Okay, thank you a lot for the explanation Hodgman but it's a bit "restricted" to specifically BSP implementation... I'm having trouble trying to mentally/visually "translate" this into working for a "basic" octree/quadtree..
We can start with an example, as a quadtree top-down view:
Could you maybe explain how this could be applied in a case such as this?
In your BSP example, I kind of understand how you can check whether another plane is "behind" or "in front of" a given plane, but in a quad tree... you'd have to check 4 planes for a given section, in all four directions, so wouldn't every other section technically be "visible" by this procedure?