// say you have a 2D map
// like so
// X X X X X X
// X X X X X X
// X X X X X X
// X X X X X X
// X X X X X X
// X X X X X X

// you can splitup the map into pieces
// as many as you see fit, but keep them
// large, and don't bother sorting them
// either

// so now you can have something like
// X X X    X X X
// X X X    X X X
// X X X    X X X
//
// X X X    X X X
// X X X    X X X
// X X X    X X X

// you generate 4 VBOs for this since you have _four_ sections
// these sections can be your PORTALS!
// lets give them ids 0-3

// now you have 4 VBOs, it can be 5,6,7,8 does not matter
// map size can be anything, you just break it down into
// smaller chunks, that are farily large, but not too
// large that the portal culling would be useless

// now you implement yout actual PVS culling
// for(vbos_in_map)
//   if(camera_is_in_portal_or_visible_to_portal)
//     add_vbo_to_instance_data

// finally your render it!
// this way only VBO's you can see or are in
// will be rendered, everything else will not