Applying gradient texture to deformed volume based on Z coordinate

Question

I'm trying to apply a fog to my grass plane, which is deformed in multiple ways (plane is subdivied + waved by hand + 2x modifier: displace. The fog is supposed to be dense at the bottom and disappear with height, based on the gradient. But the gradient is applied to the whole object, not the local Z coordinates of the volume. Fog is generated with Volume Shatter.

I know the description is complicated, so, IMAGES! What I have is rendered, what I want is with those arrows.

[1]What I 'want' presented with a surface shader: [2]What I have: [3]This is how it looks in volume shatter:

So: I want the effect from [1] to apply to volume. Please help. I'm Struggling with this for days...

I hope I didn't use wrong tags.

Answer 1

A guy from blender IRC support channel (#blender on freenode) found the answer, so, as follows!

There are TWO methods. First one is raytracing: It's really slow, and OSL depending. And casuses crashes, apperently ;)

Second method, much faster (bueno!) uses height maps instead of gradient texture.

Also, @JA12 simplified it even more, look at the nodes:

You can study it with the attached .blend file. It's amazingly simple. .blend file my mirror: mirror