假定直线斜率k在0~1之间(k的其它取值可以类似处理)，当前象素点为（xp,yp），则下一个象素点有两种可选择点P1-assumed linear slope of the k-1 between 0 (k values can be other similar processing), the current pixel (xp, yp), the next one pixel can have two points P1