When placing lights in some rooms, it is common that light bleeds through walls, and show up in other rooms close by. The obvious way to fix this is to add shadows, but shadows can be pretty expensive (especially for point lights), so it is not often a viable solution. In Amnesia we solved this through careful placement, yet bleeding can be seen in some places.
To fix this I added a new feature that is able to limit the lights range with a box. This way the light can cast light as normal but is cut off before reaching an adjacent area. This pretty much does the job of shadows, but is much cheaper.
It turned out to be pretty simple to implement as well. In the renderer, different geometrical shapes are used to render lights (spheres for point lights and pyramids for spots) which make sure the light only affects needed pixels. To implement the masking, these shapes where simply exchanged for a box and then with some small shader changes it all worked.
Without masking:
With mask:
