Takes the difference of two png files

differ.rb

require 'chunky_png'
include ChunkyPNG::Color

images = [ChunkyPNG::Image.from_file('dog1.png'), # Put yr files here
          ChunkyPNG::Image.from_file('dog2.png')]

output = ChunkyPNG::Image.new(images.first.width, images.last.height, rgb(255, 255, 255))

height = images.first.height > images.last.height ? images.last.height : images.first.height
width = images.first.width > images.last.width ? images.last.width : images.first.width
total_pixels = height * width

output = ChunkyPNG::Image.new(width, height, rgb(255, 255, 255))

puts "Analyzing diff for #{height} x #{width} result image with #{total_pixels} total pixels"

different_pixels = 0
height.times do |y|
  width.times do |x|
    rdiff = (r(images.first[x, y]) - r(images.last[x, y])).abs
    gdiff = (g(images.first[x, y]) - g(images.last[x, y])).abs
    bdiff = (b(images.first[x, y]) - b(images.last[x, y])).abs

    if rdiff + gdiff + bdiff > 0
      different_pixels += 1
    end

    output.compose_pixel(x, y, rgb(rdiff, gdiff, bdiff))
  end
end

puts "#{different_pixels} different pixels"
puts "#{ (different_pixels.to_f / total_pixels.to_f) * 100 }% difference"

output.save("diff.png")