> Could someone tell me what is the artifact in this scene?
> When you move the spot light it moves around also.


Well, you should first know that your spotlight is not *supposed* to make
any changes in your scene. Lights only interact with scattering media, and
they won't interact with emission or absorption. So don't be expecting the
spotlight to light up the media.

OK, that said, here's part of why the spotlight is causing problems. Notice
that if you changed your spotlight to a cone object of the same size, the
cone would exactly surround the artifacts. Since spotlights affect
scattering media, they're programmed to change the way media is sampled -
the media is split up into the parts *in* the light and the parts *outside*
of the light. If your sampling parameters aren't very good (that is, if they
don't create enough samples to show the media correctly), then the fact that
the spotlight splits up the media into parts may create unwanted artifacts.

Now, I was at first wondering why this should make a difference, since all
your media is of a solid density, but I think I might know. The ratio
keyword, which has a default of .9, makes it so that more samples are taken
in lit areas of media than in unlit areas.

So, you might try setting "ratio 1". If that doesn't work, then I'm not sure
what the problem is, but I'm willing to bet that specifying better media
sampling parameters and decreasing the size of the containing sphere will
minimize the problem.

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

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