z depth with globals is ickly. Mayas zdepth will render out to iff if you dont have a 32 bit format chosen.
Main issues are that mayas z depth doesnt render out to a visible range so you have to multiply it into the visible range somtimes.

Also its not anti=aliased, when maya renders z depth it just finds distance as an absolute. So to get around that you have to render z depth at double res and than scale it down with a filter, which is really not fun. Shader ftw.