Translucent frameless window problem on Kubuntu

[superpacko]

I have the main window transparent and frameless, inside i have a rounded border widget. This works perfectly fine on Windows, but when i run it on Kubuntu the mainwindow is painted with a blured grey. The translucency works only if i remove the Qt::FramelessWindowHint. (Qt 4.74 on Kubuntu 11.04)

source:

MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent, Qt::FramelessWindowHint)
{
    ui.setupUi(this);
 
    this->setAttribute(Qt::WA_TranslucentBackground,true);
 
}

MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent, Qt::FramelessWindowHint)
{
    ui.setupUi(this);

    this->setAttribute(Qt::WA_TranslucentBackground,true);
   
}

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Why could this be happening? I need the main window to be frameless and translucent in order to give my application a rounded look and feel.