QGraphicsView and QGraphicsScene speeds

[wysota]

If you could post your code here, it would be easier to solve your problem. The 40000 chips demo loads 40000 items almost instantly and handles them real time in four views, so unless you are using more than millions of items, it shouldn't be sluggish (when run on a decent machine, of course).

[jnk5y]

Ok I have a dialog with a QGraphicsView inside.
In my header i have QGraphicsScene *SampleScene;

SampleScene = new QGraphicsScene;
 
query.exec( "SELECT x,y FROM samples;" );
 
 while( query.next() )
 {
      QGraphicsItem *item = new QGraphicsRectItem ( 
        ( minX - query.value(0).toDouble() ) * -scaleX, ( maxY - query.value(1).toDouble() ) *  scaleY, 5, 5 );
 
      SampleScene->addItem(item);
 }
 
ui.graphicsView->setRenderHints( QPainter::Antialiasing );
ui.graphicsView->setScene(SampleScene);

SampleScene = new QGraphicsScene;

query.exec( "SELECT x,y FROM samples;" );

 while( query.next() )
 {
      QGraphicsItem *item = new QGraphicsRectItem ( 
        ( minX - query.value(0).toDouble() ) * -scaleX, ( maxY - query.value(1).toDouble() ) *  scaleY, 5, 5 );

      SampleScene->addItem(item);
 }

ui.graphicsView->setRenderHints( QPainter::Antialiasing );
ui.graphicsView->setScene(SampleScene);

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I would prefer dots rather than squares and should be easier to draw. I have tried ui.graphicsView->setUpdatesEnabled(false) which helped slightly but still is very slow. I have also tried QGraphicItemsGroup which I have yet to get working for some reason. I would put the items into a QList of QGraphicItems and then

SampleGroup = SampleScene->createItemGroup ( list );
SampleGroup->show();

SampleGroup = SampleScene->createItemGroup ( list );
SampleGroup->show();

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But I could never see the points. I have studied the 40000 chips example and have also implemented a custom QGraphicsItem which helped slightly in speed. Everything else looks similar to the chips example but much slower.