Could someone tell me what is the artifact in this scene?
When you move the spot light it moves around also.
I'll place a image in povray.binaries.image.

camera{ location <0,8,-15> look_at <0,0,-3> }

light_source{<0,2,8> color rgb 1 spotlight point_at <3,2,12> radius 60 falloff
120}

sphere{0,450 pigment {rgbt .5}
     interior {media{ emission<.1,.2,.3> absorption<.4,.4,.4> }}
 no_shadow hollow}

//ground
box{<-80,0,-80><80,-1,80> pigment{checker rgb 1 rgb<1,0,0>}}


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> 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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Thanks for the input!

I should have stated earlier I'm using POV3.5 RC5 windows.

 I didn't want the spotlight to interact with the media all I wanted was to put
a spotlight on an object in an object with media.(that's about as clear as mud)

 I tried your suggestion about "ratio 1" and changing some of the other sampling
parameters, I haven't found the problem yet.  I also changed the size of the
sphere, change the sphere to a box, change the pigment from transparent to
solid. All with no effect.
 I do know one thing using absorption or emission alone, there is no problem.
And the artifact will disapear at certain viewing angles. There is no problem in
POV3.1.
I took that little POV file and turn it into an animation. It's hard to tell but
the artifact looks like a gaint sphere behind the spotlight.

When I decide to ask about this question here, I thought that someone would say
that it was an old problem and tell me the fix. But the more I work on it the
more I think it's a bug.

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>  I do know one thing using absorption or emission alone, there is no
problem.

Hmmm!

Interesting. That gives me a little insight into what's happening.

Emmitting media gets brighter when there's more of it.
Absorption media gets darker when there's more of it.

When you combine the two, they fight against each other. In my experience,
absorption tends to win in the long run. I suspect that somehow, the way the
spotlight is causing your intervals to be split up, it's causing the
absorption to win in some cases and the emission to win in others.

Try fiddling with the intervals keyword. Use the command line parameter
+SP16 to get a fast render, and then set intervals to something like 100,
just to see if that removes the problem. Look for the smallest value you can
have that still keeps the problem away.

Also raise your max_trace_level... just in case.

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

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I found a work around if you make your own spotlight with a point light and a
open cone then everything works as expected.
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Have Fun!

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