For anyone who finds this later because they have the same issue, I have actually found a workaround that fixes the problem. Evidently the back end of the Plasma compositor defaults to OpenGL 2.0, and that is not working anymore. If I set the compositor backend to XRender the transparency works again. The workaround is System Settings => Display and Monitor => Compositor Then, for the Rendering backend: select XRender.

@thehans said in QOpenGLWidget::grabFrameBuffer() always ignores alpha channel?: Unfortunately I'm on Ubuntu 18.04 which is still on 5.9.5. Is there any chance of this fix being backported to earlier versions? Qt 5.9 is now in "very strict" update mode, which means in only receives security fixes. It no longer receives bug fixes. Your options are: Download and use Qt 5.12 or newer from https://www.qt.io/download-open-source, OR Apply the patch to your copy of Qt 5.9 and build it from source. I think the first option is easier.

@SGaist My solution is heavily inspired by: this old ogre wiki post (should have linked it as well, forgot about it until now). I mostly slimmed it down to what I posted as my solution, but I agree if I have time I'll try to make an example/entry. There are some things that need to be updated from the linked post to fit Ogre 2.1 since the post is old.

I think you have to add a QMultiSampleAntiAliasing to either a QRenderStateSet or a QRenderPassFilter. Did you also enable it in the OpenGL format that you set?

@kenchan I knew that. I want not stacking by z-order, I want simultaneous stacking. It works something like that by default (if opacity of items is 0.5 ) output = Merge(Merge(background, 0.5 × item1), 0.5 × item2) I want something like that output = Merge(background, 0.5 × item1, 0.5 × item2) aka simultaneous merge items to scene, so it not depends from z-order. (example — I want ⅓ background, ⅓ item1 and ⅓ item2, Indifferently to items z-order)

@mrjj No, but I will.

I did register but realized I posted with the wrong email address. Now I've corrected it and can see my post on the list. Thank you.

Thanks for sharing !

Hi, It is a limitation. http://doc.qt.io/qt-5/qopenglwidget.html "Limitations Putting other widgets underneath and making the QOpenGLWidget transparent will not lead to the expected results: The widgets underneath will not be visible. This is because in practice the QOpenGLWidget is drawn before all other regular, non-OpenGL widgets, and so see-through type of solutions are not feasible. Other type of layouts, like having widgets on top of the QOpenGLWidget, will function as expected." You have to use Qt::WA_AlwaysStackOnTop to do this.

Win7 & Qt 5.4.2 with QtCreator 3.4.1

@SGaist Thanks. I'll give it a look.

@Juaruipav Yes it is activated. Did you try by deactivating it ? May be it is a bug on Windows 8.1. Better you should report it on Qt Bug Tracker.

@tmason202 , try this one void QTSemiTransparentWidget::paintEvent(QPaintEvent* event) { QPainter painter(this); painter.setOpacity(0.5); QStyleOption opt; opt.initFrom(this); QStyle* pStyle = QApplication::style(); pStyle->drawPrimitive(QStyle::PE_Widget, &opt, &painter, this); } at least you should draw something in paint event not to just set an opacity.

Yes of course