qt - 線と QPainterPath の交点を見つける

Question

QPainterPathヒットスキャン発射体のパス (基本的には線ですが、この例では として表しています) がシーン内のアイテムと交差するポイントを特定しようとしています。QPainterPath、などによって提供される関数を使用してこの点を見つける方法があるかどうかはわかりませんQLineF。以下のコードは、私がやろうとしていることを示しています。

#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)を返しますtrueがprojectilePath.intersected(targetShape)、空のパスを返します。

これを達成する方法はありますか？

Answer 1

QPainterPath と線の交点 (QPainterPath y を x で見つける)への回答が指摘するように、QPainterPath::intersected()は塗りつぶし領域を持つパスのみを考慮します。そこでも言及されている長方形のパストリックは、次のように実装できます。

#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();
}

これはかなり良い精度 ( QLineF::length()は整数であるため、± 1 ピクセル) ですが、同じことを実現するためのより適切な方法があるかもしれません。