require 'chunky_png'
include ChunkyPNG::Color


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

output = ChunkyPNG::Image.new(images.first.width, images.last.width, WHITE)

diff = []

images.first.height.times do |y|
  images.first.row(y).each_with_index do |pixel, x|
    unless pixel == images.last[x,y]
      score = Math.sqrt(
        (r(images.last[x,y]) - r(pixel)) ** 2 +
        (g(images.last[x,y]) - g(pixel)) ** 2 +
        (b(images.last[x,y]) - b(pixel)) ** 2
      ) / Math.sqrt(MAX ** 2 * 3)

      output[x,y] = grayscale(MAX - (score * 255).round)
      diff << score
    end
  end
end

puts "pixels (total):     #{images.first.pixels.length}"
puts "pixels changed:     #{diff.length}"
puts "image changed (%): #{(diff.inject {|sum, value| sum + value} / images.first.pixels.length) * 100}%"

output.save('diff.png')