What drawVideoBackground() does?

Hi,

On my renderFrameQCAR() loop, I tried to commented this line :

//QCAR::Renderer::getInstance().drawVideoBackground();

I expected my AR to work as usual except the video would not show. But instead of that, I get a black screen. All my overlays, images I were drawing on the open gl surface are not shown. Why is that? Is there a way to show the overlays without showing the video? 

It would be too difficult to explain why I need that, but trust me that would help me so much. Thanks a lot for your help!