I'm trying to render offscreen a qml file. But after the rendering the opengl texture remain empty.
The test application can be found here but the render part look like that:

_qcontext->makeCurrent(_offscreenSurface);
    _fbo->bind();
    _quickWindow->setRenderTarget(QQuickRenderTarget::fromOpenGLTexture(_fbo->texture(), GL_RGBA, QSize(_width, _height)));
    _renderControl->beginFrame();
    _renderControl->polishItems();
    _renderControl->sync();
    _renderControl->render();
    _renderControl->endFrame();
    _fbo->toImage().save(QString("toImgage-%1.png").arg(_fbo->texture()));
    _qcontext->functions()->glBindTexture(GL_TEXTURE_2D, _fbo->texture());
    QImage img(_width, _height, QImage::Format_RGBA8888);
    _qcontext->functions()->glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, _width, _height, 0, GL_RGBA,  GL_UNSIGNED_INT_8_8_8_8_REV, img.bits());
    img.save(QString("img-%1.png").arg(_fbo->texture()));

if the line 9th line ( _fbo->toImage() ) is commented out the texture remain empty. How can I force the result to be store in the texture without calling the toImage function ?

Regards,