线段与矩形相交计算（QT方法）

不想加载geos库，故使用qt的方法，计算出矩形与线段的交集。

QLineF clipLine(QLineF line, QRectF box)
{
    QRectF lineBox(line.p1(), line.p2());
    if (!box.intersects(lineBox))
    {
        return line;
    }

    bool b0 = box.contains(line.p1());
    bool b1 = box.contains(line.p2());
    if (b0 && b1)
    {
        return line;
    }
    else
    {
        QLineF line1(box.topLeft(), box.topRight());
        QLineF line2(box.topRight(), box.bottomRight());
        QLineF line3(box.bottomLeft(), box.bottomRight());
        QLineF line4(box.topLeft(), box.bottomLeft());

        QPointF pt1, pt2, pt3, pt4;
        bool bi1 = QLineF::BoundedIntersection == line.intersect(line1, &pt1);
        bool bi2 = QLineF::BoundedIntersection == line.intersect(line2, &pt2);
        bool bi3 = QLineF::BoundedIntersection == line.intersect(line3, &pt3);
        bool bi4 = QLineF::BoundedIntersection == line.intersect(line4, &pt4);

        if (b0)
        {
            if (bi1) return QLineF(line.p1(), pt1);
            if (bi2) return QLineF(line.p1(), pt2);
            if (bi3) return QLineF(line.p1(), pt3);
            if (bi4) return QLineF(line.p1(), pt4);
        }
        else if (b1)
        {
            if (bi1) return QLineF(line.p2(), pt1);
            if (bi2) return QLineF(line.p2(), pt2);
            if (bi3) return QLineF(line.p2(), pt3);
            if (bi4) return QLineF(line.p2(), pt4);
        }
        else
        {
            QPolygonF pts;
            if (bi1) pts.append(pt1);
            if (bi2) pts.append(pt2);
            if (bi3) pts.append(pt3);
            if (bi4) pts.append(pt4);

            return QLineF(pts.first(), pts.last());
        }
    }
}