Barby::Outputter to generate "proper" EAN-13 barcodes, with guard lines and code written below

ean_13.rb

class EAN13Outputter < Barby::RmagickOutputter
    attr_writer :font_size

    def to_image(opts = {})
      with_options opts do
        canvas = Magick::Image.new(full_width, full_height)
        bars = Magick::Draw.new

        x1 = margin + offset_width
        y1 = margin + (offset_height / 2)

        guard_positions = []

        booleans_with_guard.each do |(bar, guard)|
          if bar
            x2 = x1+(xdim-1)
            y2 = y1+(height-1)
            if guard.present?
              y2 += guard_height
              guard_positions << x1
            end
            bars.rectangle(x1, y1, x2, y2)
          end
          x1 += xdim
        end

        bars.draw(canvas)

        draw_data(canvas, guard_positions)

        canvas
      end
    end

    def draw_data(canvas, guard_positions)
      draw_data_part(canvas, barcode.to_s[0, 1], margin, margin * 2.5)
      draw_data_part(canvas, barcode.to_s[1, 6], guard_positions[1], guard_positions[2])
      draw_data_part(canvas, barcode.to_s[7, 6], guard_positions[3], guard_positions[4])
    end

    def draw_data_part(canvas, part, start_x, end_x)
      text_width = text.get_type_metrics(part).width
      available_width = end_x - start_x

      centered_x = (available_width / 2) - (text_width / 2)

      text.annotate(canvas, 0, 0, start_x + centered_x, offset_height, part)
    end

    def booleans_with_guard
      raise "This doesn't look like an EAN-13 barcode" unless booleans.length == 95

      guards = [
        0, 2, # left guard
        46, 48, # center guard
        92, 94 # right guard
      ]

      booleans.map.with_index do |boolean, index|
        [boolean, guards.index(index).try(:+, 1)]
      end
    end

    def text
      @magick_text_draw ||= begin
        result = Magick::Draw.new
        result.font_family = 'arial'
        result.pointsize = @font_size
        result.gravity = Magick::SouthWestGravity
        result
      end
    end

    def font_size
      @font_size || 22
    end

    def character_height
      @character_height ||= text.get_type_metrics("M").height
    end

    def guard_height
      height * 0.15
    end

    def offset_width
      @offset_width ||= text.get_type_metrics("M").width
    end

    def offset_height
      @offset_height ||= (character_height / 2)
    end

    def full_height
      super + guard_height + (offset_height * 2)
    end

    def full_width
      super + (offset_width * 2)
    end
  end