曲线的定义有四个点：起始点、终止点（也称锚点）以及两个相互分离的中间点。滑动两个中间点，贝塞尔曲线的形状会发生变化-There are four points to define the curve: the starting point, end point (also known as the anchor) and an intermediate point between the two separated　each other. Slide the two intermediate points, the shape of the Bezier curve will change