I need to render only some part of virtual object like in this image where part of Android Robot is hidden 
For doing this I use "Stencil Testing" and i draw with OpenGL a simple rectangle.
The problem is that now I need the part "hidden" to be not static but dynamic and I have to choose the part to hidden from a binary image like this one 
where the black part needs to "hide" (like in the example before) the virtual object and virtual plane. The "mask" will be changed for every frame.
