how can make the widget transparent

how can make a widget like the http://doc.trolltech.com/qq/qq16-background.html
do someone can help me???

You can try using QWidget::setPalette
window.setPalette(QBrush(Qt::NoBrush));

Hope this helps :)

I think it's easier to do something like:
widget.setStyleSheet("background-color: transparent;");
or for semi-transparency:
widget.setStyleSheet("background-color: rgba(0, 0, 0, 50%);");