Qt painting without clearing the background

Question

I'm using a QPainter to get some graphics on a window. Unfortunately every time the paintEvent() function is called, the whole window is cleared. How can I draw without clearing? I.e. how do I leave the stuff from previous paint event untouched?

I'm using Qt4

Answer 1

You need to set the backgroundMode by using the setBackgroundMode setter. The Qt::NoBackground enum is what you are searching for.

Answer 2

Since both replies are for Qt3 only, here is the solution for Qt4. You need to call

setAttribute(Qt::WA_OpaquePaintEvent);

(for example in the constructor) following the documentation here.

Answer 3

In Qt3, set the Qt::WNoAutoErase flag from the WidgetFlags enum to avoid that the window is cleared on each paint event.

You can set this flag by passing it to the QWidget constructor.