Right.
This is not the first time I made such mistake. So the lifecycle of GraphicsScene is required for graphics view.

Thanks for the help. This is solved.

@alex_malyu

Thank you. In fact that was what I thought too, I was just worried, since I read that processEvents() [...] can cause a lot of unexpected issues if your code is not reentrant.
Here it says that it [...] makes the application react with delays to events. Furthermore the code is difficult to read and analyze, therefore this solution is only suited for short and simple problems that are to be processed in a single thread, such as splash screens and the monitoring of short operations. In response to this, I can say that, graphically speaking, the application works really well indeed, without delays (for example when I press a button to stop the execution).
Here it is said that you can call this function occasionally when your program is busy performing a long operation. But, in the event that you are running a local loop which calls this function continuously, without an event loop, the DeferredDelete events will not be processed. As far as I know, this is not my case, since I do have an event loop (I think) in the main.cpp, in which I have return a.exec();:

int main(int argc, char *argv[]) { QApplication a(argc, argv); MainWindow w(setUpParser(&a)); w.show(); return a.exec(); }

Can nobody help me out ?

@mrjj I read it the same but hope it's not the case.

Hi I dont know about mouse coors
but Just wanted to make sure you saw
Diagram Scene Example
in case you could borrow stuff from that.

Hi,

Things to check:

What GPU do you have on your system ? OS ? Qt version ? GPU drivers ? Are you rendering too much objects on your scene ?

@jcelerier said in Rendering performance problems on iMac with Retina 5K:

https://bugreports.qt.io/browse/QTBUG-61384?focusedCommentId=361794&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-361794
.

That one.

@N.Sumi
I tried to do this inside my QGraphicsItem::paint() but I get instant crash. I tried with the native brackets and without.

painter->beginNativePainting(); QOpenGLFunctions f; f.glClearColor(0.0f, 0.0f, 0.0f, 0.0f); //Black painter->endNativePainting();

I do not understand last question. I do not see any problem with mapping which rely on the item bounding rectangle.

You can have an infinite number of visually the same graphics items if you use different combination of position and bounding box parameters.

For example if you create QGraphicsEllipseItem using following rectangle/position pairs you will get the same result on the screen?
pair 1:
QRectF rect1( 0, 0, 1., 2 );
QPointF pos1(10.,20.);
pair 2:
QRectF rect1( 10,20, 1., 2 );
QPointF pos1(0.,0.);

Which way to define depends on your needs.

@alex_malyu
Thank you very much alex. I just tried what you suggested and it works perfectly.

@SGaist
I thought about slots and timer too, as suggested in the examples on qt's site, but it requires subclassing a QGraphicsItem for each item in the QGraphicsView. The problem is, I don't know the number of items that will be generated until runtime, and apart from this, creating 10.000 classes (one for each item) would be very resource intensive, I think.
Anyway, the method above, the one with
QCoreApplication::processEvents () ;
works perfectly for my purpose.
So thanks both for your politeness: it is very nice to see such a great community :)

scene = new QGraphicsScene(20, 115, 600, 600,this);
scene->setItemIndexMethod(QGraphicsScene::NoIndex);
canvas = new CanvasItem(this);
scene->addItem(canvas);

graphicsView = new QGraphicsView(this); graphicsView->setObjectName(QStringLiteral("graphicsView")); graphicsView->setGeometry(QRect(20, 115, 600, 600)); graphicsView->setMouseTracking(true); graphicsView->setScene(scene); QSizePolicy sizePolicy0 (QSizePolicy::Fixed, QSizePolicy::Fixed); sizePolicy0.setHorizontalStretch(0); sizePolicy0.setVerticalStretch(0); graphicsView->setSizePolicy(sizePolicy0); graphicsView->setFrameStyle(QFrame::Panel | QFrame::Raised); [QGraphicsView with QPixMap Background](https://forum.qt.io/topic/6313/qgraphicsview-with-qpixmap-background/4)

According this post, I use QgraphicsView::setStyleSheet() to set background, It‘s still useless.
but I comment this statement which "scene->addItem(canvas);", It's work.

//class CanvasItem :public QObject, public QGraphicsItem

Object which you create on the stack are destroyed when you leave the scope where you declared them. Holding and using a pointer to them will (at best) achieve nothing, or (at worst) cause a crash.

Create all Object deriving from QObject and QGraphicsItem using new, like so:
QGraphicsScene *gscene = new QGraphicsScene(this);
Consider what the parent should be: The object is destroyed when the parent is destroyed. In this example, it makes sense that the scene is destroyed together with the main window, so using the main window as parent makes sense.
QGraphicsView view = new QGraphicsView(this);
view->setScene(gscene);
Same for items. Except that here the parent will be the scene, or another item.

(I am aware there may be sensible exceptions to the "use new for QObjects and QGraphicsItems" rule - but it's the best way in >90 % of cases, and it's always one way you can do it)

You can try turning on background and item cache - if those items are non interactive then this will speed up.
Another suggestions:

change aliasing Use OpenGL for QGraphicsView. use BSP tree (if not already) group small items and add them as child to "parent item", then on parent item create QImage buffer and draw those items there, on paintEvent for "Parent item" draw only pixmap not actual items.

Thank you. That solved it :D
I need to set a fix scene rect for the view, because it always move the rectangle to the left boarder when i changed the size, but now everything is working :D

QGraphicsView ableiten und resizeEvent() überladen.
Darin dann per setSceneRect() die Scene-Größe setzen und alle deine items manuell layouten.
QGraphicsItem::setPos() um die Position zu setzen.
Ich nehme mal an du verwendest QGraphicsRectItems?