说明：  双缓冲技术： 另一种减小帧之间的闪烁的方法是使用双缓冲,它在许多动画applet 中被使用。 主要原理是创建一个后台图象,将一帧画入图象,然后调用drawImage() 将整个图象一次画到屏幕上去。好处是大部分绘制是离屏的。将离屏图象一次 绘至屏幕上比直接在屏幕上绘制要有效得多。 双缓冲可以使动画平滑,但有一个缺点,要分配一张后台图象,如果图象 相当大,这将需要很大一块内存
(double buffering technology : a reduced frame between the flashing method is to use double buffering, in many applet animation are used. The main principle is to create a background image of a painting into images, then called drawImage () to the entire image to a screen painted up. Most benefits are drawn from the screen. Leaving screen image to a painting directly on the screen than on the screen mapping can be much more effective. Double Buffer can make the animation smooth, but there is a drawback to allocate a background image, image considerably, which will require a lot of memory)