[Interest] WA_NoSystemBackground vs WA_OpaquePaintEvent

[Philippe]

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 <bo at 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
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