The WA_OpaquePaintEvent meaning is pretty clear actually. WA_NoSystemBackground is more subtle. As you said, when setting WA_OpaquePaintEvent, WA_NoSystemBackground does not seem to be useful. But since this is not documented so, I understand you prefer to set it anyway, together with WA_OpaquePaintEvent.
For a top level widget, with some transparency effects, WA_NoSystemBackground makes sense (the Qt doc says it itself: setting WA_TranslucentBackground auto-sets WA_NoSystemBackground). But what I don't see is: is there any valid use of WA_NoSystemBackground alone (without WA_OpaquePaintEvent), for a child widget?... Philippe On Thu, 22 Jan 2015 07:59:54 +0100 Bo Thorsen <b...@vikingsoft.eu> wrote: > Hi Philippe,Den 21-01-2015 kl. 15:32 skrev Philippe: > > The difference between these two modes is not obvious. > > When should we use *WA_NoSystemBackground* instead of *WE_OpaquePaintEvent*, > > and reciprocally? > > When should we use both? > > I could not get any clear hint while searching through the Qt source... > > WA_NoSystemBackground tells QWidget that it should not fill the window > with the standard widget background. If this is not set, it's possible > that the paint of a widget is opaque. Qt tries to figure this out > automatically. > > WA_OpaquePaintEvent says to QWidget that you know for a fact that your > painting of the window will fill up everything. The widget uses this > knowledge to optimize out some of the things it does before calling the > widget paintEvent (filling the background is one). > > WA_NoSystemBackground doesn't say that your painting of the widget will > cover the widget completely. This allows you to do some extra > transparency in your widgets or windows. > > If you have set WA_OpaquePaintEvent you don't really need > WA_NoSystemBackground because the effect is covered. Or that is at least > what it looks like when I look at the current sources. I normally set > both anyway, as there might be some subtle difference in future versions. > > WA_OpaquePaintEvent is all about optimization. For a lot of custom > widgets it's not really necessary to worry about them. Of course, we > don't like to waste cpu cycles, but the painting system in Qt is so fast > that if this is a small widget you are not going to see any difference. > But if your painting of the widget does fill the entire widget, you > should set this. > > WA_NoSystemBackground is not about optimization, it is a control you > have to set whether Qt will paint something on the widget before your > paintEvent is called. > > I hope this helps. > > Bo Thorsen, > Director, Viking Software. > > -- > Viking Software > Qt and C++ developers for hire > http://www.vikingsoft.eu > _______________________________________________ > Interest mailing list > Interest@qt-project.org > http://lists.qt-project.org/mailman/listinfo/interest _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
