Mike Williams wrote:

> [...]
> 
> What value are you using for max_trace?
> 

hmm, thanks for the suggestion, it was at the default
but changing it to 20 made no difference.

First up what I'm actually trying to do is:

difference {
        object {
                StoneWork
                texture { T }
        }
        isosurface {
                // isocylinder + pigment fn of T
                texture { T1 }
        }
}

but this change makes no difference to the problem
and I was trying to simplify when I posted.

I have a few observations though.

1. Switch to radiosity without conventional lighting
and the problem more or less disappears.

2. Swap in a simple cylinder for the isosurface and
the problem also goes away.

3. However, drop the intensity if the pigment fn
perturbing the isosurface cylinder to zero (so it's
a simple isosurface cylinder) and the problem persists.

if it helps, here's my code for the isosurface cylinder
(vim syntax highlighting converted to html):

http://thyme.homelinux.net/IsoCylinder.html

I'm probably doing something stupid :-(

-- 
Bill Hails
http://thyme.homelinux.net/

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