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I'm displaying a map built as rectangle of QGraphicsPixmapitem items (each item stands for one map tile). Because my map is quite large (around 30 MB of PNG files) I want to be able to load pixmaps on demand only when they're visible for user in QGraphicsView and unload when they became invisible.

Is there any way to figure out visible scene rectangle?

Goofy
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    I suppose this is what you're looking for: http://stackoverflow.com/questions/1355446/get-visible-rectangle-of-qgraphicsview. – Luca Carlon Nov 13 '11 at 15:25

2 Answers2

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This gives you the visible scene rectangle:

sceneRect = graphicsView.mapToScene(graphicsView.rect()).boundingRect()

In case there is a shear or rotation transformation present it gives you the bounding rectangle of the visible scene area. If you do not have such transformations (only shift or zoom) the returned rectangle is the exact scene area.

Now for your real problem of efficiently display a huge tiles map in a scene? You could load the tiles in background and first evaluate if your Qt framework isn't already optimized for big pixmap that are outside the visible range. 30 MB also doesn't sound so big that it wouldn't fit into memory.

NoDataDumpNoContribution
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QGraphicsView inherits the QWidget::geometry() function. You can use this to determine its location and size within its parent widget. (Outside of its constructor)

The QGrapicsScene can be larger than the QGraphicsView. The default QGraphicsView will add horizontal and vertical scroll bars to house the QGraphicsScene. I imagine you would like to do something like this:

//create a QGraphicsScene (for this example *scene) that is the size of your entire map.
QGraphicsScene *scene=new QGraphicsScene(0,0,mapWidth,mapHeight);
//create a QGraphicsView* named view that is the size of your visible area
//I'm assuming visibleHeight and visibleWidth do not change (this is your viewing window)
QGraphicsView *view=new QGraphicsView(0,0,visibleWidth,visibleHeight);
view->setScene(scene);

Have the user control the x and y position of the scene that triggers some custom signal like sceneMoved(int,int). Before you redraw the scene, call a slot to check the new position of the scene:

connect(this,SIGNAL(sceneMoved(int,int)),this,SLOT(drawScene(int,int)));

void SomeClass::drawScene(int newX, int newY){
//if you already have a pointer to the scene do this, or call
//QGraphicsView::scene();
    int oldX=scene->geometry()->x();
    int oldY=scene->geometry()->y();
    //now that you have your oldX, oldY, newX, and newY, visibleWidth, visibleHeight
    //you can determine what you need to redraw, what you need to delete, and what can stay
}

There is still a lot of if..else, but you get the point. I suggest trying to segment your map into squares the size of your visible area.

buster
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