar_png'
require 'pp'

# !!! convert gif file to png first
# will get 8 frames

images = Dir["task*.png"].sort.map{ |fname| ZPNG::Image.load(fname) }

w = images[0].width
h = images[0].height

sugar = SugarPNG.new :width => w

w.times do |x|
  h.times do |y|
    a = images.map{ |img| img[x,y] }
    if a.uniq.size != 1
      sugar[x,y] = :black
    end
  end
end

sugar.save "sugar.png"

system "zsteg sugar.png"