Plasmoid opacity

[AcerExtensa]

Hi all!
Do anybody know how to set opacity for plasmoid?
Background transparency like Desktop Folder plasmoid has...
I have tried with QPointer::setOpacity() but that doesn't work.

void TestPlasmoid::paintInterface(QPainter *p, const QStyleOptionGraphicsItem *option, const QRect &contentsRect)
{
 p->setOpacity(0.2);
}

void TestPlasmoid::paintInterface(QPainter *p, const QStyleOptionGraphicsItem *option, const QRect &contentsRect)
{
 p->setOpacity(0.2);
}

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