Is there a material that contributes to GI, but isn't directly visible? I am doing renders of the inside of a room. I would like to show a "bird's eye" view of the room, from above the ceiling (outside the room), but still see the interior of the room.



An easy way of doing this is to simply hide or remove the ceiling surfaces. However, this has the side effect of reducing the amount of lighting in the scene, because the ceiling no longer reflects light into the room. The top corners and edges of the walls also are no longer lit accurately.

I would like to render the scene as if the ceiling was there, but without the ceiling actually being (directly) visible.

Is there a name for this problem, a technique, or material?
You can use single sided triangles for the ceiling so that they are pointing towards the room. This way the ceiling influences the GI in the room but you can see through it when observing from outside