Cannot make transparent (translucent) background using Qt styles

I have used the Qt widgets->styles example to test this functionality. The only change i have made is that in void NorwegianWoodStyle::polish(QWidget *widget) method i have added two lines of code:-

if(widget->isWindow())

{ widget->setWindowFlags( Qt::FramelessWindowHint ); widget->setAttribute( Qt::WA_TranslucentBackground ); }

However the output is not what i have expected. its showing output with black background. However when building the same example with Qt 4.8 I am able to see the transparent background. The following screenshots in the attachment section will help you more in understanding the problem.