Constraining visibility

[inonzuk]

Hello,

I want to get the following effect of constraint visibility:
when I look at a box from 50 distance units away, it is visible.
backing up slowly it becomes blurrier until at some distance away it disappears from the view completely (let's say 100 distance units away).

I tried playing with the FOG parameters (both linear and exp/exp2) and I did not understand how to use them to get that effect. I assume I need to use the linear fog somehow, but how ?

Let's say the world size is 4000, how can I restrict my visibility to 100 distance unit away ? (i.e., every object located more than 100 unit from the tank location will not be seen by the tank).

Any advice of that would be more than welcome.
Thanks in advance
inonzuk

[trepan]

http://www.opengl.org/sdk/docs/man/xhtml/glFog.xml

BZDB variables
_fogMode { none | linear | exp | exp2 }
_fogColor { all modes }
_fogStart { linear }
_fogEnd { linear }
_fogDensity { exp & exp2 }