查找直线和QPainterPath之间的交点

Question

我正试图确定命中扫描投射物的路径（基本上是一条线，但在我的示例中我将其表示为QPainterPath）与场景中的项目相交的点。我不确定是否有办法使用QPainterPath、QLineF等提供的功能来找到这一点。下面的代码说明了我试图做的事情：

#include <QtWidgets>
bool hit(const QPainterPath &projectilePath, QGraphicsScene *scene, QPointF &hitPos)
{
    const QList<QGraphicsItem *> itemsInPath = scene->items(projectilePath, Qt::IntersectsItemBoundingRect);
    if (!itemsInPath.isEmpty()) {
        const QPointF projectileStartPos = projectilePath.elementAt(0);
        float shortestDistance = std::numeric_limits<float>::max();
        QGraphicsItem *closest = 0;
        foreach (QGraphicsItem *item, itemsInPath) {
            QPointF distanceAsPoint = item->pos() - projectileStartPos;
            float distance = abs(distanceAsPoint.x() + distanceAsPoint.y());
            if (distance < shortestDistance) {
                shortestDistance = distance;
                closest = item;
            }
        }
        QPainterPath targetShape = closest->mapToScene(closest->shape());
        // hitPos = /* the point at which projectilePath hits targetShape */
        hitPos = closest->pos(); // incorrect; always gives top left
        qDebug() << projectilePath.intersects(targetShape); // true
        qDebug() << projectilePath.intersected(targetShape); // QPainterPath: Element count=0
        // To show that they do actually intersect..
        QPen p1(Qt::green);
        p1.setWidth(2);
        QPen p2(Qt::blue);
        p2.setWidth(2);
        scene->addPath(projectilePath, p1);
        scene->addPath(targetShape, p2);
        return true;
    }
    return false;
}
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGraphicsView view;
    view.setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    QGraphicsScene *scene = new QGraphicsScene;
    view.setScene(scene);
    view.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    QGraphicsItem *target = scene->addRect(0, 0, 25, 25);
    target->setTransformOriginPoint(QPointF(12.5, 12.5));
    target->setRotation(35);
    target->setPos(100, 100);
    QPainterPath projectilePath;
    projectilePath.moveTo(200, 200);
    projectilePath.lineTo(0, 0);
    projectilePath.lineTo(200, 200);
    QPointF hitPos;
    if (hit(projectilePath, scene, hitPos)) {
        scene->addEllipse(hitPos.x() - 2, hitPos.y() - 2, 4, 4, QPen(Qt::red));
    }
    scene->addPath(projectilePath, QPen(Qt::DashLine));
    scene->addText("start")->setPos(180, 150);
    scene->addText("end")->setPos(20, 0);
    view.show();
    return app.exec();
}

projectilePath.intersects(targetShape)返回true，但projectilePath.intersected(targetShape)返回空路径。

有办法做到这一点吗？

Answer 1

正如QPainterPath和线的交点（用x找到QPainterPathy）的答案所指出的，QPainterPath:：intersected（）只适用于具有填充区域的路径。也提到的矩形路径技巧可以这样实现：

#include <QtWidgets>
/*!
    Returns the closest element (position) in a sourcePath to a target,
    using l{QPoint::manhattanLength()} to determine the distances.
*/
QPointF closestPointTo(const QPointF &target, const QPainterPath &sourcePath)
{
    Q_ASSERT(!sourcePath.isEmpty());
    QPointF shortestDistance = sourcePath.elementAt(0) - target;
    qreal shortestLength = shortestDistance.manhattanLength();
    for (int i = 1; i < sourcePath.elementCount(); ++i) {
        const QPointF distance(sourcePath.elementAt(i) - target);
        const qreal length = distance.manhattanLength();
        if (length < shortestLength) {
            shortestDistance = sourcePath.elementAt(i);
            shortestLength = length;
        }
    }
    return shortestDistance;
}
/*!
    Returns c true if a projectilePath intersects with any items in a scene,
    setting a hitPos to the position of the intersection.
*/
bool hit(const QPainterPath &projectilePath, QGraphicsScene *scene, QPointF &hitPos)
{
    const QList<QGraphicsItem *> itemsInPath = scene->items(projectilePath, Qt::IntersectsItemBoundingRect);
    if (!itemsInPath.isEmpty()) {
        const QPointF projectileStartPos = projectilePath.elementAt(0);
        float shortestDistance = std::numeric_limits<float>::max();
        QGraphicsItem *closest = 0;
        foreach (QGraphicsItem *item, itemsInPath) {
            QPointF distanceAsPoint = item->pos() - projectileStartPos;
            float distance = abs(distanceAsPoint.x() + distanceAsPoint.y());
            if (distance < shortestDistance) {
                shortestDistance = distance;
                closest = item;
            }
        }
        QPainterPath targetShape = closest->mapToScene(closest->shape());
        // QLineF has normalVector(), which is useful for extending our path to a rectangle.
        // The path needs to be a rectangle, as QPainterPath::intersected() only accounts
        // for intersections between fill areas, which projectilePath doesn't have.
        QLineF pathAsLine(projectileStartPos, projectilePath.elementAt(1));
        // Extend the first point in the path out by 1 pixel.
        QLineF startEdge = pathAsLine.normalVector();
        startEdge.setLength(1);
        // Swap the points in the line so the normal vector is at the other end of the line.
        pathAsLine.setPoints(pathAsLine.p2(), pathAsLine.p1());
        QLineF endEdge = pathAsLine.normalVector();
        // The end point is currently pointing the wrong way; move it to face the same
        // direction as startEdge.
        endEdge.setLength(-1);
        // Now we can create a rectangle from our edges.
        QPainterPath rectPath(startEdge.p1());
        rectPath.lineTo(startEdge.p2());
        rectPath.lineTo(endEdge.p2());
        rectPath.lineTo(endEdge.p1());
        rectPath.lineTo(startEdge.p1());
        // Visualize the rectangle that we created.
        scene->addPath(rectPath, QPen(QBrush(Qt::blue), 2));
        // Visualize the intersection of the rectangle with the item.
        scene->addPath(targetShape.intersected(rectPath), QPen(QBrush(Qt::cyan), 2));
        // The hit position will be the element (point) of the rectangle that is the
        // closest to where the projectile was fired from.
        hitPos = closestPointTo(projectileStartPos, targetShape.intersected(rectPath));
        return true;
    }
    return false;
}
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGraphicsView view;
    QGraphicsScene *scene = new QGraphicsScene;
    view.setScene(scene);
    view.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    QGraphicsItem *target = scene->addRect(0, 0, 25, 25);
    target->setTransformOriginPoint(QPointF(12.5, 12.5));
    target->setRotation(35);
    target->setPos(100, 100);
    QPainterPath projectilePath;
    projectilePath.moveTo(200, 200);
    projectilePath.lineTo(0, 0);
    projectilePath.lineTo(200, 200);
    QPointF hitPos;
    if (hit(projectilePath, scene, hitPos)) {
        scene->addEllipse(hitPos.x() - 2, hitPos.y() - 2, 4, 4, QPen(Qt::red));
    }
    scene->addPath(projectilePath, QPen(Qt::DashLine));
    scene->addText("start")->setPos(180, 150);
    scene->addText("end")->setPos(20, 0);
    view.show();
    return app.exec();
}

这有很好的精度（±1像素，因为QLineF:：length（）是一个整数），但可能有一种更简洁的方法可以实现同样的效果。

Answer 2

仅供记录（如果有其他人介入）。上面的答案非常好。如果第一个点已经是最接近的点，那么closestPoint函数中可能会出现一个小错误。它应该返回elementAt（0），而不是elementAt（0）-target。

以下是固定功能：

QPointF closestPointTo(const QPointF &target, const QPainterPath &sourcePath)
{
    Q_ASSERT(!sourcePath.isEmpty());
    QPointF shortestDistance;
    qreal shortestLength = std::numeric_limits<int>::max();
    for (int i = 0; i < sourcePath.elementCount(); ++i) {
        const QPointF distance(sourcePath.elementAt(i) - target);
        const qreal length = distance.manhattanLength();
        if (length < shortestLength) {
            shortestDistance = sourcePath.elementAt(i);
            shortestLength = length;
        }
    }
    return shortestDistance;
}