    # File lib/compass/commands/generate_grid_background.rb, line 75
          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
            logger.yellow do
              $stderr.puts "Unless you need to check layouts in legacy browsers, it's preferable"
              $stderr.puts "to use the pure CSS3-based grid background mixin:"
              $stderr.puts
              $stderr.puts "http://compass-style.org/reference/compass/layout/grid_background/"
            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