Easiest way for offscreen rendering with QOpenGLWidget

Question

I have a hidden QOpenGLWidget (Qt 5.4.2, NOT QGLWidget) and I want to basically continually do grab() or grabFramebuffer() to get its content (and write it to disk). The wid

Answer 1

For offscreen rendering we can use also QOffscreenSurface.

Here is worked class based on Bim's MyGLWindow (not all is tested):

Header OpenGlOffscreenSurface.h:

#ifndef OPENGLOFFSCREENSURFACE_H
#define OPENGLOFFSCREENSURFACE_H

#pragma once

#include <QtCore/QObject>
#include <QtGui/QScreen>
#include <QtGui/QOffscreenSurface>
#include <QtGui/QPaintEvent>
#include <QtGui/QResizeEvent>
#include <QtGui/QOpenGLPaintDevice>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QOpenGLFunctions_3_0>
#include <QtGui/QOpenGLFramebufferObject>
#include <QtGui/QSurfaceFormat>
#include <QtWidgets/QWidget>
#include <QOpenGLShaderProgram>

#include <atomic>
#include <mutex>

class OpenGlOffscreenSurface
    : public QOffscreenSurface
{
    Q_OBJECT

public:
    /// @brief Constructor. Creates a render window.
    /// @param targetScreen Target screen.
    /// @param size Initial size of a surface buffer.
    /// this is because before the FBO and off-screen surface haven't been created.
    /// By default this uses the QWindow::requestedFormat() for OpenGL context and off-screen
    /// surface.
    explicit OpenGlOffscreenSurface(
            QScreen* targetScreen = nullptr,
            const QSize& size = QSize (1, 1));

    /// @brief Destructor.
    virtual ~OpenGlOffscreenSurface();

    /// @brief Check if the window is initialized and can be used for rendering.
    /// @return Returns true if context, surface and FBO have been set up to start rendering.
    bool isValid() const;

    /// @brief Return the context used in this window.
    /// @return The context used in this window or nullptr if it hasn't been created yet.
    QOpenGLContext* context() const;

    /// @brief Return the OpenGL function object that can be used the issue OpenGL commands.
    /// @return The functions for the context or nullptr if it the context hasn't been created yet.
    QOpenGLFunctions* functions() const;

    /// @brief Return the OpenGL off-screen frame buffer object identifier.
    /// @return The OpenGL off-screen frame buffer object identifier or 0 if no FBO has been created
    /// yet.
    /// @note This changes on every resize!
    GLuint framebufferObjectHandle() const;

    /// @brief Return the OpenGL off-screen frame buffer object.
    /// @return The OpenGL off-screen frame buffer object or nullptr if no FBO has been created yet.
    /// @note This changes on every resize!
    const QOpenGLFramebufferObject* getFramebufferObject() const;

    /// @brief Return the QPaintDevice for paint into it.
    QPaintDevice* getPaintDevice() const;

    /// @brief Return the OpenGL off-screen frame buffer object identifier.
    /// @return The OpenGL off-screen frame buffer object identifier or 0 if no FBO has been created
    /// yet.
    void bindFramebufferObject();

    /// @brief Return the current contents of the FBO.
    /// @return FBO content as 32bit QImage. You might need to swap RGBA to BGRA or vice-versa.
    QImage grabFramebuffer();

    /// @brief Makes the OpenGL context current for rendering.
    /// @note Make sure to bindFramebufferObject() if you want to render to this widgets FBO.
    void makeCurrent();

    /// @brief Release the OpenGL context.
    void doneCurrent();

    /// @brief Copy content of framebuffer to back buffer and swap buffers if the surface is
    /// double-buffered.
    /// If the surface is not double-buffered, the frame buffer content is blitted to the front
    /// buffer.
    /// If the window is not exposed, only the OpenGL pipeline is glFlush()ed so the framebuffer can
    /// be read back.
    void swapBuffers();

    /// @brief Use bufferSize() instead size() for get a size of a surface buffer. We can't override size() as it is not virtual.
    QSize bufferSize() const;

    /// @brief Resize surface buffer to newSize.
    void resize(const QSize& newSize);

    /// @brief Resize surface buffer to size with width w and height h.
    /// @param w Width.
    /// @param h Height.
    void resize(int w, int h);

public slots:
    /// @brief Lazy update routine like QWidget::update().
    void update();

    /// @brief Immediately render the widget contents to framebuffer.
    void render();

signals:
    /// @brief Emitted when swapBuffers() was called and bufferswapping is done.
    void frameSwapped();

    /// @brief Emitted after a resizeEvent().
    void resized();

protected:
    virtual void exposeEvent(QExposeEvent* e);
    virtual void resizeEvent(QResizeEvent* e);
    virtual bool event(QEvent* e) override;

//    virtual int metric(QPaintDevice::PaintDeviceMetric metric) const override;

    /// @brief Called exactly once when the window is first exposed OR render() is called when the
    /// widget is invisible.
    /// @note After this the off-screen surface and FBO are available.
    virtual void initializeGL() = 0;

    /// @brief Called whenever the window size changes.
    /// @param width New window width.
    /// @param height New window height.
    virtual void resizeGL(
            int width,
            int height) = 0;

    /// @brief Called whenever the window needs to repaint itself. Override to draw OpenGL content.
    /// When this function is called, the context is already current and the correct framebuffer is
    /// bound.
    virtual void paintGL() = 0;

    //      /// @brief Called whenever the window needs to repaint itself. Override to draw QPainter
    // content.
    //      /// @brief This is called AFTER paintGL()! Only needed when painting using a QPainter.
    //      virtual void paintEvent(QPainter & painter) = 0;

private:
    Q_DISABLE_COPY(OpenGlOffscreenSurface)
    /// @brief Initialize the window.
    void initializeInternal();

    /// @brief Internal method that does the actual swap work, NOT using a mutex.
    void swapBuffersInternal();

    /// @brief Internal method that checks state and makes the context current, NOT using a mutex.
    void makeCurrentInternal();

    /// @brief Internal method to grab content of a specific framebuffer.
    QImage grabFramebufferInternal(QOpenGLFramebufferObject* fbo);

    /// @brief (Re-)allocate FBO and paint device if needed due to size changes etc.
    void recreateFBOAndPaintDevice();

    /// @brief False before the window was first exposed OR render() was called.
    std::atomic_bool m_initialized;
    /// @brief False before the overridden initializeGL() was first called.
    bool m_initializedGL = false;
    /// @brief True when currently a window update is pending.
    std::atomic_bool m_updatePending;
    /// @brief Mutex making sure not grabbing while drawing etc.
    std::mutex m_mutex;

    /// @brief OpenGL render context.
    QOpenGLContext* m_context = nullptr;
    /// @brief The OpenGL 2.1 / ES 2.0 function object that can be used the issue OpenGL commands.
    QOpenGLFunctions* m_functions = nullptr;
    /// @brief The OpenGL 3.0 function object that can be used the issue OpenGL commands.
    QOpenGLFunctions_3_0* m_functions_3_0 = nullptr;
    /// @brief OpenGL paint device for painting with a QPainter.
    QOpenGLPaintDevice* m_paintDevice = nullptr;
    /// @brief Background FBO for off-screen rendering when the window is not exposed.
    QOpenGLFramebufferObject* m_fbo = nullptr;
    /// @brief Background FBO resolving a multi sampling frame buffer in m_fbo to a frame buffer
    /// that can be grabbed to a QImage.
    QOpenGLFramebufferObject* m_resolvedFbo = nullptr;

    /// @brief Shader used for blitting m_fbo to screen if glBlitFrameBuffer is not available.
    QOpenGLShaderProgram* m_blitShader;

    QSize m_size;
};

#endif  // OPENGLOFFSCREENSURFACE_H

Source OpenGlOffscreenSurface.cpp:

#include "OpenGlOffscreenSurface.h"

#include <QtCore/QCoreApplication>
#include <QtGui/QPainter>

OpenGlOffscreenSurface::OpenGlOffscreenSurface(
        QScreen*     targetScreen,
        const QSize& size)
    : QOffscreenSurface(targetScreen)
    , m_size(size)
{
    setFormat(QSurfaceFormat::defaultFormat());
    m_initialized = false;
    m_updatePending = false;
    create();  // Some platforms require this function to be called on the main (GUI) thread
    initializeInternal();
}


OpenGlOffscreenSurface::~OpenGlOffscreenSurface()
{
    // to delete the FBOs we first need to make the context current
    m_context->makeCurrent(this);
    // destroy framebuffer objects
    if (m_fbo) {
        m_fbo->release();
        delete m_fbo;
        m_fbo = nullptr;
    }
    if (m_resolvedFbo) {
        m_resolvedFbo->release();
        delete m_resolvedFbo;
        m_resolvedFbo = nullptr;
    }
    // destroy shader
    delete m_blitShader;
    m_blitShader = nullptr;
    // free context
    m_context->doneCurrent();
    delete m_context;
    m_context = nullptr;
    // free paint device
    delete m_paintDevice;
    m_paintDevice = nullptr;
    m_initialized = false;
    m_updatePending = false;
    destroy();
}


QOpenGLContext* OpenGlOffscreenSurface::context() const
{
    return (m_context);
}


QOpenGLFunctions* OpenGlOffscreenSurface::functions() const
{
    return (m_functions);
}


GLuint OpenGlOffscreenSurface::framebufferObjectHandle() const
{
    return (m_fbo ? m_fbo->handle() : 0);
}


const QOpenGLFramebufferObject* OpenGlOffscreenSurface::getFramebufferObject() const
{
    return (m_fbo);
}


QPaintDevice* OpenGlOffscreenSurface::getPaintDevice() const
{
    return (m_paintDevice);
}


void OpenGlOffscreenSurface::bindFramebufferObject()
{
    if (m_fbo) {
        m_fbo->bind();
    } else {
        QOpenGLFramebufferObject::bindDefault();
    }
}


bool OpenGlOffscreenSurface::isValid() const
{
    return (m_initialized && m_context && m_fbo);
}


void OpenGlOffscreenSurface::makeCurrent()
{
    makeCurrentInternal();
}


void OpenGlOffscreenSurface::makeCurrentInternal()
{
    if (isValid()) {
        m_context->makeCurrent(this);
    } else {
        throw ("OpenGlOffscreenSurface::makeCurrent() - Window not yet properly initialized!");
    }
}


void OpenGlOffscreenSurface::doneCurrent()
{
    if (m_context) {
        m_context->doneCurrent();
    }
}


QImage OpenGlOffscreenSurface::grabFramebuffer()
{
    std::lock_guard <std::mutex> locker(m_mutex);
    makeCurrentInternal();
    // blit framebuffer to resolve framebuffer first if needed
    if (m_fbo->format().samples() > 0) {
        // check if we have glFrameBufferBlit support. this is true for desktop OpenGL 3.0+, but not
        // OpenGL ES 2.0
        if (m_functions_3_0) {
            // only blit the color buffer attachment
            m_functions_3_0->glBindFramebuffer(GL_READ_FRAMEBUFFER, m_fbo->handle());
            m_functions_3_0->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, m_resolvedFbo->handle());
            m_functions_3_0->glBlitFramebuffer(0, 0, bufferSize().width(),
                    bufferSize().height(), 0, 0, bufferSize().width(),
                    bufferSize().height(), GL_COLOR_BUFFER_BIT, GL_NEAREST);
            m_functions_3_0->glBindFramebuffer(GL_FRAMEBUFFER, 0);
        } else {
            // we must unbind the FBO here, so we can use its texture and bind the default
            // back-buffer
            m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_resolvedFbo->handle());
            // now use its texture for drawing in the shader
            // --> bind shader and draw textured quad here
            // bind regular FBO again
            m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
        }
        // check if OpenGL errors happened
        if (GLenum error = m_functions->glGetError() != GL_NO_ERROR) {
            qDebug() << "OpenGlOffscreenSurface::grabFramebuffer() - OpenGL error" << error;
        }

        // now grab from resolve FBO
        return (grabFramebufferInternal(m_resolvedFbo));
    } else {
        // no multi-sampling. grab directly from FBO
        return (grabFramebufferInternal(m_fbo));
    }
}  // OpenGlOffscreenSurface::grabFramebuffer


QImage OpenGlOffscreenSurface::grabFramebufferInternal(QOpenGLFramebufferObject* fbo)
{
    QImage image;
    // bind framebuffer first
    m_functions->glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo->handle());
    if (m_functions_3_0) {
        m_functions_3_0->glReadBuffer(GL_COLOR_ATTACHMENT0);
    }
    GLenum internalFormat = fbo->format().internalTextureFormat();
    bool hasAlpha = internalFormat == GL_RGBA || internalFormat == GL_BGRA
                    || internalFormat == GL_RGBA8;
    if (internalFormat == GL_BGRA) {
        image = QImage(fbo->size(), hasAlpha ? QImage::Format_ARGB32 : QImage::Format_RGB32);
        m_functions->glReadPixels(0, 0, fbo->size().width(),
                fbo->size().height(), GL_BGRA, GL_UNSIGNED_BYTE, image.bits());
    } else if ((internalFormat == GL_RGBA) || (internalFormat == GL_RGBA8)) {
        image = QImage(fbo->size(), hasAlpha ? QImage::Format_RGBA8888 : QImage::Format_RGBX8888);
        m_functions->glReadPixels(0, 0, fbo->size().width(),
                fbo->size().height(), GL_RGBA, GL_UNSIGNED_BYTE, image.bits());
    } else {
        qDebug() << "OpenGlOffscreenSurface::grabFramebuffer() - Unsupported framebuffer format"
                 << internalFormat << "!";
    }
    m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());

    return (image.mirrored());
}  // OpenGlOffscreenSurface::grabFramebufferInternal


void OpenGlOffscreenSurface::swapBuffers()
{
    swapBuffersInternal();
    emit frameSwapped();
}


void OpenGlOffscreenSurface::swapBuffersInternal()
{
    // blit framebuffer to back buffer
    m_context->makeCurrent(this);
    // make sure all paint operation have been processed
    m_functions->glFlush();
    // check if we have glFrameBufferBlit support. this is true for desktop OpenGL 3.0+, but not
    // OpenGL ES 2.0
    if (m_functions_3_0) {
        // if our framebuffer has multi-sampling, the resolve should be done automagically
        m_functions_3_0->glBindFramebuffer(GL_READ_FRAMEBUFFER, m_fbo->handle());
        m_functions_3_0->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
        // blit all buffers including depth buffer for further rendering
        m_functions_3_0->glBlitFramebuffer(0, 0, bufferSize().width(),
                bufferSize().height(), 0, 0, bufferSize().width(),
                bufferSize().height(), GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT,
                GL_NEAREST);
        m_functions_3_0->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
    } else {
        // we must unbind the FBO here, so we can use its texture and bind the default back-buffer
        m_functions->glBindFramebuffer(GL_FRAMEBUFFER, 0);
        // now use its texture for drawing in the shader
        // --> bind shader and draw textured quad here
        // bind regular FBO again
        m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
    }
    // check if OpenGL errors happened
    if (GLenum error = m_functions->glGetError() != GL_NO_ERROR) {
        qDebug() << "OpenGlOffscreenSurface::swapBuffersInternal() - OpenGL error" << error;
    }
    // now swap back buffer to front buffer
    m_context->swapBuffers(this);
}  // OpenGlOffscreenSurface::swapBuffersInternal


void OpenGlOffscreenSurface::recreateFBOAndPaintDevice()
{
    if (m_context && ((m_fbo == nullptr) || (m_fbo->size() != bufferSize()))) {
        m_context->makeCurrent(this);
        // free old FBOs
        if (m_fbo) {
            m_fbo->release();
            delete m_fbo;
            m_fbo = nullptr;
        }
        if (m_resolvedFbo) {
            m_resolvedFbo->release();
            delete m_resolvedFbo;
            m_resolvedFbo = nullptr;
        }
        // create new frame buffer
//        QOpenGLFramebufferObjectFormat format = QGLInfo::DefaultFramebufferFormat();
//        format.setSamples(QGLInfo::HasMultisamplingSupport(m_context) ? 4 : 0);
        QOpenGLFramebufferObjectFormat format;
        format.setSamples(0);

        m_fbo = new QOpenGLFramebufferObject(bufferSize(), format);
        if (!m_fbo->isValid()) {
            throw ("OpenGlOffscreenSurface::recreateFbo() - Failed to create background FBO!");
        }
        // clear framebuffer
        m_fbo->bind();
        m_functions->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
        m_fbo->release();
        // if multi sampling is requested and supported we need a resolve FBO
        if (format.samples() > 0) {
            // create resolve framebuffer with only a color attachment
            format.setAttachment(QOpenGLFramebufferObject::NoAttachment);
            format.setSamples(0);
            m_resolvedFbo = new QOpenGLFramebufferObject(bufferSize(), format);
            if (!m_resolvedFbo->isValid()) {
                throw ("OpenGlOffscreenSurface::recreateFbo() - Failed to create resolve FBO!");
            }
            // clear resolve framebuffer
            m_resolvedFbo->bind();
            m_functions->glClear(GL_COLOR_BUFFER_BIT);
            m_resolvedFbo->release();
        }
    }
    // create paint device for painting with QPainter if needed
    if (!m_paintDevice) {
        m_paintDevice = new QOpenGLPaintDevice;
    }
    // update paint device size if needed
    if (m_paintDevice->size() != bufferSize()) {
        m_paintDevice->setSize(bufferSize());
    }
}  // OpenGlOffscreenSurface::recreateFBOAndPaintDevice


void OpenGlOffscreenSurface::initializeInternal()
{
    if (!m_initialized.exchange(true)) {
        // create OpenGL context. we set the format requested by the user (default:
        // QWindow::requestedFormat())
        m_context = new QOpenGLContext(this);
        m_context->setFormat(format());
        if (m_context->create()) {
            m_context->makeCurrent(this);
            // initialize the OpenGL 2.1 / ES 2.0 functions for this object
            m_functions = m_context->functions();
            m_functions->initializeOpenGLFunctions();
            // try initializing the OpenGL 3.0 functions for this object
            m_functions_3_0 = m_context->versionFunctions <QOpenGLFunctions_3_0>();
            if (m_functions_3_0) {
                m_functions_3_0->initializeOpenGLFunctions();
            } else {
                // if we do not have OpenGL 3.0 functions, glBlitFrameBuffer is not available, so we
                // must do the blit
                // using a shader and the framebuffer texture, so we need to create the shader
                // here...
                // --> allocate m_blitShader, a simple shader for drawing a textured quad
                // --> build quad geometry, VBO, whatever
            }
            // now we have a context, create the FBO
            recreateFBOAndPaintDevice();
        } else {
            m_initialized = false;
            delete m_context;
            m_context = nullptr;
            throw ("Failed to create OpenGL context!");
        }
    }
}  // OpenGlOffscreenSurface::initializeInternal


void OpenGlOffscreenSurface::update()
{
    // only queue an update if there's not already an update pending
    if (!m_updatePending.exchange(true)) {
        QCoreApplication::postEvent(this, new QEvent(QEvent::UpdateRequest));
    }
}


void OpenGlOffscreenSurface::render()
{
    std::lock_guard <std::mutex> locker(m_mutex);
    // check if we need to initialize stuff
    initializeInternal();
    // check if we need to call the user initialization
//    makeCurrent(); // TODO: may be makeCurrent() must be here, as noted for QOpenGLWidget.initializeGL()
    if (!m_initializedGL) {
        m_initializedGL = true;
        initializeGL();
    }
    // make context current and bind framebuffer
    makeCurrent();
    bindFramebufferObject();
    // call user paint function
    paintGL();
    doneCurrent();
    // mark that we're done with updating
    m_updatePending = false;
}  // OpenGlOffscreenSurface::render


void OpenGlOffscreenSurface::exposeEvent(QExposeEvent* e)
{
    // render window content if window is exposed
    render();
}  // OpenGlOffscreenSurface::exposeEvent


void OpenGlOffscreenSurface::resizeEvent(QResizeEvent* e)
{
    // call base implementation
    resize(e->size());
    emit resized();
}


void OpenGlOffscreenSurface::resize(const QSize& newSize)
{
    m_mutex.lock();
    // make context current first
    makeCurrent();

    m_size = QSize(newSize);

    // update FBO and paint device
    recreateFBOAndPaintDevice();
    m_mutex.unlock();
    // call user-defined resize method
    resizeGL(bufferSize().width(), bufferSize().height());
}  // OpenGlOffscreenSurface::resize


void OpenGlOffscreenSurface::resize(
        int w,
        int h)
{
    resize(QSize(w, h));
}


bool OpenGlOffscreenSurface::event(QEvent* event)
{
    switch (event->type()) {
        case QEvent::UpdateLater:
            update();
            return (true);

        case QEvent::UpdateRequest:
            render();
            return (true);

        default:
            return (false);
    }  // switch
}  // OpenGlOffscreenSurface::event


QSize OpenGlOffscreenSurface::bufferSize() const
{
    return (m_size);
}

Using OpenGlOffscreenSurface:

Header ExamplePaintSurface.h:

#ifndef EXAMPLEPAINTSURFACE_H
#define EXAMPLEPAINTSURFACE_H

#include "OpenGlOffscreenSurface.h"

class ExamplePaintSurface
    : public OpenGlOffscreenSurface
{
public:
    explicit ExamplePaintSurface(
            QScreen* targetScreen = nullptr,
            const QSize& size = QSize (1, 1));

    virtual ~ExamplePaintSurface() override;

protected:
    virtual void initializeGL() override;

    virtual void resizeGL(
            int width,
            int height) override;

    virtual void paintGL() override;
};


#endif  // EXAMPLEPAINTSURFACE_H

Source ExamplePaintSurface.cpp:

#include "ExamplePaintSurface.h"

#include <QPainter>

ExamplePaintSurface::ExamplePaintSurface(
        QScreen*     targetScreen,
        const QSize& size)
    : OpenGlOffscreenSurface(targetScreen, size) {}


ExamplePaintSurface::~ExamplePaintSurface() {}


void ExamplePaintSurface::initializeGL() {}


void ExamplePaintSurface::resizeGL(int width, int height) {}


void ExamplePaintSurface::paintGL()
{
    QPainter painter(getPaintDevice());
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);

    painter.drawText(20, 40, "Test"); // <-- drawing here

    painter.end();
}

Source main.cpp:

#include <QApplication>

#include "ExamplePaintSurface.h"


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    ExamplePaintSurface paintSurface;
    paintSurface.resize(300, 200);
    paintSurface.render();
    QImage image = paintSurface.grabFramebuffer();
    image.save(QString("image.png"));

    return a.exec();
}

Part example with QOffscreenSurface is from here:

#include <QGuiApplication>
#include <QSurfaceFormat>
#include <QOpenGlContext>
#include <QOffscreenSurface>
#include <QOpenGLFunctions_4_3_Core>
#include <QDebug>

int main(int argc, char* argv[])
{
   QGuiApplication a(argc, argv);

   QSurfaceFormat surfaceFormat;
   surfaceFormat.setMajorVersion(4);
   surfaceFormat.setMinorVersion(3);

   QOpenGLContext openGLContext;
   openGLContext.setFormat(surfaceFormat);
   openGLContext.create();
   if(!openGLContext.isValid()) return -1;

   QOffscreenSurface surface;
   surface.setFormat(surfaceFormat);
   surface.create();
   if(!surface.isValid()) return -2;

   openGLContext.makeCurrent(&surface);

   QOpenGLFunctions_4_3_Core f;
   if(!f.initializeOpenGLFunctions()) return -3;

   qDebug() << QString::fromLatin1((const char*)f.glGetString(GL_VERSION));

   return 0;
}

And here is an aswer about it:

For offscreen rendering, try rendering into a QOpenGLFramebufferObject, which can be converted into a QImage, which in turn can easily be saved to disk.

For that however, you still need a surface to render onto (as required by QOpenGLContext::makeCurrent()), so your only choice is indeed using a QWindow or a QGLWidget to get such a surface.

In Qt 5.1, there will be a new class called QOffscreenSurface, which will probably be most suitable for your usecase. You would use QOffscreenSurface to get a surface for your OpenGL context, and then render into a FBO using QOpenGLFramebufferObject, and then call QOpenGLFramebufferObject::toImage() to get access to the pixels.

Answer 2

UPDATE 2: The corresponding bug in QOpenGLWidget seems to be fixed in Qt 5.10, so I suggest to simply use the class again. Although you might want to wait for this bug to also get fixed...
UPDATE 1: Added 3, best solution using a custom QWindow-derived class

1 - QOpenGLWidget
If a hidden QOpenGLWidget does allocate a framebuffer (not sure if this happens), there is still no way to bind it manually, because you can not get the buffer id. Additionally none of the necessary functions initializeGL(), resizeGL() and paintGL are called and none of the functions grab(), grabFramebuffer and render() are working correctly. Here is (imo) a workaround to draw the widget offscreen. You call paintGL directly after setting up all the necessary stuff:

class GLWidget: public QOpenGLWidget
{
public:
    GLWidget(QWidget * parent = nullptr);
private:
    bool m_isInitialized = false;
    QOpenGLFramebufferObject m_fbo = nullptr;
};

void GLWidget::drawOffscreen()
{
        //the context should be valid. make sure it is current for painting
    makeCurrent();
    if (!m_isInitialized)
    {
        initializeGL();
        resizeGL(width(), height());
    }
    if (!m_fbo || m_fbo->width() != width() || m_fbo->height() != height())
    {
        //allocate additional? FBO for rendering or resize it if widget size changed
        delete m_fbo;
        QOpenGLFramebufferObjectFormat format;
        format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
        m_fbo = new QOpenGLFramebufferObject(width(), height(), format);
        resizeGL(width(), height());
    }

    //#1 DOES NOT WORK: bind FBO and render() widget
    m_fbo->bind();
    QOpenGLPaintDevice fboPaintDev(width(), height());
    QPainter painter(&fboPaintDev);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
    render(&painter);
    painter.end();
    //You could now grab the content of the framebuffer we've rendered to
    QImage image1 = m_fbo->toImage();
    image1.save(QString("fb1.png"));
    m_fbo->release();
    //#1 --------------------------------------------------------------

    //#2 WORKS: bind FBO and render stuff with paintGL() call
    m_fbo->bind();
    paintGL();
    //You could now grab the content of the framebuffer we've rendered to
    QImage image2 = m_fbo->toImage();
    image2.save(QString("fb2.png"));
    m_fbo->release();
    //#2 --------------------------------------------------------------

    //bind default framebuffer again. not sure if this necessary
    //and isn't supposed to use defaultFramebuffer()...
    m_fbo->bindDefault();
    doneCurrent();
}

void GLWidget::paintGL()
{
    //When doing mixed QPainter/OpenGL rendering make sure to use a QOpenGLPaintDevice, otherwise only OpenGL content is visible!
    //I'm not sure why, because according to the docs (http://doc.qt.io/qt-5/topics-graphics.html) this is supposed to be the same...
    QOpenGLPaintDevice fboPaintDev(width(), height());
    QPainter painter(&fboPaintDev);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
    //This is what you'd use (and what would work) if the widget was visible
    //QPainter painter;
    //painter.begin(this);

    //now start OpenGL painting
    painter.beginNativePainting();
    glClearColor(0.5f, 0.0f, 0.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    ...
    painter.endNativePainting();
    //draw non-OpenGL stuff with QPainter
    painter.drawText(20, 40, "Foo");
    ...
    painter.end();
}

2 - QGraphicsView with QOpenGLWidget viewport
Here render() works as expected when you provide it with an QOpenGLPaintDevice:

MainWindow::MainWindow()
{
    scene = new QGraphicsScene;
    hiddenView = new QGraphicsView(scene);
    hiddenGLWidget = new QOpenGLWidget;
    hiddenView->setViewport(hiddenGLWidget);
    //hiddenView->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    //hiddenView->show();
}

void MainWindow::screenshot()
{
    //try regular grab functions
    QPixmap pixmap1 = hiddenView->grab(); //image with scrollbars, no OpenGL content
    pixmap1.save("bla1.png");
    QPixmap pixmap2 = hiddenGLWidget->grab(); //produces an empty image
    pixmap2.save("bla2.png");
    //try grabbing only the QOpenGLWidget framebuffer
    QImage image1 = hiddenGLWidget->grabFramebuffer(); //null image
    image1.save("bla3.png");

    //WORKS: render via FBO
    hiddenGLWidget->makeCurrent();
    QOpenGLFramebufferObjectFormat format;
    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    QOpenGLFramebufferObject * fbo = new QOpenGLFramebufferObject(hiddenView->width(), hiddenView->height(), format);
    fbo->bind();
    QOpenGLPaintDevice fboPaintDev(hiddenView->width(), hiddenView->height());
    QPainter painter(&fboPaintDev);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
    hiddenView->render(&painter); //WORKS and captures mixed OpenGL and non-OpenGL QGraphicsitems
    //hiddenView->repaint(); //does not work
    //hiddenView->scene()->render(&painter); //does not work
    //hiddenGLWidget->paintGL(); //might work. can not call, protected
    //hiddenGLWidget->render(&painter); //does not work
    //hiddenGLWidget->repaint(); //does not work
    painter.end();
    QImage image2 = fbo->toImage();
    image2.save("bla4.png");
    fbo->release();
    delete fbo;
}

3 - How to render to and grab an image from a hidden QOpenGLWidget
A better overall solution is to use a custom QWindow with a QSurface::OpenGLSurface type. Create an extra QOpenGLContext, an extra background QOpenGLFramebufferObject you will draw to, and a QOpenGLShaderProgram to blit the framebuffer to the backbuffer. If you want multisampling, you might need a resolve QOpenGLFramebufferObject too, to convert the multisampled framebuffer to a non-multisampled one. The class interface can be similar to QOpenGLWidget (virtual initializeGL(), resizeGL(), paintGL() for users). Reimplement exposeEvent(), resizeEvent() and event() (you might need to reimplement metric() too).
A semi-complete implementation:

Header:

#pragma once

#include <QtCore/QObject>
#include <QtGui/QScreen>
#include <QtGui/QWindow>
#include <QtGui/QPaintEvent>
#include <QtGui/QResizeEvent>
#include <QtGui/QOpenGLPaintDevice>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QOpenGLFunctions_3_0>
#include <QtGui/QOpenGLFramebufferObject>
#include <QtGui/QSurfaceFormat>
#include <QtWidgets/QWidget>

#include <atomic>
#include <mutex>

class MyGLWindow : public QWindow
{
    Q_OBJECT

public:
    /// @brief Constructor. Creates a render window.
    /// @param targetScreen Target screen.
    /// this is because before the FBO and off-screen surface haven't been created.
    /// By default this uses the QWindow::requestedFormat() for OpenGL context and off-screen surface.
    explicit MyGLWindow(QScreen * targetScreen = nullptr);

    /// @brief Constructor. Creates a render window.
    /// @param parent Parent window.
    /// this is because before the FBO and off-screen surface haven't been created.
    /// By default this uses the QWindow::requestedFormat() for OpenGL context and off-screen surface.
    explicit MyGLWindow(QWindow * parent);

    /// @brief Destructor.
    virtual ~MyGLWindow();

    /// @brief Create a container widget for this window.
    /// @param parent Parent widget.
    /// @return Returns a container widget for the window.
    QWidget * createWidget(QWidget * parent = nullptr);

    /// @brief Check if the window is initialized and can be used for rendering.
    /// @return Returns true if context, surface and FBO have been set up to start rendering.
    bool isValid() const;

    /// @brief Return the context used in this window.
    /// @return The context used in this window or nullptr if it hasn't been created yet.
    QOpenGLContext * context() const;

    /// @brief Return the OpenGL function object that can be used the issue OpenGL commands.
    /// @return The functions for the context or nullptr if it the context hasn't been created yet.
    QOpenGLFunctions * functions() const;

    /// @brief Return the OpenGL off-screen frame buffer object identifier.
    /// @return The OpenGL off-screen frame buffer object identifier or 0 if no FBO has been created yet.
    /// @note This changes on every resize!
    GLuint framebufferObjectHandle() const;

    /// @brief Return the OpenGL off-screen frame buffer object.
    /// @return The OpenGL off-screen frame buffer object or nullptr if no FBO has been created yet.
    /// @note This changes on every resize!
    const QOpenGLFramebufferObject * getFramebufferObject() const;

    /// @brief Return the OpenGL off-screen frame buffer object identifier.
    /// @return The OpenGL off-screen frame buffer object identifier or 0 if no FBO has been created yet.
    void bindFramebufferObject();

    /// @brief Return the current contents of the FBO.
    /// @return FBO content as 32bit QImage. You might need to swap RGBA to BGRA or vice-versa.
    QImage grabFramebuffer();

    /// @brief Makes the OpenGL context current for rendering.
    /// @note Make sure to bindFramebufferObject() if you want to render to this widgets FBO.
    void makeCurrent();

    /// @brief Release the OpenGL context.
    void doneCurrent();

    /// @brief Copy content of framebuffer to back buffer and swap buffers if the surface is double-buffered.
    /// If the surface is not double-buffered, the frame buffer content is blitted to the front buffer.
    /// If the window is not exposed, only the OpenGL pipeline is glFlush()ed so the framebuffer can be read back.
    void swapBuffers();

    public slots:
    /// @brief Lazy update routine like QWidget::update().
    void update();
    /// @brief Immediately render the widget contents to framebuffer.
    void render();

signals:
    /// @brief Emitted when swapBuffers() was called and bufferswapping is done.
    void frameSwapped();
    /// @brief Emitted after a resizeEvent().
    void resized();

protected:
    virtual void exposeEvent(QExposeEvent *e) override;
    virtual void resizeEvent(QResizeEvent *e) override;
    virtual bool event(QEvent *e) override;
    //      virtual int metric(QPaintDevice::PaintDeviceMetric metric) const override;

    /// @brief Called exactly once when the window is first exposed OR render() is called when the widget is invisible.
    /// @note After this the off-screen surface and FBO are available.
    virtual void initializeGL() = 0;
    /// @brief Called whenever the window size changes.
    /// @param width New window width.
    /// @param height New window height.
    virtual void resizeGL(int width, int height) = 0;
    /// @brief Called whenever the window needs to repaint itself. Override to draw OpenGL content.
    /// When this function is called, the context is already current and the correct framebuffer is bound.
    virtual void paintGL() = 0;
    //      /// @brief Called whenever the window needs to repaint itself. Override to draw QPainter content.
    //      /// @brief This is called AFTER paintGL()! Only needed when painting using a QPainter.
    //      virtual void paintEvent(QPainter & painter) = 0;

private:
    Q_DISABLE_COPY(QGLWindow)

    /// @brief Initialize the window.
    void initializeInternal();
    /// @brief Internal method that does the actual swap work, NOT using a mutex.
    void swapBuffersInternal();
    /// @brief Internal method that checks state and makes the context current, NOT using a mutex.
    void makeCurrentInternal();
    /// @brief Internal method to grab content of a specific framebuffer.
    QImage grabFramebufferInternal(QOpenGLFramebufferObject * fbo);

    /// @brief (Re-)allocate FBO and paint device if needed due to size changes etc.
    void recreateFBOAndPaintDevice();

    /// @brief False before the window was first exposed OR render() was called.
    std::atomic_bool m_initialized;
    /// @brief False before the overridden initializeGL() was first called.
    bool m_initializedGL = false;
    /// @brief True when currently a window update is pending.
    std::atomic_bool m_updatePending;
    /// @brief Mutex making sure not grabbing while drawing etc.
    std::mutex m_mutex;

    /// @brief OpenGL render context.
    QOpenGLContext * m_context = nullptr;
    /// @brief The OpenGL 2.1 / ES 2.0 function object that can be used the issue OpenGL commands.
    QOpenGLFunctions * m_functions = nullptr;
    /// @brief The OpenGL 3.0 function object that can be used the issue OpenGL commands.
    QOpenGLFunctions_3_0 * m_functions_3_0 = nullptr;
    /// @brief OpenGL paint device for painting with a QPainter.
    QOpenGLPaintDevice * m_paintDevice = nullptr;
    /// @brief Background FBO for off-screen rendering when the window is not exposed.
    QOpenGLFramebufferObject * m_fbo = nullptr;
    /// @brief Background FBO resolving a multi sampling frame buffer in m_fbo to a frame buffer
    /// that can be grabbed to a QImage.
    QOpenGLFramebufferObject * m_resolvedFbo = nullptr;

    /// @brief Shader used for blitting m_fbo to screen if glBlitFrameBuffer is not available.
    QOpenGLShaderProgram * m_blitShader;
};

Source:

#include "MyGLWindow.h"

#include <QtCore/QCoreApplication>
#include <QtGui/QPainter>

MyGLWindow::MyGLWindow(QScreen * targetScreen)
    : QWindow(targetScreen)
{
    //Set Qt::Widget flag to make sure the window resizes properly the first time
    //when used as a widget via MyGLWindow::createWidget()!
    setFlags(Qt::Widget);
    setSurfaceType(QSurface::OpenGLSurface);
    setFormat(QGLInfo::DefaultSurfaceFormat());
    m_initialized = false;
    m_updatePending = false;
    create();
    initializeInternal();
}

MyGLWindow::MyGLWindow(QWindow * parent)
    : QWindow(parent)
{
    //Set Qt::Widget flag to make sure the window resizes properly the first time
    //when used as a widget via MyGLWindow::createWidget()!
    setFlags(Qt::Widget);
    setSurfaceType(QSurface::OpenGLSurface);
    setFormat(QGLInfo::DefaultSurfaceFormat());
    m_initialized = false;
    m_updatePending = false;
    create();
    initializeInternal();
}

MyGLWindow::~MyGLWindow()
{
    //to delete the FBOs we first need to make the context current
    m_context->makeCurrent(this);
    //destroy framebuffer objects
    if (m_fbo)
    {
        m_fbo->release();
        delete m_fbo;
        m_fbo = nullptr;
    }
    if (m_resolvedFbo)
    {
        m_resolvedFbo->release();
        delete m_resolvedFbo;
        m_resolvedFbo = nullptr;
    }
    //destroy shader
    delete m_blitShader;
    m_blitShader = nullptr;
    //free context
    m_context->doneCurrent();
    delete m_context;
    m_context = nullptr;
    //free paint device
    delete m_paintDevice;
    m_paintDevice = nullptr;
    m_initialized = false;
    m_updatePending = false;
}

QWidget * MyGLWindow::createWidget(QWidget * parent)
{
    QWidget * container = QWidget::createWindowContainer(this, parent);
    return container;
}

QOpenGLContext * MyGLWindow::context() const
{
    return m_context;
}

QOpenGLFunctions * MyGLWindow::functions() const
{
    return m_functions;
}

GLuint MyGLWindow::framebufferObjectHandle() const
{
    return m_fbo ? m_fbo->handle() : 0;
}

const QOpenGLFramebufferObject * MyGLWindow::getFramebufferObject() const
{
    return m_fbo;
}

void MyGLWindow::bindFramebufferObject()
{
    if (m_fbo)
    {
        m_fbo->bind();
    }
    else
    {
        QOpenGLFramebufferObject::bindDefault();
    }
}

bool MyGLWindow::isValid() const
{
    return (m_initialized && m_context && m_fbo);
}

void MyGLWindow::makeCurrent()
{
    makeCurrentInternal();
}

void MyGLWindow::makeCurrentInternal()
{
    if (isValid())
    {
        m_context->makeCurrent(this);
    }
    else
    {
        throw("MyGLWindow::makeCurrent() - Window not yet properly initialized!");
    }
}

void MyGLWindow::doneCurrent()
{
    if (m_context)
    {
        m_context->doneCurrent();
    }
}

QImage MyGLWindow::grabFramebuffer()
{
    std::lock_guard<std::mutex> locker(m_mutex);
    makeCurrentInternal();
    //blit framebuffer to resolve framebuffer first if needed
    if (m_fbo->format().samples() > 0)
    {
        //check if we have glFrameBufferBlit support. this is true for desktop OpenGL 3.0+, but not OpenGL ES 2.0
        if (m_functions_3_0)
        {
            //only blit the color buffer attachment
            m_functions_3_0->glBindFramebuffer(GL_READ_FRAMEBUFFER, m_fbo->handle());
            m_functions_3_0->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, m_resolvedFbo->handle());
            m_functions_3_0->glBlitFramebuffer(0, 0, width(), height(), 0, 0, width(), height(),
                                                          GL_COLOR_BUFFER_BIT,
                                                          GL_NEAREST);
            m_functions_3_0->glBindFramebuffer(GL_FRAMEBUFFER, 0);
        }
        else
        {
            //we must unbind the FBO here, so we can use its texture and bind the default back-buffer
            m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_resolvedFbo->handle());
            //now use its texture for drawing in the shader
            --> bind shader and draw textured quad here
            //bind regular FBO again
            m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
        }
        //check if OpenGL errors happened
        if (GLenum error = m_functions->glGetError() != GL_NO_ERROR)
        {
            qDebug() << "MyGLWindow::grabFramebuffer() - OpenGL error" << error;
        }
        //now grab from resolve FBO
        return grabFramebufferInternal(m_resolvedFbo);
    }
    else
    {
        //no multi-sampling. grab directly from FBO
        return grabFramebufferInternal(m_fbo);
    }
}

QImage MyGLWindow::grabFramebufferInternal(QOpenGLFramebufferObject * fbo)
{
    QImage image;
    //bind framebuffer first
    m_functions->glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo->handle());
    if (m_functions_3_0)
    {
        m_functions_3_0->glReadBuffer(GL_COLOR_ATTACHMENT0);
    }
    GLenum internalFormat = fbo->format().internalTextureFormat();
    bool hasAlpha = internalFormat == GL_RGBA || internalFormat == GL_BGRA || internalFormat == GL_RGBA8;
    if (internalFormat == GL_BGRA)
    {
        image = QImage(fbo->size(), hasAlpha ? QImage::Format_ARGB32 : QImage::Format_RGB32);
        m_functions->glReadPixels(0, 0, fbo->size().width(), fbo->size().height(), GL_BGRA, GL_UNSIGNED_BYTE, image.bits());
    }
    else if (internalFormat == GL_RGBA || internalFormat == GL_RGBA8)
    {
        image = QImage(fbo->size(), hasAlpha ? QImage::Format_RGBA8888 : QImage::Format_RGBX8888);
        m_functions->glReadPixels(0, 0, fbo->size().width(), fbo->size().height(), GL_RGBA, GL_UNSIGNED_BYTE, image.bits());
    }
    else
    {
        qDebug() << "MyGLWindow::grabFramebuffer() - Unsupported framebuffer format" << internalFormat << "!";
    }
    m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
    return image.mirrored();
}

void MyGLWindow::swapBuffers()
{
    swapBuffersInternal();
    emit frameSwapped();
}

void MyGLWindow::swapBuffersInternal()
{
    if (isExposed() && isVisible())
    {
        //blit framebuffer to back buffer
        m_context->makeCurrent(this);
        //make sure all paint operation have been processed
        m_functions->glFlush();
        //check if we have glFrameBufferBlit support. this is true for desktop OpenGL 3.0+, but not OpenGL ES 2.0
        if (m_functions_3_0)
        {
            //if our framebuffer has multi-sampling, the resolve should be done automagically
            m_functions_3_0->glBindFramebuffer(GL_READ_FRAMEBUFFER, m_fbo->handle());
            m_functions_3_0->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
            //blit all buffers including depth buffer for further rendering
            m_functions_3_0->glBlitFramebuffer(0, 0, width(), height(), 0, 0, width(), height(),
                                                          GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT,
                                                          GL_NEAREST);
            m_functions_3_0->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
        }
        else
        {
            //we must unbind the FBO here, so we can use its texture and bind the default back-buffer
            m_functions->glBindFramebuffer(GL_FRAMEBUFFER, 0);
            //now use its texture for drawing in the shader
            --> bind shader and draw textured quad here
            //bind regular FBO again
            m_functions->glBindFramebuffer(GL_FRAMEBUFFER, m_fbo->handle());
        }
        //check if OpenGL errors happened
        if (GLenum error = m_functions->glGetError() != GL_NO_ERROR)
        {
            qDebug() << "MyGLWindow::swapBuffersInternal() - OpenGL error" << error;
        }
        //now swap back buffer to front buffer
        m_context->swapBuffers(this);
    }
    else
    {
        //not visible. only flush the pipeline so we can possibly grab the FBO later
        m_context->makeCurrent(this);
        m_functions->glFlush();
    }
}

void MyGLWindow::recreateFBOAndPaintDevice()
{
    const QSize deviceSize = size() * devicePixelRatio();
    if (m_context && (m_fbo == nullptr || m_fbo->size() != deviceSize))
    {
        m_context->makeCurrent(this);
        //free old FBOs
        if (m_fbo)
        {
            m_fbo->release();
            delete m_fbo;
            m_fbo = nullptr;
        }
        if (m_resolvedFbo)
        {
            m_resolvedFbo->release();
            delete m_resolvedFbo;
            m_resolvedFbo = nullptr;
        }
        //create new frame buffer
        QOpenGLFramebufferObjectFormat format = QGLInfo::DefaultFramebufferFormat();
        format.setSamples(QGLInfo::HasMultisamplingSupport(m_context) ? 4 : 0);
        m_fbo = new QOpenGLFramebufferObject(deviceSize, format);
        if (!m_fbo->isValid())
        {
            throw("MyGLWindow::recreateFbo() - Failed to create background FBO!");
        }
        //clear framebuffer
        m_fbo->bind();
        m_functions->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
        m_fbo->release();
        //if multi sampling is requested and supported we need a resolve FBO
        if (format.samples() > 0)
        {
            //create resolve framebuffer with only a color attachment
            format.setAttachment(QOpenGLFramebufferObject::NoAttachment);
            format.setSamples(0);
            m_resolvedFbo = new QOpenGLFramebufferObject(deviceSize, format);
            if (!m_resolvedFbo->isValid())
            {
                throw("MyGLWindow::recreateFbo() - Failed to create resolve FBO!");
            }
            //clear resolve framebuffer
            m_resolvedFbo->bind();
            m_functions->glClear(GL_COLOR_BUFFER_BIT);
            m_resolvedFbo->release();
        }
    }
    //create paint device for painting with QPainter if needed
    if (!m_paintDevice)
    {
        m_paintDevice = new QOpenGLPaintDevice;
    }
    //update paint device size if needed
    if (m_paintDevice->size() != deviceSize)
    {
        m_paintDevice->setDevicePixelRatio(devicePixelRatio());
        m_paintDevice->setSize(deviceSize);
    }
}

void MyGLWindow::initializeInternal()
{
    if (!m_initialized.exchange(true))
    {
        //create OpenGL context. we set the format requested by the user (default: QWindow::requestedFormat())
        m_context = new QOpenGLContext(this);
        m_context->setFormat(format());
        if (m_context->create())
        {
            m_context->makeCurrent(this);
            //initialize the OpenGL 2.1 / ES 2.0 functions for this object
            m_functions = m_context->functions();
            m_functions->initializeOpenGLFunctions();
            //try initializing the OpenGL 3.0 functions for this object
            m_functions_3_0 = m_context->versionFunctions<QOpenGLFunctions_3_0>();
            if (m_functions_3_0)
            {
                m_functions_3_0->initializeOpenGLFunctions();
            }
            else
            {
                //if we do not have OpenGL 3.0 functions, glBlitFrameBuffer is not available, so we must do the blit
                //using a shader and the framebuffer texture, so we need to create the shader here...
                --> allocate m_blitShader, a simple shader for drawing a textured quad
                --> build quad geometry, VBO, whatever
            }
            //now we have a context, create the FBO
            recreateFBOAndPaintDevice();
        }
        else
        {
            m_initialized = false;
            delete m_context;
            m_context = nullptr;
            throw("Failed to create OpenGL context!");
        }
    }
}

void MyGLWindow::update()
{
    //only queue an update if there's not already an update pending
    if (!m_updatePending.exchange(true))
    {
        QCoreApplication::postEvent(this, new QEvent(QEvent::UpdateRequest));
    }
}

void MyGLWindow::render()
{
    std::lock_guard<std::mutex> locker(m_mutex);
    //check if we need to initialize stuff
    initializeInternal();
    //check if we need to call the user initialization
    if (!m_initializedGL)
    {
        m_initializedGL = true;
        initializeGL();
    }
    //make context current and bind framebuffer
    makeCurrent();
    bindFramebufferObject();
    //call user paint function
    paintGL();
    doneCurrent();
    //mark that we're done with updating
    m_updatePending = false;
}

void MyGLWindow::exposeEvent(QExposeEvent * e)
{
    //call base implementation
    QWindow::exposeEvent(e);
    //render window content if window is exposed
    if (isExposed()/* && isVisible()*/)
    {
        render();
    }
}

void MyGLWindow::resizeEvent(QResizeEvent *e)
{
    //call base implementation
    QWindow::resizeEvent(e);
    m_mutex.lock();
    //make context current first
    makeCurrent();
    //update FBO and paint device
    recreateFBOAndPaintDevice();
    m_mutex.unlock();
    //call user-defined resize method
    resizeGL(e->size().width(), e->size().height());
    emit resized();
}

bool MyGLWindow::event(QEvent *event)
{
    switch (event->type())
    {
        case QEvent::UpdateLater:
            update();
            return true;
        case QEvent::UpdateRequest:
            render();
            return true;
        default:
            return QWindow::event(event);
    }
}