QGraphicsView::itemAt() - How to correctly pick QGraphicsItem?

tomas.soltys

How does QGraphicsView::itemAt() works?

I have a class DiagramView derived from QGraphicsView and following method:

void DiagramView::mousePressEvent(QMouseEvent *mouseEvent)
{
    if (QGraphicsItem *item = this->itemAt(mouseEvent->pos()))
    {
        qDebug() << "Hit " << mouseEvent->pos() << mapToScene(mouseEvent->pos()) << item->boundingRect() << item;
    }
    else
    {
        qDebug() << "Miss" << mouseEvent->pos() << mapToScene(mouseEvent->pos());
    }
}

Here is the output from above when I performed 4 mouse clicks going from top to bottom.:

Miss QPoint(672,424) QPointF(259,-2)
Hit  QPoint(672,426) QPointF(259,0)  QRectF(0,0 130x54) QGraphicsItem(0x560853ca75e0, pos=250,0)
Hit  QPoint(672,443) QPointF(259,17) QRectF(0,0 130x54) QGraphicsItem(0x560853ca75e0, pos=250,0)
Miss QPoint(672,451) QPointF(259,25)

Note that my item is a rectangle of size 130x54 and same is its bounding rectangle. Somehow I am not able to score hit in whole rectangle but only in small part of it. Last click QPointF(259,17) was definitely inside the bounding rectangle QRectF(0,0 130x54) with pos=250,0
Same happens in x direction.

What am I missing?

Asperamanca

Seems you are mixing coordinate systems.
boundingRect is in 'item' coordinates. That doesn't tell you the item's position within the scene.
Map the boundingRect to scene coordinates using QGraphicsItem::mapToScene, then you can compare it the the scene coordinates of your mouse position

tomas.soltys

@Asperamanca thanks for your reply. When I map to scene it gets only more obvious that something is oddly wrong.

void DiagramView::mousePressEvent(QMouseEvent *mouseEvent)
{
    if (QGraphicsItem *item = this->itemAt(mouseEvent->pos()))
    {
        qDebug() << "Hit " << mouseEvent->pos() << mapToScene(mouseEvent->pos()) << item->mapToScene(item->boundingRect());
    }
    else
    {
        qDebug() << "Miss" << mouseEvent->pos() << mapToScene(mouseEvent->pos());
    }
}

Output:

Miss QPoint(680,423) QPointF(267,-3)
Hit  QPoint(679,426) QPointF(266,0)  QPolygonF(QPointF(250,0)QPointF(380,0)QPointF(380,54)QPointF(250,54)QPointF(250,0))
Hit  QPoint(679,430) QPointF(266,4)  QPolygonF(QPointF(250,0)QPointF(380,0)QPointF(380,54)QPointF(250,54)QPointF(250,0))
Hit  QPoint(675,450) QPointF(262,24) QPolygonF(QPointF(250,0)QPointF(380,0)QPointF(380,54)QPointF(250,54)QPointF(250,0))
Miss QPoint(675,451) QPointF(262,25)

I actually happens for any item at any position exactly same. As if the view/scene was able to "see" only part of the item, as if it was down-scaled scaled and keeping its origin.

I paint the rectangle in following way:

painter->drawRoundedRect(QRectF(0,0,130,54),5,5);
setPos(250,0);

Asperamanca

How do you set the boundingRect?
Does the item have a shape as well?

itemAt primarily uses shape(), boundingRect() is only a fallback if shape is not set.

tomas.soltys

@Asperamanca

QRectF GraphicsNodeItem::boundingRect() const
{
    return QRectF(0,0,130,54).normalized();
}

QPainterPath GraphicsNodeItem::shape() const
{
    QPainterPath path;
    path.addRect(this->boundingRect());
    return path;
}

Even if I set bounding rectangle to some ridiculous size itemAt() will always return item only if clicked within small rectangle originating in the top left corner of the bounding rectangle.

Asperamanca

@tomas-soltys
That looks weird. What version of Qt are you using?
Can you try the behavior with a stock QGraphicsRectItem, using the setRect function to set your "boundingRect"?

tomas.soltys

@Asperamanca

QGraphicsRectItem works nicely.

QGraphicsRectItem *rectItem = new QGraphicsRectItem;
rectItem->setRect(0, 0, 130, 54);
rectItem->setPos(250,0);

Qt version is 6.3.1 and I am experiencing this behavior on Linux, MacOS and Windows.

Asperamanca

@tomas-soltys said in QGraphicsView::itemAt() - How to correctly pick QGraphicsItem?:

GraphicsNodeItem

Your code for boundingRect and shape is pretty much identical to the code of QGraphicsRectItem (ignoring the extras that are needed when using a pen as border).
Is your GraphicsNodeItem a base class? Could boundingRect() be overloaded and thus behave differently?

tomas.soltys

@Asperamanca

Yes it is a base class.

class GraphicsNodeItem : public QGraphicsItem
{

    public:

        GraphicsNodeItem(QGraphicsItem *parent = nullptr);
        QRectF boundingRect() const override;
        QPainterPath shape() const override;
};

class GraphicsPersonItem : public GraphicsNodeItem
{
    public:

        explicit GraphicsPersonItem(const FPerson &person, GraphicsNodeItem *parent = nullptr);
        void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
};

I have noticed that GraphicsNodeItem::shape() is called only when successful hit, whereas GraphicsNodeItem::boundingRect() is being called constantly. Is this correct behavior?

Asperamanca

@tomas-soltys
I think boundingRect is called first, if that indicates a hit, the shape is checked.

So if you click far outside of a circle, the click is outside of it's boundingRect and shape() is not called.
If you click into the center of the circle, boundingRect indicates a hit, and shape confirms it.
If you click at the corner of the circle, the click may be inside the boundingRect, but outside of the shape.

tomas.soltys

@Asperamanca
I have found working solution. Since size of my item depends on QFontMetrics used by QPainter I am setting final dimensions inside GraphicsPersonItem::paint() method. Once I added this->prepareGeometryChange(); at the beginning my picking started to work correctly.

Asperamanca

If possible, calculate the geometry before paint, e,g. when setting text and font. This will save you an extra paint cycle.