QGraphicsItem scaling produces too large bounding rect - c++

Question

Problem: When using a QGraphicsItem with the flag QGraphicsItem::ItemIgnoresTransformations, the View does not scale properly and shows unnecessary scroll bars.
To reproduce, place a QGraphicsView on a Form and use this code:
#include <QGraphicsScene>
#include <QGraphicsTextItem>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QGraphicsScene* scene = new QGraphicsScene();
// Set the scene to the view. Has to be done before
// transformation in order for the problem to occur.
ui->graphicsView->setScene(scene);
// Add some text, make it transformation-invariant.
QGraphicsTextItem* txt = scene->addText("Hello World!");
txt->setFlag(QGraphicsItem::ItemIgnoresTransformations);
// Scale the scene, re-calculate the bounding rect.
ui->graphicsView->scale(10, 5);
QRectF rect = scene->itemsBoundingRect();
scene->setSceneRect(rect);
}
This also works with other Items like QGraphicsEllipseItem or QGraphicsRectItem.
Without setting the flag (simply comment out the line txt->setFlag...), the output is as expected:
However, when the flag is set, I would expect the scroll bar to disappear, because the text clearly fits into the view. But instead it looks like this:
I know that the Scene does only automatically grow, but not shrink to its content, so I am explicitly setting the Scene Rect at the end. But even this does not help.
It seems to me like this is a bug in Qt, but maybe I also simply misunderstood something. Any idea what the problem (and solution) is?
Using Qt 5.5, Ubuntu 14.04.
PS: Yes, the scene is never freed. This is of course no production code ;-)

Answer 1

I had a similar issue with addLine and addEllipse on top of an QImage.
The problem seems to come from the QGraphicsItem::ItemIgnoresTransformations flag that seems to enlarge the scene to hold the "transformed" object placement, but then ignores the transform for placement and size, however the damage is already done because it has incorrectly changed the scene scale.
I have a work-around that might be able to help you.
If you reset the scene scale to a known correct value after positioning the objects that ignore the transform. It will avoid your problem.
For example:
m_firstImageScene->setSceneRect(m_firstImageRect);