how to remove extra margins arounding a QPixmap?

问题

I draw a QRectF in the paint function of my class and set a QPixmap as brush for it. I build an object from a class containing this QRectF. When I put this item in my scene and set background for the scene the QRectF appears. It also occurs for a QPixmap that I add to the scene. What can I do to remove the extra margins?

void MyQgraphicsObject::paint(QPainter *painter, )
{
    QRectF rec(0,0,50,60);
    QPixmap pi(":picture/im/super.jpg");
    pi=pi.scaled(50,60);
    painter->setBrush(QBrush(pi));
    painter->setPen(Qt::NoPen);
    painter->drawRoundedRect(rec,10,10);
}
////////////////////
QPixmap a(":picture/im/cloud.jpg");
scene->addPixmap(a);
scene->setbackground(Qt::blue);

来源：https://stackoverflow.com/questions/24475305/how-to-remove-extra-margins-arounding-a-qpixmap