[NTripleOne]

Yes, you're entirely correct - that is precisely why it's concave - as Parallax mapping is purely the illusion of depth, instead of adding any actual depth or height, this means that the perceived depth effect is constrained to the surface of the object.

If the pyramid shape were to extend past the surface of the texture, it would look fantastic up until the point that the viewing angle of the surface would cause the pyramid to visibly protrude past the edge, at which point anything extending beyond those boundaries would be lost.

Here's a terrible mspaint example, please ignore my awful consistency, it's early and I've not had any caffeine yet.

http://i.imgur.com/iC7czF5.png