Is there any easy way to put kind of a "mask" in front of the camera in an Ogre 3d application, so that parts of the scene are always covered by that mask? The idea is that I want to have an effect as if you would look through a keyhole, or as if you would look through a tube. There should be a black border around the center of the screen, so that the user only sees that center.
Ideally, I would be able to dynamically change the diameter of this circle through which the user looks on the scene. Thus that black border on the sides of the scene would become wider or narrower.
Any ideas how to implement that in Ogre?