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

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.