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require 'chunky_png'

images = [
  ChunkyPNG::Image.from_file('1.png'),
  ChunkyPNG::Image.from_file('2.png')
]

diff = []

images.first.height.times do |y|
  images.first.row(y).each_with_index do |pixel, x|
    diff << [x,y] unless pixel == images.last[x,y]
  end
end

puts "pixels (total):     #{images.first.pixels.length}"
puts "pixels changed:     #{diff.length}"
puts "pixels changed (%): #{(diff.length.to_f / images.first.pixels.length) * 100}%"

x, y = diff.map{ |xy| xy[0] }, diff.map{ |xy| xy[1] }

images.last.rect(x.min, y.min, x.max, y.max, ChunkyPNG::Color.rgb(0,255,0))
images.last.save('diff.png')