# File lib/compass/commands/generate_grid_background.rb, line 72
      def perform
        unless options[:grid_dimensions] =~ /^(\d+)\+(\d+)(?:x(\d+))?$/
          puts "ERROR: '#{options[:grid_dimensions]}' is not valid."
          puts "Dimensions should be specified like: 30+10x20"
          puts "where 30 is the column width, 10 is the gutter width, and 20 is the (optional) height."
          return
        end
        column_width = $1.to_i
        gutter_width = $2.to_i
        height = $3.to_i if $3
        filename = options[:grid_filename] || projectize("#{project_images_subdirectory}/grid.png")
        GridBuilder.new(options.merge(:column_width => column_width, :gutter_width => gutter_width, :height => height, :filename => filename, :working_path => self.working_path)).generate!
      end