widget cannot be hide if the desktop and window background are set to be transparent

I use QT4.6 to design GUI for media player in embeded platform, Becuase I need to play some video, so I set the desktop background to transparent and set the window background to transparent for video display. But the widgets in the window (for exmaple, icon or label, button, etc.) cannot be hiden, and the mouse moving track will be displayed.