How to refresh QGraphicsItem bounding rectangle after scaling?

clostridium_difficile

In my application I scale the item, but bounding rectangle does not change. How to make bounding rectangle change when scaling an item?

mrjj

Hi
How did you implement it ?

clostridium_difficile

void ScaleTool::Update(QPointF currentPoint)
{
qreal currentDistance = std::sqrt(std::pow(rect().topLeft().x() - currentPoint.x(), 2) + std::pow(rect().topLeft().y() - currentPoint.y(), 2));
layer->setScale(currentDistance / startDistance);

setRect(layer->boundingRect());
setPos(layer->pos());
}

ScaleTool is derrived from QGraphicsRectItem. I expect line setRect(layer->boundingRect()); to set rectangle to fit layer's bounding rectangle, but it does not change (i checked it in qDebug).

mrjj

Hi
Normally you override the virtual function and return the rect

QRectF CircleItem::boundingRect() const
{
    qreal penWidth = 1;
    return QRectF(-radius - penWidth / 2, -radius - penWidth / 2,
                  diameter + penWidth, diameter + penWidth);
}

so does setRect(layer->boundingRect());
set a rect you then return in that boundingRect?

clostridium_difficile

Okay, I forgot to reimplement bounding rectangle. But, how can I "refresh" it once it changes?

mrjj

@clostridium_difficile
well could you not just return QRectF QGraphicsRectItem::rect() const
since you set it in Update ?

QRectF ScaleTool::boundingRect() const
{  
    return rect();
}

However, i would check what the base class (QGraphicsRectItem) actually returns
as it might already return rect and what is missing is a call to
https://doc.qt.io/qt-5/qgraphicsitem.html#prepareGeometryChange

clostridium_difficile

solution to my problem was returning mapped to scene bounding rectangle:

QRectF ImageLayer::boundingRect() const
{
	return mapRectToScene(rect());
}

mrjj

@clostridium_difficile
ah yes good catch. boundingRect is ofc in expected to be in scene coordinates.