Qt获取多边形(QGraphicsPolygonItem)或Qt图形组件与直线(QLineF)的交点

有时需要获取直线与各种图形的交点，包括多边形和各种Qt图形框。

例如上图中，要想使连接线始终在多边形的边上，且能指向多边形中心，那么我们就要获取连线AB与多边形的交点。

1.多边形(QGraphicsPolygonItem)与直线(QLineF)的交点

//已知点和多边形
//A、B点可通过多边形的boundingRect().width()/2获取;
QPointF A;
QPointF B;
QGraphicsPolygonItem yellowItem;

QLineF lineAB(A,B);  //AB连线
QPointF arrowPoint; //设置交点
QPolygonF yellowtPolygon = yellowItem->polygon();
QPointF p1 = yellowtPolygon.first() + yellowItem->pos();
//遍历各边连线
for (int i = 1; i < yellowtPolygon.count(); ++i) {
	QPointF p2 = yellowtPolygon.at(i) + yellowItem->pos();
	QLineF polyLine = QLineF(p1, p2);

	//核心：判断是否相交
	QLineF::IntersectType intersectType =
		polyLine.intersect(lineAB, &arrowPoint);
	if (intersectType == QLineF::BoundedIntersection)
		break;
	p1 = p2;
}

//arrowPoint 即为交点

从上面代码可以看到，一个多边形和直线的交点的实现，就是遍历直线与所有边的联系，推而广之，所有Qt的图形化组件，比如QPushButton，QQGraphicsTextItem等，只要有边界的图形化组件都能获取其与直线的交点。即遍历所有边与直线的交点即可。

2.Qt图形组件与直线(QLineF)的交点

QPointF A;
QPointF B;
QLineF lineAB(A,B);  //AB连线
Q普通组件 m_CommentItem;

qreal commentWidth  = m_CommentItem->boundingRect().width();
qreal commentHeight = m_CommentItem->boundingRect().height();

QPointF intersectPoint;
//四个边-四条线
QLineF line1(0,0,commentWidth,0);
QLineF line2(0,0,0,commentHeight);
QLineF line3(commentWidth,0,commentWidth,commentHeight);
QLineF line4(0,commentHeight,commentWidth,commentHeight);
QList<QLineF> lineList;
lineList.append(line1);
lineList.append(line2);
lineList.append(line3);
lineList.append(line4);

//遍历四条线
foreach(QLineF oneline,lineList)
{
	QLineF::IntersectType intersectType =
		oneline.intersect(lineAB, &intersectPoint);
	if (intersectType == QLineF::BoundedIntersection)
		break;
}

//intersectPoint 即为交点

3.可参考例子：

Qt Examples and Demos -> Graphics View ->Diagram Scene