[osg-users] QtQuickFrameBufferObject and GraphicsContexts

[Antoine Rennuit]

Hi all

I have integrated OSG within QQuick 2 via QtQuickFramebufferObject, it is working great with complex scenes and within a complex application behind the hood, except for 1 point: I sometimes observe flickering of the whole window (it is not only the 3d view that flickers but the whole application window - check the video here (https://youtu.be/NlAxvTAVpl0)). It is not periodic, only every now and then, it can get crazy for a few seconds and then calms down.

In QtQuick the windowing system is actually rendered in OpenGL (hence the complexity). QtQuickFramebufferObject provides you with the tools to design a custom 3d item, and is thus the preferred way to implement a 3d view.

QtQuickFramebufferObject has you define a renderer and create a Qt compatible GL framebuffer in a function called by the QtQuick framework


Code:
QOpenGLFramebufferObject* OsgRenderer::createFramebufferObject(const QSize &size)
{
    QOpenGLFramebufferObjectFormat format;
    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    return new QOpenGLFramebufferObject(size, format);
}



it then has you write a render function also called by the QtQuick 2 framework, this is where I call my OSG rendering

Code:

void OsgRenderer::render()
{
    assert(m_osgItem);

    if ( !m_osgItem->getViewer() )
        return;

    // Without this line the model is not displayed in the second
    // and subsequent frames.
    QOpenGLContext::currentContext()->functions()->glUseProgram(0);

    // Ask OSG to render.
    m_osgItem->getViewer()->frame(); // WARNING: non-blocking (executed in a thread of its own - in a thread-safe way).

    // Reset OpenGl state for QtQuick.
    m_osgItem->window()->resetOpenGLState();
}



The QtQuick framework then merges the framebuffer in which I rendered with its own rendering to display the final image.

Now I suspect my flickering issue comes from a problem of GL context mix-up between QtQuick and OSG.

So I have inherited osgViewer::GraphicsWindowEmbedded to use as custom GraphicsContext which binds and releases the framebuffer created above:

Code:

bool OsgWindow::makeCurrentImplementation()
{
    if (!m_renderer)
        return false;

    if (!m_renderer->framebufferObject())
        return false;

    return m_renderer->framebufferObject()->bind();
}

bool OsgWindow::releaseContextImplementation()
{
    if (!m_renderer)
        return false;

    if (!m_renderer->framebufferObject())
        return false;

    return m_renderer->framebufferObject()->release();
}



Does that ensure me that the OSG rendering in frame() is written to this FBO?

Also as far as I know, the GL context is kept in OsgWindow (it inherits osgViewer::GraphicsWindowEmbedded as stated above), but is it reapplied at each frame? i.e. the GL context is modified by QtQuick's own GL rendering but am I sure that it is put back in place for OSG's correct rendering when I call frame()? If not, is there a way for me to enforce OSG's GL context is not modified by QtQuick?

Thanks a lot,

Antoine

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http://forum.openscenegraph.org/viewtopic.php?p=71500#71500