Sometimes, we may want paint in QWidget without QPainter, for example, using DirectX api. This document will tell you how to reach it.

###0. setAttribute(Qt::WA_PaintOnScreen, true)

Document says that if you want to use native painting, the attribute should be true. Otherwise you will see the fuck flicker.

###1. Let paint engine be 0

Thus QPainter is disabled. Otherwise you will see the fuck flicker again.
This page is helpful: http://lists.trolltech.com/qt4-preview-feedback/2005-04/thread00609-0.html
It seems that let paintEngine() return 0 is from here.

###2. Reimplement paintEvent

Oberviously, you should do something here. You can begin the painting in paintEvent

###3. Reimplement showEvent

This is important if you change the WindowStaysOnTopHint flag. As far as i know, some paint engines use the resource depend on the window. If window changes, resource must be recreated again. For example, render target in direct2d, device context in gdi . If the window flag WindowStaysOnTopHint changes, the widget will hide then show again, maybe that's the reason. If you forget this step, then the widget will never update again after the WindowStaysOnTopHint changes.