关于中心画圆算法，通过计算x = 0到 y = 0 的1/4椭圆的范围，然后通过对称原理得到其他3/4个点的信息。(R2表示椭圆厂轴的半径） -Center Circle on the algorithm, by calculating the x = 0 to y = 0 1/4 of the scope of an oval, and then adopted the principle of symmetry other 3/4 point information. (R2 that