# File lib/graster.rb, line 70
  def update_config
    @scale = @config[:dpi].map{|n| 1.0/n }
    @offset = @config[:offset]

    if @image
      2.times {|i| @config[:tile_size][i] ||= @image.size[i]*@scale[i] }
      @tile_interval = 2.times.map {|i|
        @config[:tile_size][i] + @config[:tile_spacing][i]
      }
    end

    @on_range = Range.new Image.f_to_pix(@config[:on_range].first),
                          Image.f_to_pix(@config[:on_range].last)
  end