[Development] [QML] Avoiding graphics flicker in Quick2

Sean Harmer sean.harmer at kdab.com
Fri Aug 5 10:53:53 CEST 2016


Hi,

Can you try a plain OpenGL test application with RGBA8 framebuffer and see if 
that exhibits the same flashing/flickering problem. If so, it's a 
driver/hardware issue.

If you are forced into then using a 16-bit framebuffer, then you can avoid the 
banding by using dithering in your gradients. Either apply the dithering offline 
and store the gradients as images or write a custom material that applies the 
dithering in the fragment shader.

Cheers,

Sean

On Friday 05 August 2016 08:12:59 Denis Shienkov wrote:
> Hi all,
> 
> I have an idea how to avoid flickering, but I don't know about results:
> what if I will discard the Open GL support in favor to Quick2 render?
> 
> I have read this documentation: http://doc.qt.io/QtQuick2DRenderer/
> which says about the "Qt Quick 2D Renderer" module.
> 
> So, my things is:
> 
> 1. Download && build && install  this module:
> 
> git://code.qt.io/qt/qtdeclarative-render2d.git
> 
> 2. Use this env variable:
> 
> export QMLSCENE_DEVICE=softwarecontext
> 
> 3. Run my app...
> 
> 
> But, I  don't understand:
> 
> 1) What of engine is used to 2D rendering in case if I have EGLFS only?
> 
> 2) Should I re-build Qt5 too with the "--no-opengl" option (or I can
> keep the "-opengl es2" option) ?
> 
> 04.08.2016 16:10, Denis Shienkov пишет:
> > Hi Robin,
> > 
> > > Repeating my last answer...
> > > http://lists.qt-project.org/pipermail/development/2016-July/026736.html
> > 
> > I'm sorry, but I did not receive (did not see) your last answer.
> > 
> > > I'm going to guess you're using eglfs or something like that
> > 
> > Yes, I use EGLFS via X11 on Linux Apalis T30 board, where are used
> > NVIDIA's drivers.
> > 
> > Seems, the flickering is when are used:
> > 
> > export QT_QPA_EGLFS_FORCE888=1
> > 
> > ENV variable... (I need this variable to smooth the Image's
> > gradients... even X11 started with 24-bit depth).
> > 
> > PS: But, I'm not sure about QT_QPA_EGLFS_FORCE888... :(
> > 
> > > Using an overlay to debug item positions (QSG_VISUALIZE=overdraw)
> > 
> > could be handy for trying to rule
> > this out, as if it is specific to the application,
> > 
> > Ok, many thanks, I will try it.
> > 
> > 
> > 2016-08-04 14:57 GMT+03:00 Robin Burchell <robin+qt at viroteck.net
> > 
> > <mailto:robin+qt at viroteck.net>>:
> >     Repeating my last answer...
> >     
> >     http://lists.qt-project.org/pipermail/development/2016-July/026736.htm
> >     l
> >     
> >     On Thu, Aug 4, 2016, at 01:50 PM, Denis Shienkov wrote:
> >     > {quote}
> >     > 
> >     > Hi all.
> >     > 
> >     > I have some QML application, where just use QQuickView as QML
> >     
> >     container.
> >     
> >     > This application uses Qt 5.7.x and running on Linux embedded
> >     
> >     board with
> >     
> >     > the
> >     > NVIDIA core (Toradex Apalis T30).
> >     > 
> >     > But sometimes I got flickering in process of animation. A flickering
> >     > represents in form of a "white" strips with full-screen width
> >     
> >     and 1/5-1/6
> >     
> >     > screen heigth. This flickers happens quickly and frequently
> >     
> >     (every ~20-60
> >     
> >     > seconds, randomly)...
> >     > 
> >     > I have seen this "Avoiding graphics flicker in Qt / QML" [1]
> >     
> >     blog where
> >     
> >     > are
> >     > used QML with Qt 4.x,
> >     > where were used this trics:
> >     > 
> >     > {code}
> >     > int main(int argc, char **argv)
> >     > {
> >     > 
> >     >     ...
> >     >  
> >     >  view.setAttribute(Qt::WA_OpaquePaintEvent);
> >     >  view.setAttribute(Qt::WA_NoSystemBackground);
> >     >  view.viewport()->setAttribute(Qt::WA_OpaquePaintEvent);
> >     >  view.viewport()->setAttribute(Qt::WA_NoSystemBackground);
> >     >  
> >     >     ...
> >     > 
> >     > }
> >     > {code}
> >     > 
> >     > But, in my case I have use Quick v2, with QQuickView instead of
> >     > QDeclarativeView,
> >     > which does not allowed to setup any attributes.
> >     > 
> >     > So, my questions is: is there are any way to setup
> >     > Qt::WA_OpaquePaintEvent
> >     > and/or
> >     > Qt::WA_NoSystemBackground in Quck2? Or, maybe is it possible to use
> >     > another
> >     > tricks?
> >     > 
> >     > BR,
> >     > Denis
> >     
> >     [1]https://blog.rburchell.com/2011/11/avoiding-graphics-flicker-in-qt-> >     qml.html>     
> >     > {quote}
> >     > 
> >     > This flickering present on some HDMI displays...
> >     > 
> >     > We currently have this flags:
> >     > 
> >     > {code}
> >     > ...
> >     > view.setFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint);
> >     > ...
> >     > {code}
> >     > 
> >     > Maybe is it an reasons of flikering? Has someone any ideas?
> >     > 
> >     > BR,
> >     > Denis
> >     > _______________________________________________
> >     > Development mailing list
> >     > Development at qt-project.org <mailto:Development at qt-project.org>
> >     > http://lists.qt-project.org/mailman/listinfo/development

-- 
Dr Sean Harmer | sean.harmer at kdab.com | Managing Director UK
KDAB (UK) Ltd, a KDAB Group company
Tel. +44 (0)1625 809908; Sweden (HQ) +46-563-540090
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