QWindow on a non-GUI thread

[rtavakko]

Hi guys,

I have a project where there is a subclass of QWindow rendering content on a thread. It runs well other than the assert error I get when rendering.:

Program: ..._0_MSVC2017_64bit-Debug\source\GLRenderer\debug\Qt5Cored.dll
Module: 5.12.0
File: kernel\qcoreapplication.cpp
Line: 578

ASSERT failure in QCoreApplication::sendEvent: "Cannot send events to objects owned by a different thread. Current thread 0x0x28a6e997e10. Receiver '' (of type 'OpenGLRenderWindow') was created in thread 0x0x28a7bc0d190", file kernel\qcoreapplication.cpp, line 578

Header file:

#ifndef OPENGLRENDERWINDOW_H
#define OPENGLRENDERWINDOW_H

#include <QWindow>
#include <QOpenGLContext>

class OpenGLRenderWindow : public QWindow
{
public:
    OpenGLRenderWindow(QScreen* outputScreen,
const QSurfaceFormat& surfaceFormat,
QOpenGLContext* sharedContext);

    virtual ~OpenGLRenderWindow();

    bool event(QEvent *event) override;
    void exposeEvent(QExposeEvent* event) override;

    void resizeEvent(QResizeEvent *event) override;

    const QSurfaceFormat& getOpenGLFormat();
    QOpenGLContext* getOpenGLContext();

protected:
    void swapSurfaceBuffers();

    bool makeContextCurrent();
    void doneContextCurrent();

    QSurfaceFormat openGLFormat;
    QOpenGLContext* openGLContext;

public slots:
    void renderFrame();
};

#endif // OPENGLRENDERWINDOW_H

CPP:

#include "openglrenderwindow.h"

OpenGLRenderWindow::OpenGLRenderWindow(
QScreen *outputScreen,
const QSurfaceFormat &surfaceFormat,
QOpenGLContext *sharedContext) :
    QWindow(outputScreen),
    openGLFormat(surfaceFormat),
    openGLContext(nullptr)
{
    //Create surface
    setSurfaceType(QWindow::OpenGLSurface);
    setFormat(openGLFormat);

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

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

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

OpenGLRenderWindow::~OpenGLRenderWindow()
{
}

bool OpenGLRenderWindow::event(QEvent *event)
{
    switch (event->type())
    {
        case QEvent::UpdateRequest:
        renderFrame();
        return true;
        default:
        return QWindow::event(event);
    }
}

void OpenGLRenderWindow::exposeEvent(QExposeEvent *event)
{
    if(isExposed())
        renderFrame();
}

void OpenGLRenderWindow::resizeEvent(QResizeEvent *event)
{
    if(!makeContextCurrent())
        return;

    unsigned int w = event->size().width();
    unsigned int h = event->size().height();

    QWindow::resize(QSize(w,h));

    swapSurfaceBuffers();
    doneContextCurrent();
}

const QSurfaceFormat &OpenGLRenderWindow::getOpenGLFormat()
{
    return openGLFormat;
}

QOpenGLContext *OpenGLRenderWindow::getOpenGLContext()
{
    return openGLContext;
}

void OpenGLRenderWindow::swapSurfaceBuffers()
{
    openGLContext->swapBuffers(this);
}

bool OpenGLRenderWindow::makeContextCurrent()
{
    //Initialize OpenGL context & other resources
    return openGLContext->makeCurrent(this);
}

void OpenGLRenderWindow::doneContextCurrent()
{
    openGLContext->doneCurrent();
}

void OpenGLRenderWindow::renderFrame()
{
    if(!isExposed())
        return;

    if(!makeContextCurrent())
        return;

    //Rendering code here

    swapSurfaceBuffers();
    doneContextCurrent();
}

displayWindow = new OpenGLRenderWindow(mainOutputScreen,surfaceFormat,nullptr);
displayWindow->show();
displayWindow->moveToThread(thread);

I have this exact same setup for an offscreen renderer subclassing QOffscreenSurface which works without issues (difference being the I call create() in the constructor of that class).

It seems like there is no way around getting resize and expose events for a QWindow, but I don't quite know how to fix this.

Is there something wrong with the order I create and move the resources?
Is there a way to disable events to this window? It renders pretty well despite of the pop-up error.

[SGaist]

Hi,

IIRC, you can move your QOpenGLContext to a different thread but not the QWindow.

[rtavakko]

@SGaist Thanks for replying.

Are you aware of any alternatives to QWindow where using a thread is possible?

I do most of my rendering on a QOffscreenSurface subclass and the QWindow is only displaying the result. But since it sits on the GUI thread, any events there cause it to drop frames and stall.

[SGaist]

Maybe the QQuickRenderControl example might help.

[rtavakko]

@SGaist Thanks! I will play around with that example