I want to render my animated scene with only 1 object visible - where other objects pass over/infront of it I want a gap. - ie. Imagine cube A is the only thing i want to be visible in my render, cube B passes across cube A and I want the shape of cube B to block part of cube A (leaving a hole/gap in A) even though cube B is invisible.
I guess another way of describing whAt I need is - a way of rendering an alpha channel of how a group of objects(i want to be invisible) affect(block from view) a visible object.
I hope this doesnt sound stupid but its hard to decribe without being too long winded.
I have been trying all kinds of multipass render settings and compositing tags but cant get it right.
I think you'll have to do two renders: one with A and a multipass render with the other objects, where you assign object IDs and separate passes for the other objects. Then you can assemble the first render with the object ID passes in AE or similar.