html5中getContext()方法和CanvasRenderingContext2D对象详解

http://www.w3school.com.cn/htmldom/met_canvas_getcontext.asp   getContext方法详解

                    

http://www.w3school.com.cn/htmldom/dom_obj_canvasrenderingcontext2d.asp   CanvasRenderingContext2D对象详解