> No, POV-Ray supports NURBS as much as most other renderers support them 
> - by rendering meshes generated from them.

Sure, POV-Ray supports triangle meshes - but you'd need an external 
application to generate a triangle mesh from a NURBS mesh (of course SDL 
could do this, but it's quite slow when you need to tesselate thousands 
of NURBS patches).
The scene file containing triangle meshes is much larger than the file 
describing just the NURBS, this can be a problem for distributing the 
scenes via the internet (e.g. for the Internet Movie Project).
Another problem when tesselating "outside" of POV-Ray are the trim curves...

> It is possible to directly raytrace NURBS of course but that is rarely done.

I've read some papers about directly ray-tracing bezier-patches (I think 
NURBS would work quite similar) - The conclusion was that it's also just 
an approximation, and a tesselated triangle mesh of the same quality 
would render faster, so there would be no real advantage in raytracing 
"directly".

> No.  No algorithm commonly used to directly raytrace NURBS would allow 
> displacement mapping.  So the above statement does not make sense.

I meant the tesselation approach (as used in POV-Ray for 
bezier-patches).  Theoretically it should be possible to add 
displacement mapping to the bezier-patch tesselation code.
The big advantage of doing displacement mapping within POV-Ray would be 
that all the patterns that can be used for bump-mapping could be re-used.

Don't get me wrong. Iso-Surfaces are a really cool feature of POV-Ray 
and offer a lot of possibilities. Its just a bit of an overkill to use 
this feature for displacement mapping...

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