Relation between QTransform scale and boundingRect.size() - c++

Question

I have little concern about what is relation between QTransform scale and width and height return values in boundingRect() method of QGraphicsItem.
Actually I want to scale QGraphicsItem as its boundingRect size. i.e. if size of my item initially 100,100 that I am passing in boundingRect() method after that I am increasing the size of item by mousemove event. If my increased width and height is 400,300 respectively is my scale factors 4,3?
Any help will be appreciable.
This is code
this->setPos(minMax().first.x(), minMax().first.y());
qreal w = minMax().second.x() - minMax().first.x();
qreal h = minMax().second.y() - minMax().first.y();
qreal scaleFactorW = w / boundingRect().width();
qreal scaleFactorH = h / boundingRect().height();
QTransform trans;
trans.scale(scaleFactorW, scaleFactorH);
setTransform(trans);
bottomPoints = QPointF(w, h);
minMax function is:
float xMin = 0, xMax = 0, yMin = 0, yMax = 0;
QList<double> xValues, yValues;
xValues << shaper[0]->scenePos().x() << shaper[1]->scenePos().x() << shaper[2]->scenePos().x() << shaper[3]->scenePos().x() << shaper[4]->scenePos().x() << shaper[5]->scenePos().x() << shaper[6]->scenePos().x() << shaper[7]->scenePos().x();
yValues << shaper[0]->scenePos().y() << shaper[1]->scenePos().y() << shaper[2]->scenePos().y() << shaper[3]->scenePos().y() << shaper[4]->scenePos().y() << shaper[5]->scenePos().y() << shaper[6]->scenePos().y() << shaper[7]->scenePos().y();
qSort(xValues.begin(), xValues.end());
qSort(yValues.begin(), yValues.end());
xMin = xValues.first();
xMax = xValues.last();
yMin = yValues.first();
yMax = yValues.last();
return qMakePair(QPointF(xMin, yMin), QPointF(xMax, yMax));
shaper is qgraphicsitem by which I am resizing item.
Thanks :)

Answer 1

Thanks for showing your code.
As I said before,
trans.scale(scaleFactorW, scaleFactorH);
won't change the size returned by QGraphicsItem::boundingRect.
But in fact, QGraphicsItem::setScale has the same behaviour and the boundingRect() for the item doesn't change either.
QTransform::scale and QGraphicsItem::setScale are not the same, but both are useful to change the image size. Well, in the case of QTransform, you're scaling the coordinate system.
I think an example is the best way of explaining myself.
(this inherits QGraphicsItem)
qWarning() << "QGraphicsItem::scale(): " << this->scale();
QRectF br = this->boundingRect();
qWarning() << "QGraphicsItem::boundingRect() size / x / y / w / h: " << br.size() << " / "
<< br.x() << " / "
<< br.y() << " / "
<< br.width() << " / "
<< br.height();
QTransform trans = this->transform();
trans.scale(2.0, 2.0);
this->setTransform(trans);
/*
Comment trans.scale(2.0, 2.0) and uncomment the following line
to check the difference using the logs.
*/
// this->setScale(2.0);
qWarning() << "QGraphicsItem::scale(): " << this->scale();
br = this->boundingRect();
qWarning() << "QGraphicsItem::boundingRect() size / x / y / w / h: " << br.size() << " / "
<< br.x() << " / "
<< br.y() << " / "
<< br.width() << " / "
<< br.height();
qWarning() << "boundingRect * item_scale: " << this->boundingRect().size() * this->scale();