> 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.
>
>

If you want sometning circular, the disk may be what you need:
disk{<0,0,0>, <0,0,-1>, 1 pigment{spherical colour_map{[0 rgb 
background_colour][clock background_colour filter 1] } scale Some_Scale 
translate Somewhere hollow}
If your scene use any media or fog, the hollow is needed it you want the 
media or fog to act predictably. A disk have an interior that is the 
same as that of a plane and it's not limited to the visible part.
You may also use a plane similarly.

This, in an animation, will start with a totaly transparent disk that 
will become increasingly opaque in a circular gradient. The center will 
stay transparent. Using the background's colour and manipulating the 
filter component ensure that the mix will be concistent.

Another way would be with some media. The density of a media can be 
exactly controled by a pigment of any function. The spherical pattern 
start with a value of zero at the origin and drop to zero at a radius of 
1. This can be changed by using a colour_map and an alternate wave type 
for the way it changes, and by scaling to adjust it to your needs.



Alain

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