gistfile1.rb

#!/usr/bin/env ruby

require 'rmagick'

include Magick

[".jpg", ".png", ".gif"].each do |image_extension|
  
  `mkdir -p ./left`
  `mkdir -p ./right`
  list = ImageList.new(*Dir["**/*#{image_extension}"])
  
  list.each do |image|
    extension = File.extname(image.filename)
    basename = File.basename(image.filename, extension)
    width,height = image.columns, image.rows
    new_width = width / 2.0
    image.crop(0, 0, new_width, height, true).write("./left/#{basename}_left#{extension}")
    image.crop(new_width, 0, new_width, height, true).write("./right/#{basename}_right#{extension}")
  end
  
end