> 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.
>

Oups! It's should have been interior_texture, and it's definitively a 
feature found in 3.6. Not sure, but it may have been there in 3.5 or 
earlier.

You should upgrade to version 3.7. It's somewhat faster, can take 
advantage of all cores on modern computer, have several bug fixes, 
introduce some new features like subsurface light transport and high 
dynamic range images as source and output option.
It also have some improvements for focal blur and antialiasing. The irid 
code is totaly redone as it was broken in version 3.6: Just add this to 
some finish to see what's the problem (also try with turbulence 10, 1, 
0.1, 0.01):
irid{0.2 thickness 0.2 turbulence 0.001}




Alain

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