Thanks for the suggestion CA and the reply and explanation Oshyan.
So there are 2 ways of rendering the terrain.
1) render-time displacements
2) pre-calculating displacements (which needs a lot of memory)
Both of them won't differ much in rendertime, which I can imagine.
Don't understand me wrong. I'm a patient person and don't bother waiting ages for a render to finish, but it feels a little bit useless that (in my case) 75% of the rendertime is due to rendering pixels which aren't visible at the end.
What I meant to say is that, either which of the 2 methods you use, why does the renderer 'choose' to render things first BEHIND objects/hills or the background instead of rendering first what faces the camera? So, building up the image from front till the back. Or am I now talking about 100% depth-based occlusion?