> If I take the fog away all looks good but WHY does the fog make my hollow
> transparent cone visible?  Better yet how do I keep it from making my
> transparent cone visible?

This is a limitation of transparent fog. What happens is that every time the
ray passes through a transparent surface, fog up to that point is
calculated. When the ray passes through no surfaces and hits the sky sphere,
you get 80% blackness (which is essentially invisible since your sky sphere
is so bright). But when it passes through two surfaces (into the cone and
back out), you get 80% blackness combined with 80% of what's left blackness
combined with 80% of what's left after that blackness, giving you 99.2%
blackness. (Actually a little less than that since not all of these three
rays pass through an infinite distance.)

The solution is to not use transparent fog when you need transparent
surfaces. Now, if I remember correctly, you're making a light cycle arena
and you wanted fog in the arena but you didn't want it to cover up your
background, right? I assume that's why you're using transparency in the fog.
Another solution you might try is ground fog. Use fog_type 2 with fog_offset
and fog_alt (check the documentation) to make your fog only appear within
the arena and hardly be visible above it. Then you may be able to ditch the
transparency and the effects won't be noticeable in the background. I tried

  fog {
    distance 500
    color rgb <0.0, 0.0, 0.0>
    fog_type 2
    fog_offset 0 // constant density below y=0
    fog_alt 2 // rate at which it fades off above y=0
  }

and got pretty good results. If you can see the fog too much around the base
of the cone when you put media in the cone, then try putting the cone
farther away or decreasing fog_alt or fog_offset.

 - Slime
 [ http://www.slimeland.com/ ]

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