Find the intersection between the line and QPainterPath

Question

I am trying to determine the point at which the hitcan removal path (basically a string, but I represented it as QPainterPath in my example) intersects with an element in my scene. I'm not sure if there is a way to find this point using the functions provided by QPainterPath , QLineF , etc. The code below shows what I'm trying to do:

 #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) returns true , but projectilePath.intersected(targetShape) returns an empty path.

Is there any way to achieve this?

Answer 1

As an answer to the intersection point of QPainterPath and the string (find QPainterPath y by x) indicates, QPainterPath :: intersected () only supports paths that have padding areas. The rectangular path trick, which is also mentioned there, can be implemented as follows:

 #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(); } 

This is pretty good accuracy (± 1 pixel, since QLineF :: length () is an integer), but there may be an easier way to achieve the same.

Answer 2

Just for the record (and if someone else is taking steps here). The above answer is excellent. There is only a small error in the closeestPoint function, which can happen if the first point is already the closest. It should return elementAt (0) instead of elementAt (0) - target.

Here is a fixed function:

 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; }