利用中点画圆算法绘制一个圆,中点画圆法是利用圆心在坐标原点圆的方程，将每个像素的中点坐标代入圆的方程得到d,再通过d的值来确定中点与圆的位置。-Utilization of stipple round draw a circle algorithm。Stipple the center circle method is to use in the origin of coordinates the equation of circle, the midpoint of each pixel co