hi Alain,

thank you for replying.

On 11/03/2014 02:50, Alain wrote:
> Looking at your function, I suspect ... box {<-9, -2, -9>, <9, 2, 9>}

> Add an identical box with pigment{rgbt<1, 0.1, 0.1, 1>} to your scene if
> it looks like there are missing parts.

> You can also add inside_texture{pigment{rgb<1,0,0>}finish{ambient 1}}.
> That way, any hole should appear in bright red.
> 
> Next, the camera needs to be located outside that box.

I followed your advice and used those bounds, initially with colour to
verify the camera placing.  I also placed the box with the transparency.

no joy though, still all black; I even managed to lose the "*something*"
found by Scott. :-(

after some more googling I downloaded Ingo's MMGM and had a look at his
p_umbilic.pov file, which works nicely (and so much faster than the
parametric).  what is very confusing though is that Ingo's version swaps
the x and z functions when compared to the formulas in Wikipedia and
Blaise Montandon's program, and also that BM's formulae incorporate the
pi and use -1..1 bounds while the others use -pi..pi bounds, a different
way of scaling I assume?

I shall have to burn some more 'midnight oil', my aim is to use the
parametric because, AIUI, the surface can then be decorated with a
texture which itself is the result of a computation (needed, I think, to
replicate the Hilbert curve engraved on the original artwork).

btw, inside_texture is not available in v3.6.

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