Drawing on Qt ImageViewer example

Question

I'm new to Qt and started developing an app based on one of the sample projects qt provides: "Image Viewer".

Here's the examle project itself: http://doc.qt.io/qt-4.8/qt-widgets-imageviewer-example.html

To make sure I didn't do anything wrong thying to extend project's functionality, I created a new project and simply copied files from example project (see section "Files" at the top of "Image Viewer Example" page)

Then, I edited imageviewer.h file:

added

#include <QPainter>

and

protected:
    void paintEvent(QPaintEvent *);

to ImageViewer class.

Then I defined paintEvent in imageviewe.cpp:

void ImageViewer::paintEvent(QPaintEvent * e)
{
    QPainter p(this);

    p.drawLine(0,0,100,100);
} 

I expected that to draw a line in the top-left corner, but it didn't.

It seems to me, that the point is that scrollArea overlaps the line, making it "invisible". So, I commented out the line

setCentralWidget(scrollArea);

within ImageViewer::ImageViewer() definition.

The line appeared, but the images were not showed, of coruse.

Then I tried drawing on scrollArea itself, changing paintEvent code:

void ImageViewer::paintEvent(QPaintEvent * e)
{
    QPainter p(scrollArea);

    p.drawLine(0,0,100,100);
}

which resulted in messages like

QPainter::begin: Paint device returned engine == 0, type: 1

Tried googling it, but the solutions have nothing to do with scrollArea overlapping the line (or just don't work).

Would appreciate any help on this.