When is QGLWidget's paintGL called?

Question

All I can find is "whenever the widget needs to be painted."

When is that, specifically?

Ironsmith

Answer 1

When you call updateGL() on your widget (or update()), or just Qt decides to redraw your widget. Reasons why Qt might choose to ask for a redraw include:

• your widget gets resized
• your widget is hidden and shown again
• your widget is minimized and then restored
• something else is put in front of your widget and then moved away
• the Moon is in the third quarter
• a distant butterfly had flapped its wings
• ...

In short, you have very little control about when Qt asks for a repaint. Just be sure to paint fast! :-)

Rubefaction

Complimentary info: paintGL() is not even called "instantly" when you call updateGL(). Instead, Qt waits and call it later when appropriate, so that successive calls of updateGL() only result in one call of paintGL(). – Clever

Thanks! I was not aware of the update() and updateGL() functions. Is this all from experience? – Ironsmith

Do you mean, if that's a complete list of when Qt asks for repaints, apart from user-generated ones (i.e. calling update())? – Rubefaction

I'm just wondering where you got your info, really, since I wasn't able to find anything about it online. :) – Ironsmith

Docs and source code. Docs say: Each widget performs all painting operations from within its paintEvent() function. This is called whenever the widget needs to be redrawn, either as a result of some external change or when requested by the application. – Rubefaction

So I guess "some external change" in that quote could be expanded to "whatever external change Qt's implementers have decided requires a redraw". And it tends to be sensible, and the redraw is fast anyway so it doesn't matter that it's called often. And if I want to see the real nitty gritty details of when it repaints, I can check the source code. – Ironsmith