# File gglib/tile.rb, line 60
    def centerOn(x, y)
      hwidth=((@x2-@x1)/2).ceil.abs
      hheight=((@y2-@y1)/2).ceil.abs
      @x2=x+hwidth
      @y2=y+hheight
      @y1=y-hheight
      @x1=x-hwidth
    end