Draw over an image, without replacing the pixels, save drawn image as standalone

[Charvi]

One more feature, sorry to extend in the same question but it's because the context has already been set.

In order to implement scaling on the wheel event I did the following:

void
LT_DrawingCanvas::wheelEvent(QWheelEvent *event)
{
    QWidget::wheelEvent(event);
    scaleFactor += ((float)event->delta()/1200.0);
    update();
}

void
LT_DrawingCanvas::wheelEvent(QWheelEvent *event)
{
    QWidget::wheelEvent(event);
    scaleFactor += ((float)event->delta()/1200.0);
    update();
}

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and in the paintEvent:

void
LT_DrawingCanvas::paintEvent(QPaintEvent *pe)
{
    QPainter p(this);
    p.scale(scaleFactor, scaleFactor);
    p.drawPixmap(0, 0, m_base);
    p.drawImage(0, 0, m_mask);
}

void
LT_DrawingCanvas::paintEvent(QPaintEvent *pe)
{
    QPainter p(this);
    p.scale(scaleFactor, scaleFactor);
    p.drawPixmap(0, 0, m_base);
    p.drawImage(0, 0, m_mask);
}

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My widget hierarchy is as below:
Mainwindow
- Grid Layout
- Scrollarea
- MyCustomDrawing Canvas Widget (created just like the code wysota suggested above)

I am not getting the scrollbars on scaling the image. I realize I am drawing the scaled image again, instead of resizing the widget. I tried using resize on the widget but it didn't work. What I am doing wrong ?