Qt painting without clearing the background

问题

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

回答1:

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

回答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.

回答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.

来源：https://stackoverflow.com/questions/1644514/qt-painting-without-clearing-the-background