# encoding: UTF-8
require 'RMagick'
require_relative '../diff'

module Simulacrum
  # The RMagicDiff class implements image diffing using ImageMagick
  class RMagicDiff < Simulacrum::Diff
    def delta
      @delta * 100
    end

    private

    def compare
      a = Magick::Image.read(@a_path)
      b = Magick::Image.read(@b_path)
      @image, @delta = square_root_mean_squared(a, b)
    end

    def square_root_mean_squared(a, b)
      # Calculate the Square Root Mean Squared Error for the comparison of the
      # two images.
      #
      # Gets the color difference for each pixel, and square it, average all
      # the squared differences, then return the square root.
      #
      # http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=17284
      a[0].compare_channel(b[0], Magick::MeanSquaredErrorMetric)
    end
  end
end