Canvas.getContextCanvas.getContext(string contextType)输入contextType string输出RenderingContextCanvas.getContextCanvas.getContext(string contextType)获取画布对象的绘图上下文 输入contextType string上下文类型 值说明2d2d 绘图上下文webglwebgl 绘图上下文 输出RenderingContext绘图上下文