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rho_err = abs(line.rho())-img.width()/2
rho指的是：原点到目标直线的距离。
目标直线
也就是过原点做一条目标直线的垂线，这个垂线的长度。
为什么要用这个一个值？因为是和霍夫变换的算法有关的，霍夫变换的算法，就是转换为(rho, ang)的域