Drawing line to QImage

I am trying to draw line to QImage and show it in Qlabel. However I have some issues that I cannot solve.

    QPixmap px;    px.fromImage (imgRaw);  // define in header file QImage imgRaw;     QPainter p (&px);    p.setPen (Qt::red);    p.drawLine (mouseStart_X, mouseStart_Y, mouseReleased_X, mouseReleased_Y);    p.end ();    ui->lblRightImg->setPixmap (px);    ui->lblRightImg->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);    ui->lblRightImg->setScaledContents(true);

When I used this code above it gives such error :

    QPainter::begin: Paint device returned engine == 0, type: 2    QPainter::setPen: Painter not active    QPainter::end: Painter not active, aborted

Then I change my code because it tries to draw in null pixmap so after changing code like this:

    QPixmap px(100, 100);    px.fromImage (imgRaw);  // define in header file QImage imgRaw;

Then it gives noisy image(black and gray broken image)

Could you please help me to solve this issue ?

EDIT :

Also tried:

QPixmap px = QPixmap::fromImage (imgRaw);

Then it gives same image without any drawing..