First problem solved.

1) comment out the atmosphere.

2) In your cloud texture, replace the finish either by:

   finish {ambient1 diffuse 1}	// no radiosity

or:

   finish {emission 1 diffuse 1}	// radiosity

3) In your cloud texture, replace the scale by:

   scale ring_width/2000

Now the clouds become visible above the camera. The scale is important 
for this view. The Ringworld is so incredibly huge that what looks right 
from space is only a grey expanse seen from ground level. Cloud patterns 
like on Earth would look tiny on the Ringworld.

4) The problem is your atmosphere which generates the artefact in the 
foreground of the image. This seems probably to be an epsilon effect but 
I am not so sure about that as without atmosphere the scene renders 
properly. I would like the opinion of experts on this.

In any case replace the scattering by:

   scattering {4, <0.1, 0.3, 1.0> * 1000000/ring_radius}

This will make the atmosphere visible.

See the attached image for the state of the art.


Thomas

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