def rx(corner)
      case corner
      when :tl; reg_off
      when :tr; page_width - reg_crop - reg_off
      when :br; page_width - reg_crop - reg_off
      when :bl; reg_off
      end
    end
def rx(corner)
      case corner
      when :tl; reg_off
      when :tr; page_width - reg_crop - reg_off
      when :br; page_width - reg_crop - reg_off
      when :bl; reg_off
      end
    end
iterationless x registration