Transparent background and rounded edges

Hello guys, how are you?

I'm newbie in the Qt and i'm with a problem. Well, i'm working with Qt embedded and formerly i used DirectFB in my project, with it, i used transparent backgrounds and rounded edges without problems, however, was necessary to stop use DirectFB and to use just FB from Linux kernel.

Well, this part with DirectFB i don't know how work, because is engineers's function. I just know a overview.

As i work with UI, my problem is that after to alter DirectFB to FB, no works more the transparent background together with rounded edges, moreover, works sometimes. But the most strange is that normally my screen shows some strange pixels instead of rounded edges. Or the transparency of the my widget seems doesn't work and the widget seems like a normal rectangle.

Anyway, the big problem is don't make transparent background and rounded edges to work together.

Note: when i change the background to any color, the rounded edges works.
Note 2: the development is to set-top boxes.

Anybody know how to resolve that problem?

Sorry my english, i'm learning...

Thanks.