QT新手，想问下大佬们为啥我的drawPixmap画不出来，drawLine却可以

代码如下

void MainWindow::paintEvent(QPaintEvent * qPEvent){
    QPalette pal;
    QPixmap backgroundPix;
    QPainter painter(this);
    backgroundPix.fill(QColor(0,0,0));
    painter.drawLine(QPoint(0,0),QPoint(280,280));
    painter.drawPixmap(0,0,100,100,backgroundPix);
}

QPixmap backgroundPix 未指定宽和高：

QPixmap backgroundPix(100,100);

感谢大佬