HI,
note QPixmap::grabWindow
and ScreenShot Example,
u may cheat to have the same effect by grabbing desktop under your window and paint your png on background, that is copy of desktop.
Many applications do so(especially older) -it can be seen when moving a window or changing something in background because some of applications grabs desktop only once.
i also read posts on this forum, that shows setMask works also for mainWindow's (mainly related to rounded corners of communicators).
almost forgot, did you checked autoFillBackground property of your window?
there's also Shaped Clock example in qt manual that does exactly what you need...
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