[webkit-qt] WebGL rendering offscreen

Jorge Fernandez Monteagudo jorgefm at cirsa.com
Sun Nov 9 01:25:04 PST 2014

Hi all!

This is my first message in this list. Maybe this question has been asked before several times
but I've not been able to find a link with an answer, then I go to the source :)

I'm trying to embed a WebGL page render in my OpenGL application. I've develop a little test case
using Qt 5.3.2, when I'm able to show a static html page rendered in a OpenGL texture in a QGLWidget.
This texture comes from a QGLFramebufferObject. I use the QWebPage 'repaintRequested' signal to
do the FBO generation. This is the code:

QGLFramebufferObject *mp_fbo;
QWebPage *mp_Page;
QPainter m_Painter;

void TGSWebNavigator::onRepaintRequested(const QRect& _rect)
        // Paint the view in the FBO.
        m_Painter.begin( mp_fbo );
        mp_Page->mainFrame()->render( &m_Painter, _rect );

        // Repaint QGLWidget with the new FBO.

If I try to do the same with an URL pointing to a WebGL I get OpenGL errors. The problem I think is related
to OpenGL contexts because the OpenGL widget has his own context and the WebGL rendering is done
is another one. In a static html page no new context is created when rendering is done. I've set several
traces to show the current context:

        qDebug() << "[TGSWebNavigator] onRepaintRequested 1... ctx" << QGLContext::currentContext();
        m_Painter.begin( mp_fbo );
        qDebug() << "[TGSWebNavigator] onRepaintRequested 2... ctx" << QGLContext::currentContext();
        mp_Page->mainFrame()->render( &m_Painter, _rect );
        qDebug() << "[TGSWebNavigator] onRepaintRequested 3... ctx" << QGLContext::currentContext();
        qDebug() << "[TGSWebNavigator] onRepaintRequested 4... ctx" << QGLContext::currentContext();

When I start seeing different context in trace 2 and 3, the OpenGL errors begins.

I've tried using 'QGLPixelBuffer' because you can set a shared widget which you want to share textures
(my QGLWidget) with no luck. The only solution comes from replace the FBO with a QImage, render
on it and finally change the texture I use in the QGLWidget with the contents from the QImage but this
it's not an option, the perfomance is very affected.

Then my question is: are there any options? Can I set the context where the WebGL rendering is done?
Or, at least make it share with a given context? Maybe I'm totally wrong and there is other ways to do
what I'm trying to do... Any comment, link, info is welcome!

Best regards,

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