DDA的模拟算法。指定的显示窗口的屏幕划分成64X64的区域；每个区域有一个象素，是位于区域中心半径为r的圆。进行DDA直线段绘制时，算法中选中的象素就用窗口中相应的圆代替。-DDA simulation algorithm. The specified display window of the screen is divided into 64X64 areas　each region has a pixel is located in the Regional Center radius r circle. DDA to draw a straight line segment, the algorithm used to select the pixel on the window, replaced by the corresponding circle.