I am sorry for digging such a old thread but this feature would be really useful. Some people are stating that this feature does not exist in most of the game engines and it would be costly but I cannot agree with that. Unity3D and has this feature and it comes with no additional rendering costs. In fact, you can get performance gains if you decrease amount of layers that your camera is rendering. I believe that Cryengine also have this feature.
Possible uses:
- camera that renders FPP hands+weapon
only and composites its render target
over normal game image - camera that renders only some
fractions of the game and is
generating custom rendering buffer
(for example, you want to render
characters only or foliage only, or
just your enemies which are added to
the “enemies” layer)