[Interest] Attribute Qt::AA_UseOpenGLES must be set before QCoreApplication is created.

[Giuseppe D'Angelo]

Il 18/10/19 11:28, Thomas Sevaldrud ha scritto:

> I doesn't actually appear to have any negative consequences. Everything 
> works as before, but our users are complaining about the warning :)
> 
> The reason that these attributes are set after creating the QApplication 
> is that I have a fallback system where I can try for software rendering 
> if the ANGLE setup fails for some reason (crappy drivers, etc). So if my 
> GLContext with AA_UseOpenGLES fails, I try again with 
> AA_UseSoftwareOpenGL. This also appears to work nicely.

The principle is that once Qt "thinks" you are using Desktop GL, setting 
that attribute may or may not make it switch over to ANGLE, and vice 
versa. In other words there comes a point in time after which setting 
the attribute becomes meaningless (in the specific case: typically after 
the first GL context has been created, but this is undocumented and 
should not be relied upon).

What Qt is warning about is that you're touching a setting that may or 
may not have any effect (depending on the OS, what you did so far in the 
application, which modules of Qt you're using, the day of the week, the 
moon phase), so don't it.

What you could maybe do is to create a Q(Gui)Application, do your tests 
to detect which GL way to use, and if you need to switch to ANGLE or 
software then

1) destroy the QGuiApplication object
2) set all the attributes you need
3) create QGuiApplication again and proceed

(Or, similarly: save some settings and restart the application with the 
new settings)


> I don't actually use the QApplication for anything, I'm only using Qt as 
> a GL wrapper with no QML, Gui, Network or anything. So I tried to simply 
> remove the QGuiApplication, but then it crashes during 
> QOpenGLContext::create(), so I guess it needs to exist after all.

Touching any and I mean any GUI class requires a QGuiApplication object, 
anything else is not supported (and will likely crash).

HTH,

-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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