How to use QGraphicsBlurEffect on a widget?

[ffii4455]

Hi

I want to use some effects on my widget.Such as QGraphicsBlurEffect. But the final outcome like this:
QQ截图20120217211221.png

The background color change to black...
The code just like this:

QGraphicsBlurEffect *effect = new QGraphicsBlurEffect;
this->setGraphicsEffect(effect);
//use in constructed function,but if i use  in Slot function,it works without black backgroundcolor...but the effect just effects on the units(buttons,lineedits...) of the widget.The other area is no effect..

QGraphicsBlurEffect *effect = new QGraphicsBlurEffect;
this->setGraphicsEffect(effect);
//use in constructed function,but if i use  in Slot function,it works without black backgroundcolor...but the effect just effects on the units(buttons,lineedits...) of the widget.The other area is no effect..

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I want some help.Thanks~~
Environment: QT 4.7.3 + QtCreator in WindowsXP