Color picking, QOpenGL & back buffer

Question

There are some mysteries, implementing a simple colorPicking with QOpenGL in-built tools.

Context : I've an application, owning its own OGL widget. For some reasons (multi-widgets), I had to change my QGLWidget by a QOpenGLWidget which allows me easily to have many OpenGL contexts without (a priori) any problems. This change actually broke my color Picking and I'm then investigating:

I previously did this so as to get my object:

glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);


// render every object in our scene
ShaderLib::bindShader(ShaderLib::PICKING_SHADER);
{
    for(auto const& _3dobject : model_->getObjects())
        _3dobject.second->draw(projection_, cameraview_, true);
}
ShaderLib::unbind();

glFlush();
glFinish();

// get color information from frame buffer
float pixel[4];
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);

glReadPixels(event->x(), viewport[3] - event->y(), 1, 1, GL_RGBA,  GL_FLOAT, pixel)

This perfectly worked with the QGLWidget. I could get the pixel, then the matching object. I saved my pixels in QImage to confirm and I exactly had what was expected.

After changing QGLWidget by QOpenGLWidget :

Then, with the QOpenGLWidget, the above code didn't work. Worst : glReadPixels seems to not reading in the back framebuffer. How do I know ? I simply displayed the whole supposed buffer read via glReadPixels, as before, and it gives me a partial screenshot of my application but not my QOPenGLWidget :O, what means that now, glReadPixels has a different behavior according to QGLWidget or QOpenGLWidget !

Well. Never give up !

I try to get the framebuffer through QOpenGLWidget::grabFrameBuffer(); It creates a QImage of the... I don't know what buffer.

glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);


// render every object in our scene
ShaderLib::bindShader(ShaderLib::PICKING_SHADER);
{
    for(auto const& _3dobject : model_->getObjects())
        _3dobject.second->draw(projection_, cameraview_, true);
}
ShaderLib::unbind();

glFlush();
glFinish();

QImage fb = grabFramebuffer();

This prints me an image of the framebuffer drawn in my paintGL() function, which is different from the one rendered in my MousePressEvent() (where I render with a specific 'picking' shader...

Hoped you followed everything. To sum up :

• Does anyone understand why glReadPixels gives a different result between the two 'painters' used ? I certainly missed something
• Does anyone gets how double-buffering is working with QOpenGLWidget ? It seems that the user cannot really choose what's happening.