In the meantime i stumbled upon the fog feature. It does exactly what I want
except that I can only define a plane but not really another geometry:
fog{ fog_type 2
distance 3
color White
fog_offset -5.5
fog_alt 2.0
turbulence 1.8
up <-0.5,1,0>
}
This would be great if I could somehow define a center point where the fog
then starts expanding like a sphere, obscuring everything.
