def initialize_bitmap_representation(width_, height_, color = nil)

      color ||= Color.from_rgb(0,0,0)

      @width = width_
      @height = height_
      @array = color.to_s*@width*@height
    end
def initialize_bitmap_representation(width_, height_, color = nil)

      color ||= Color.from_rgb(0,0,0)

      @width = width_
      @height = height_
      @array = color.to_s*@width*@height
    end
Initialize the representation of bitmap with the given width and height (used internally)