Right, just to illustrate this suspected precision glitch, here's 7 images
of a simplified Ringworld (please excuse the crudity of the composite).

The Ring is a hollow open cylinder 1 million units tall and 90 million units
in radius, with a light at the centre. I've translated it by 90 million y
so that the origin lies on the Ring surface with the x axis at a tangent.

The first three images are, left to right, a camera y = 100, 10, 1.
The middle image is camera y = 0.
The final three images are, left to right, a camera y = -1, -10, -100.

Firstly, Thomas, do you see the same thing with your Ring?
Secondly, can anyone clarify what exactly is happening here? Is it a
precision problem, or something else?

code:

camera { location <0, 100, 0>
         up <0, 1, 0>
         right <4/3, 0, 0>
         direction <0, 0, FoV_75>
         look_at <-1000, 0, 0> }

#declare Ring = union {
  light_source { <0, 0, 0> color White }
  cylinder { <0, 0, -500000>, <0, 0, 500000>, 90000000 open hollow
             pigment { color SkyBlue }
             finish { ambient 0.5 } } }

object { Ring translate <0, 90000000, 0> }


Bill, confused.

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Preview of image 'ringtest.png'