Transparent background can't erase the old when using qpainter in paintevent

[ipx]

I use QT4.8 in embeded linux arm platform

the below layer of /dev/fb0 is video,So I want the whole QT program is transparent!

when I update the OSD(developed by QT) every second with different content, I found the new content was stacks with the old.
the old can't be erased auto even by hand if the background or the fill color is transparent!
And hide() and show() doesn't work in the same time,It will show always except not update the content when hide().
but if I set background opaque, it will ok!

How can I make the old content transparecy or erase them.

thanks, I really want your help!!!

below is the code:

Window::Window(): QWidget()
{

setWindowFlags(Qt::FramelessWindowHint);
setAttribute(Qt::WA_TranslucentBackground,true);
setAttribute(Qt::WA_NoSystemBackground);
setFixedSize(100, 100);
textPen = QPen(Qt::red);
textFont.setPixelSize(40);
QTimer *timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(animate()));
timer->start(1000);

}
void Window::animate()
{

strTime = (QTime::currentTime()).toString(tr("hh:mm:ss"));
update();

}
void Window::paintEvent(QPaintEvent *event)
{

QPainter painter;
painter.begin(this);
painter.setBackgroundMode(Qt::TransparentMode);
painter.setRenderHint(QPainter::Antialiasing);
painter.setPen(textPen);
painter.setFont(textFont);
painter.fillRect(event->rect(), QColor(0,0,0,0));
painter.drawText(QRect(0, 0, 100, 100), Qt::AlignCenter, strTime);
painter.end();

}
int main(int argc, char *argv[])
{

QWSServer::setBackground(QBrush(QColor(0,0,0,0)));
QApplication app(argc, argv);
Window window;
window.show();
return app.exec();

}

[ipx]

who can help me? God!

[lanz]

Before calling

painter.fillRect(event->rect(), QColor(0,0,0,0));

painter.fillRect(event->rect(), QColor(0,0,0,0));

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Set painter composition mode to Source. This will disable alpha blending for subsequent draw operations. Don't forget to reset it back to SourceOver though.

painter.setCompositionMode (QPainter::CompositionMode_Source);
painter.fillRect(event->rect(), Qt::transparent);
painter.setCompositionMode (QPainter::CompositionMode_SourceOver);

painter.setCompositionMode (QPainter::CompositionMode_Source);
painter.fillRect(event->rect(), Qt::transparent);
painter.setCompositionMode (QPainter::CompositionMode_SourceOver);

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