可以重新实现paintGL()，在其中通过QPainter绘图。通过update()重绘。
也可以使用通常QWidget的paintEvent()方法实现绘图，通过update()重绘。
下面是官方文档的节选：
Painting Techniques
As described above, subclassQOpenGLWidget to render pure 3D content in the following way:
- Reimplement the initializeGL() andresizeGL() functions to set up the OpenGL state and provide a perspective transformation.
- Reimplement paintGL() to paint the 3D scene, calling only OpenGL functions.
It is also possible to draw 2D graphics onto aQOpenGLWidget subclass usingQPainter:
- In paintGL(), instead of issuing OpenGL commands, construct aQPainter object for use on the widget.
- Draw primitives using QPainter's member functions.
- Direct OpenGL commands can still be issued. However, you must make sure these are enclosed by a call to the painter's beginNativePainting() and endNativePainting().
When performing drawing usingQPainter only, it is also possible to perform the painting like it is done for ordinary widgets: by reimplementingpaintEvent().
- Reimplement the paintEvent() function.
- Construct a QPainter object targeting the widget. Either pass the widget to the constructor or theQPainter::begin() function.
- Draw primitives using QPainter's member functions.
- Painting finishes then the QPainter instance is destroyed. Alternatively, callQPainter::end() explicitly.