# File gtk/sample/gtk-demo/drawingarea.rb, line 173
    def draw_brush(widget, x, y)
      update_rect = Gdk::Rectangle.new(x - 3, y - 3, 6, 6)

      # Paint to the pixmap, where we store our state
      @pixmap.draw_rectangle(widget.style.black_gc,
                            true,
                            update_rect.x, update_rect.y,
                            update_rect.width, update_rect.height)

      # Now invalidate the affected region of the drawing area.
      widget.window.invalidate(update_rect, false)
    end