[QtEmbedded] Translucent QGraphicsView with animated child widgets

[zuck]

Hi,

I'm working with Qt 4.6 and I've a QGraphicsView with some animated QGraphicsItems (I'm using QML). I want to make the window background transparent, so I can see the desktop under it.

I've tried this piece of code:

MainWindow::MainWindow(QWidget* parent)
    :   QmlView(parent)
{
    this->setWindowFlags(Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_NoSystemBackground);
    this->viewport()->setAutoFillBackground(false);
}

MainWindow::MainWindow(QWidget* parent)
    :   QmlView(parent)
{
    this->setWindowFlags(Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_NoSystemBackground);
    this->viewport()->setAutoFillBackground(false);
}

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It works, but only for static widgets: the animated widget paints a trail on the window surface.

So, how can I erase this trail?
For example, how does KDE work with desktop semi-transparent animated widgets?