Is there a way to map a world space position to a ramp?  Say for example I have a room and I want the bottom of the room and to be mapped to the bottom of the ramp and everything at the top of the room mapped to the top of the ramp.  So if the ramp went from black to white, objects on the floor with this ramp material would be black, while objects at the ceiling would be white (and everything in between would be graded).