QOpenGLWidget with shared context

rtavakko

In a nutshell, my goal is to have an OpenGL surface that has ownership of a group of QWidgets. The OpenGL surface will display a texture generated by a QOffscreenSurface. The child QWidgets act as UI elements that allow the user to change certain areas of this texture (similar to the concept of a mask).

I tried doing this with a QWindow to make the OpenGL side of things a bit easier but there were problems with z order since a QWindow is not a widget and has to be packaged using a window container.

I'm now trying to do this using a QOpenGLWidget. This looks promising; the child QWidgets are correctly shown on top of the background OpenGL texture. The issue is that I cannot share the QOpenGLWidget's context correctly.

This is my current InitializeGL code:

void GLWidget::initializeGL()
{
    //In order to share OpenGL resources, the context has to be recreated
    if(!initialized){
        QOpenGLContext* openGLContext = context();
        if(openGLContext) delete openGLContext;

        //Allocate memory for the context object and prepare to create it
        openGLContext = new QOpenGLContext(this);
        openGLContext->setFormat(openGLFormat);
        if(shareContext)
            openGLContext->setShareContext(shareContext);

        //Make sure the context is created & is sharing resources with the shared context
        bool contextCreated = openGLContext->create();
        assert(contextCreated);

        if(shareContext){
            bool sharing = QOpenGLContext::areSharing(openGLContext,shareContext);
            assert(sharing);
        }

        makeCurrent();
    }

    //Other initialization code if needed
}

This class is a subclass of QOpenGLWidget and QOpenGLFunctions. I pass the OpenGL surface format and shared context to the QOpenGLWidget constructor and recreate its context in the InitializeGL method.

I get a crash when calling QOpenGLFunctions::initializeOpenGLFunctions() which seems to suggest that even though I'm recreating the context, the widget's surface has a format that is compatible with the original context and not the new one.

My stack trace:

1   QOpenGLFunctions_4_0_Compatibility::glVertexAttrib4ubv Qt5Guid                 0x7ffa2732ee2d 
2   QOpenGLFunctions_4_0_Compatibility::glVertexAttrib4ubv Qt5Guid                 0x7ffa2732f129 
3   QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282efbcd 
4   GLWidget::initializeGL                                 glwidget.cpp 135 0x7ff6e1b16286 
5   QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282f1a0a 
6   QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282efef5 
7   QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa28295a31 
8   QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282f01c2 
9   QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa28232c4f 
10  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282302a8 
11  QStateMachinePrivate::processedPendingEvents           Qt5Cored                0x7ffa26982aa0 
12  QStateMachinePrivate::processedPendingEvents           Qt5Cored                0x7ffa269809d2 
13  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a592e 
14  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a5c90 
15  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a5c26 
16  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a41ff 
17  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a5cb1 
18  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a5c26 
19  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a41ff 
20  QTreeWidget::findItems                                 Qt5Widgetsd             0x7ffa282a5cb1 
... <More>

Is what I'm doing completely wrong? Any ideas on how to do this correctly?

Note: I can't wait for the QOpenGLWidget to get created and take its context to share with the other OpenGL surfaces. This is because of the order in which the object are created.

Cheers!

Chris Kawa

If sharing all OpenGL contexts is an option for you then you can set the global Qt::AA_ShareOpenGLContexts application attribute before instantiating QApplication. This way you won't have to do anything else to have all your contexts shared.

rtavakko

@Chris-Kawa That would work for multiple QOpenGLWidgets in the same application. My use case involves sharing the context of one QOpenGLWidget with that of a QOffscreenSurface