QOpenGLWidget update and grab in thread

[MfromOlsztyn]

How to do it correctly?

1. Create Thread
2. Create in thread object based on QOpenGLWidget
3. in Thread call grab() and take correct picture

I open example hellogl2 and add this sample code in main.cpp before return app.exec();

    QShortcut *shortcutF4 = new QShortcut(QKeySequence(Qt::Key_F4), &mainWindow, nullptr, nullptr, Qt::ApplicationShortcut);
    QObject::connect(shortcutF4, &QShortcut::activated, &mainWindow, [&]()
    {
        qDebug() << "F4";
        auto *t = new std::thread([]()
        {
             GLWidget glWidget;
             glWidget.resize(400,400);
             glWidget.grab().save("grab_hellogl2_in_thread_01.png");
             // change something
             glWidget.setXRotation(60*3);
             glWidget.grab().save("grab_hellogl2_in_thread_02.png");
             // change something
             glWidget.setYRotation(75*4);
             glWidget.grab().save("grab_hellogl2_in_thread_03.png");

        });
        t->join();
    });

    return app.exec();

But I see warning "Attempting to create QWindow-based QOffscreenSurface outside the gui thread. Expect failures."

How to do it correctly?

[ollarch]

@MfromOlsztyn All the GUI stuff must be done into main thread.

[MfromOlsztyn]

@ollarch
I don't want to show anything in the GUI

I want render off-screen, it is possible in openGL.

[Christian Ehrlicher]

@MfromOlsztyn said in QOpenGLWidget update and grab in thread:

I don't want to show anything in the GUI

But GLWidget is a widget and therefore needs to be created and access in the main thread as corectly told you already by @ollarch .

[wrosecrans]

@MfromOlsztyn You can't touch a widget from a secondary thread. The goal just isn't correct. There is no way to do that correctly. Widgets are always "GUI" even if it isn't shown.

Threaded OpenGL is potentially a complex topic, but you'll need an offscreen surface that isn't a widget.

[MfromOlsztyn]

@wrosecrans I look at https://doc.qt.io/qt-6/qopenglwidget.html#threading

*Performing offscreen rendering on worker threads, 
for example to generate textures that are then used 
in the GUI/main thread in paintGL(), are supported by exposing 
the widget's QOpenGLContext so that additional contexts 
sharing with it can be created on each thread.*

I don't want to touch main from background thread ,
and I don't want to touch background thread from main.
Two independent threads, no interaction

Another way: create second process without GUI and use QOpenGLWidget in invisible APP, is it possible ?

I'm looking for a starting example
A lot of people ask, but I don't see a solution

[Chris Kawa]

@MfromOlsztyn As others said you can't render to a widget in any thread but main. For rendering in another thread use QOffscreenSurface instead of QOpenGLWidget, which creates a windowless surface and is specifically designed for this purpose.