Displaying pixmaps and memory leak ??

[tonytony]

Ok so clearly this is not the way to do things...

I'm struggling to work out how to draw a QImage or QPixmap onto the GUI though. I can find loads of examples but they are not QT creator applications using the QGraphicsView object. I can set the scene etc but how to set it to use my image without using something like addPixmap which doesn't remove the old pixmap?

[MorrisLiang]

Basically, calling QGraphicsScene::addPixmap() will return a QGraphicsPixmapItem. So all you have to do is to hold the QGraphicsPixmapItem pointer when adding a pixmap. When adding another pixmap, delete the previous QGraphicsPixmapItem first.

void MainWindow::display_image()
{
   ...
        lpixmap = QPixmap::fromImage(limage);
        if(myPixmapItem)
                delete myPixmapItem; 
        myPixmapItem= left_cam_view->addPixmap(lpixmap);
}

void MainWindow::display_image()
{
   ...
        lpixmap = QPixmap::fromImage(limage);
        if(myPixmapItem)
                delete myPixmapItem; 
        myPixmapItem= left_cam_view->addPixmap(lpixmap);
}

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the myPixmapItem is a member of MainWindow, a pointer to QGraphicsPixmapItem. Remember to set it to 0 when constructing a MainWindow Object.



Another way is,

void MainWindow::init()
{
     ...
     myPixmapItem = new QGraphicsPixmapItem();
     left_cam_view->addItem(myPixmapItem);
     ...
}
 
void MainWindow::display_image()
{
    ...
    QPixmap lpixmap = QPixmap::fromImage(image);
    myPixmapItem->setPixmap(lpixmap);
    ...
}

void MainWindow::init()
{
     ...
     myPixmapItem = new QGraphicsPixmapItem();
     left_cam_view->addItem(myPixmapItem);
     ...
}

void MainWindow::display_image()
{
    ...
    QPixmap lpixmap = QPixmap::fromImage(image);
    myPixmapItem->setPixmap(lpixmap);
    ...
}

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In this way, you will only have a QGraphicsPixmapItem and no more creating and deleting. This one would be better solution. I'm sure calling QGraphicsPixmapItem::setPixmap() will clear the previous QPixmap.

Some other things are that you're creating a QImage and a QPixmap in the stack every time the display_image() is called. This will cause constructing and destructing them. I think it's better to make them members of the MainWindow. And then you don't have to construct and destruct them. It shouldn't take too much memory, since QPixmap is implicit sharing.

[tonytony]

Ok so before I saw the last post I discovered that clearing the scene fixed the problem, but it looks like the last post gives a more elegant solution. Thanks for you help with this!