# File lib/ruby-prof/printers/call_stack_printer.rb, line 180
    def copy_image_files
      if @output.is_a?(File)
        target_dir = File.dirname(@output.path)
        image_dir = File.join(File.dirname(__FILE__), '..', 'images')
        %w(empty plus minus).each do |img|
          source_file = "#{image_dir}/#{img}.png"
          target_file = "#{target_dir}/#{img}.png"
          FileUtils.cp(source_file, target_file) unless File.exist?(target_file)
        end
      end
    end