图形画椭圆与圆弧中点算法类似：确定一个象素后，接着在两个候选象素的中点计算一个判别式的值，由判别式的符号确定更近的点-To draw a ellipse,similar to the middle point of arc:after determine a pixel,calculate the value of a judge expression,according to its sign,determine the nearer point.