How to Get Vertical Line from Point and Line

Description

QPoint Line::VerticalPoint(QPoint pt)
{
QPointF ptCross = pt;
double dtY = static_cast<double>(y1() - y2());
double dtX = static_cast<double>(x1() - x2());
double k,b,m;
/**  0 = kx +b -y;  对应垂线方程为 -x -ky + m = 0;(mm为系数) */
if(abs(dtX - 0) < 1e-10){
ptCross = QPointF(x1(),pt.y());
goto _END;
}

if(abs(dtY - 0) < 1e-10){
ptCross = QPointF(pt.x(),y1());
goto _END;
}

k = dtY/dtX;
b = (y1()-k*x1());
m = pt.x() + k*pt.y();

ptCross.setX((m-k*b)/(k*k + 1));
ptCross.setY(k*ptCross.x()+b);

_END:
return ptCross.toPoint();
}